The role of the cerebral capillaries in acute ischemic stroke: the extended penumbra model

No-reflow after recanalization in ischemic stroke: From pathomechanisms to therapeutic strategies

Endovascular reperfusion therapy is the primary strategy for acute ischemic stroke. No-reflow is a common phenomenon, which is defined as the failure of microcirculatory reperfusion despite clot removal by thrombolysis or mechanical embolization. It has been reported that up to 25% of ischemic strokes suffer from no-reflow, which strongly contributes to an increased risk of poor clinical outcomes. No-reflow is associated with functional and structural alterations of cerebrovascular microcirculation, and the injury to the microcirculation seriously hinders the neural functional recovery following macrovascular reperfusion. Accumulated evidence indicates that pathology of no-reflow is linked to adhesion, aggregation, and rolling of blood components along the endothelium, capillary stagnation with neutrophils, astrocytes end-feet, and endothelial cell edema, pericyte contraction, and vasoconstriction. Prevention or treatment strategies aim to alleviate or reverse these pathological changes, including targeted therapies such as cilostazol, adhesion molecule blocking antibodies, peroxisome proliferator-activated receptors (PPARs) activator, adenosine, pericyte regulators, as well as adjunctive therapies, such as extracorporeal counterpulsation, ischemic preconditioning, and alternative or complementary therapies. Herein, we provide an overview of pathomechanisms, predictive factors, diagnosis, and intervention strategies for no-reflow, and attempt to convey a new perspective on the clinical management of no-reflow post-ischemic stroke.

Prediction of pseudoprogression in post-treatment glioblastoma using dynamic susceptibility contrast-derived oxygenation and microvascular transit time heterogeneity measures

OBJECTIVES

To evaluate the added value of MR dynamic susceptibility contrast (DSC)-perfusion-weighted imaging (PWI)-derived tumour microvascular and oxygenation information with cerebral blood volume (CBV) to distinguish pseudoprogression from true progression (TP) in post-treatment glioblastoma.

METHODS

This retrospective single-institution study included patients with isocitrate dehydrogenase (IDH) wild-type glioblastoma and a newly developed or enlarging measurable contrast-enhancing mass within 12 weeks after concurrent chemoradiotherapy. CBV, capillary transit time heterogeneity (CTH), oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO2) were obtained from DSC-PWI. Predictors were selected using univariable logistic regression, and performance was measured with adjusted diagnostic odds with tumour volume and area under the curve (AUC) of receiver operating characteristics analysis.

RESULTS

A total of 103 patients were included (mean age, 59.6 years; 59 women), with 67 cases of TP and 36 cases of pseudoprogression. Pseudoprogression exhibited higher CTH (4.0 vs. 3.4, p = .019) and higher OEF (12.7 vs. 10.7, p = .014) than TP, but a similar CBV (1.48 vs. 1.53, p = .13) and CMRO2 (7.7 vs. 7.3s, p = .598). Independent of tumour volume, both high CTH (adjusted odds ratio [OR] 1.52; 95% confidence interval [CI]: 1.11-2.09, p = .009) and high OEF (adjusted OR 1.17; 95% CI:1.03-1.33, p = .016) were predictors of pseudoprogression. The combination of CTH, OEF, and CBV yielded higher diagnostic performance (AUC 0.71) than CBV alone (AUC 0.65).

CONCLUSION

High intratumoural capillary transit heterogeneity and high oxygen extraction fraction derived from DSC-PWI have enhanced the diagnostic value of CBV in pseudoprogression of post-treatment IDH-wild type glioblastoma.

CLINICAL RELEVANCE STATEMENT

In the early post-treatment stage of glioblastoma, pseudoprogression exhibited both high oxygen extraction fraction and high capillary transit heterogeneity and these dynamic susceptibility contrast-perfusion weighted imaging derived parameters have added value in cerebral blood volume-based noninvasive differentiation of pseudoprogression from true progression.

KEY POINTS

• Capillary transit time heterogeneity and oxygen extraction fraction can be measured noninvasively through processing of dynamic susceptibility contrast imaging. • Pseudoprogression exhibited higher capillary transit time heterogeneity and higher oxygen extraction fraction than true progression. • A combination of cerebral blood volume, capillary transit time heterogeneity, and oxygen extraction fraction yielded the highest diagnostic performance (area under the curve 0.71).

Acute Ischemic Stroke in the Clinic and the Laboratory: Targets for Translational Research

Ischemic stroke research has enabled significant advancements in diagnosis, treatment, and management of this debilitating disease, yet challenges remain standing in the way of better patient prognoses. In this narrative review, a fictional case illustrates challenges and uncertainties that medical professionals still face - penumbra identification, lack of neuroprotective agents, side-effects of tissue plasminogen activator, dearth of molecular biomarkers, incomplete microvascular reperfusion or no-reflow, post-recanalization hyperperfusion, blood pressure management and procedural anesthetic effects. The current state of the field is broadly reviewed per topic, with the aim to introduce a broad audience (scientist and clinician alike) to recent successes in translational stroke research and pending scientific queries that are tractable for preclinical assessment. Opportunities for co-operation between clinical and experimental stroke experts are highlighted to increase the size and frequency of strides the field makes to improve our understanding of this disease and ways of treating it.

Exploring vascular contributions to cognitive impairment and dementia (ENIGMA): protocol for a prospective observational study

BACKGROUND

Post-stroke cognitive impairment (PSCI) is common. However, the underlying pathophysiology remains largely unknown. Understanding the role of microvascular changes and finding markers that can predict PSCI, could be a first step towards better screening and management of PSCI. Capillary dysfunction is a pathological feature of cerebral small vessel disease and may play a role in the mechanisms underlying PSCI. Extracellular vesicles (EVs) are secreted from cells and may act as disease biomarkers. We aim to investigate the role of capillary dysfunction in PSCI and the associations between EV characteristics and cognitive function one year after acute ischemic stroke (AIS) and transient ischemic attack (TIA).

METHODS

The ENIGMA study is a single-centre prospective clinical observational study conducted at Aarhus University Hospital, Denmark. Consecutive patients with AIS and TIA are included and followed for one year with follow-up visits at three and 12 months. An MRI is performed at 24 h and 12 months follow-up. EV characteristics will be characterised from blood samples drawn at 24 h and three months follow-up. Cognitive function is assessed three and 12 months after AIS and TIA using the Repeatable Battery for the Assessment of Neuropsychological Status.

DISCUSSION

Using novel imaging and molecular biological techniques the ENIGMA study will provide new knowledge about the vascular contributions to cognitive decline and dementia.

TRIAL REGISTRATION

The study is retrospectively registered as an ongoing observational study at ClinicalTrials.gov with the identifier NCT06257823.

Micro-ultrasound based characterization of cerebrovasculature following focal ischemic stroke and upon short-term rehabilitation

Notwithstanding recanalization treatments in the acute stage of stroke, many survivors suffer long-term impairments. Physical rehabilitation is the only widely available strategy for chronic-stage recovery, but its optimization is hindered by limited understanding of its effects on brain structure and function. Using micro-ultrasound, behavioral testing, and electrophysiology, we investigated the impact of skilled reaching rehabilitation on cerebral hemodynamics, motor function, and neuronal activity in a rat model of focal ischemic stroke. A 50 MHz micro-ultrasound transducer and intracortical electrophysiology were utilized to characterize neurovascular changes three weeks following focal ischemia elicited by endothelin-1 injection into the sensorimotor cortex. Sprague-Dawley rats were rehabilitated through tray reaching, and their fine skilled reaching was assessed via the Montoya staircase. Focal ischemia led to a sustained deficit in forelimb reaching; and increased tortuosity of the penetrating vessels in the perilesional cortex; with no lateralization of spontaneous neuronal activity. Rehabilitation improved skilled reaching; decreased cortical vascularity; was associated with elevated peri- vs. contralesional hypercapnia-induced flow homogenization and increased perilesional spontaneous cortical neuronal activity. Our study demonstrated neurovascular plasticity accompanying rehabilitation-elicited functional recovery in the subacute stage following stroke, and multiple micro-ultrasound-based markers of cerebrovascular structure and function modified in recovery from ischemia and upon rehabilitation.

Assessment of Small Vessel Function Using 7T MRI in Patients With Sporadic Cerebral Small Vessel Disease: The ZOOM@SVDs Study

BACKGROUND AND OBJECTIVES

Cerebral small vessel disease (cSVD) is a major cause of stroke and dementia, but little is known about disease mechanisms at the level of the small vessels. 7T-MRI allows assessing small vessel function in vivo in different vessel populations. We hypothesized that multiple aspects of small vessel function are altered in patients with cSVD and that these abnormalities relate to disease burden.

METHODS

Patients and controls participated in a prospective observational cohort study, the ZOOM@SVDs study. Small vessel function measures on 7T-MRI included perforating artery blood flow velocity and pulsatility index in the basal ganglia and centrum semiovale, vascular reactivity to visual stimulation in the occipital cortex, and reactivity to hypercapnia in the gray and white matter. Lesion load on 3T-MRI and cognitive function were used to assess disease burden.

RESULTS

Forty-six patients with sporadic cSVD (mean age ± SD 65 ± 9 years) and 22 matched controls (64 ± 7 years) participated in the ZOOM@SVDs study. Compared with controls, patients had increased pulsatility index (mean difference 0.09, p = 0.01) but similar blood flow velocity in basal ganglia perforating arteries and similar flow velocity and pulsatility index in centrum semiovale perforating arteries. The duration of the vascular response to brief visual stimulation in the occipital cortex was shorter in patients than in controls (mean difference -0.63 seconds, p = 0.02), whereas reactivity to hypercapnia was not significantly affected in the gray and total white matter. Among patients, reactivity to hypercapnia was lower in white matter hyperintensities compared with normal-appearing white matter (blood-oxygen-level dependent mean difference 0.35%, p = 0.001). Blood flow velocity and pulsatility index in basal ganglia perforating arteries and reactivity to brief visual stimulation correlated with disease burden.

DISCUSSION

We observed abnormalities in several aspects of small vessel function in patients with cSVD indicative of regionally increased arteriolar stiffness and decreased reactivity. Worse small vessel function also correlated with increased disease burden. These functional measures provide new mechanistic markers of sporadic cSVD.

Hyperperfusion profiles after recanalization differentially associate with outcomes in a rat ischemic stroke model

Futile recanalization hampers prognoses of ischemic stroke after successful mechanical thrombectomy, hypothetically through post-recanalization perfusion deficits, onset-to-groin delays and sex effects. Clinically, acute multiparametric imaging studies remain challenging. We assessed possible relationships between these factors and disease outcome after experimental cerebral ischemia-reperfusion, using translational MRI, behavioral testing and multi-model inference analyses. Male and female rats (N = 60) were subjected to 45-/90-min filament-induced transient middle cerebral artery occlusion. Diffusion, T2- and perfusion-weighted MRI at occlusion, 0.5 h and four days after recanalization, enabled tracking of tissue fate, and relative regional cerebral blood flow (rrCBF) and -volume (rrCBV). Lesion areas were parcellated into core, salvageable tissue and delayed injury, verified by histology. Recanalization resulted in acute-to-subacute lesion volume reductions, most apparently in females (n = 19). Hyperacute normo-to-hyperperfusion in the post-ischemic lesion augmented towards day four, particularly in males (n = 23). Tissue suffering delayed injury contained higher ratios of hypoperfused voxels early after recanalization. Regressed against acute-to-subacute lesion volume change, increased rrCBF associated with lesion growth, but increased rrCBV with lesion reduction. Similar relationships were detected for behavioral outcome. Post-ischemic hyperperfusion may develop differentially in males and females, and can be beneficial or detrimental to disease outcome, depending on which perfusion parameter is used as explanatory variable.

L-Lactate Treatment at 24 h and 48 h after Acute Experimental Stroke Is Neuroprotective via Activation of the L-Lactate Receptor HCA1

Stroke is the main cause for acquired disabilities. Pharmaceutical or mechanical removal of the thrombus is the cornerstone of stroke treatment but can only be administered to a subset of patients and within a narrow time window. Novel treatment options are therefore required. Here we induced stroke by permanent occlusion of the distal medial cerebral artery of wild-type mice and knockout mice for the lactate receptor hydroxycarboxylic acid receptor 1 (HCA1). At 24 h and 48 h after stroke induction, we injected L-lactate intraperitoneal. The resulting atrophy was measured in Nissl-stained brain sections, and capillary density and neurogenesis were measured after immunolabeling and confocal imaging. In wild-type mice, L-lactate treatment resulted in an HCA1-dependent reduction in the lesion volume accompanied by enhanced angiogenesis. In HCA1 knockout mice, on the other hand, there was no increase in angiogenesis and no reduction in lesion volume in response to L-lactate treatment. Nevertheless, the lesion volumes in HCA1 knockout mice-regardless of L-lactate treatment-were smaller than in control mice, indicating a multifactorial role of HCA1 in stroke. Our findings suggest that L-lactate administered 24 h and 48 h after stroke is protective in stroke. This represents a time window where no effective treatment options are currently available.

Neurovascular Uncoupling Is Linked to Microcirculatory Dysfunction in Regions Outside the Ischemic Core Following Ischemic Stroke

Background Normal brain function depends on the ability of the vasculature to increase blood flow to regions with high metabolic demands. Impaired neurovascular coupling, such as the local hyperemic response to neuronal activity, may contribute to poor neurological outcome after stroke despite successful recanalization, that is, futile recanalization. Methods and Results Mice implanted with chronic cranial windows were trained for awake head-fixation before experiments. One-hour occlusion of the anterior middle cerebral artery branch was induced using single-vessel photothrombosis. Cerebral perfusion and neurovascular coupling were assessed by optical coherence tomography and laser speckle contrast imaging. Capillaries and pericytes were studied in perfusion-fixed tissue by labeling lectin and platelet-derived growth factor receptor β. Arterial occlusion induced multiple spreading depolarizations over 1 hour associated with substantially reduced blood flow in the peri-ischemic cortex. Approximately half of the capillaries in the peri-ischemic area were no longer perfused at the 3- and 24-hour follow-up (45% [95% CI, 33%-58%] and 53% [95% CI, 39%-66%] reduction, respectively; P<0.0001), which was associated with contraction of an equivalent proportion of peri-ischemic capillary pericytes. The capillaries in the peri-ischemic cortex that remained perfused showed increased point prevalence of dynamic flow stalling (0.5% [95% CI, 0.2%-0.7%] at baseline, 5.1% [95% CI, 3.2%-6.5%] and 3.2% [95% CI, 1.1%-5.3%] at 3- and 24-hour follow-up, respectively; P=0.001). Whisker stimulation at the 3- and 24-hour follow-up led to reduced neurovascular coupling responses in the sensory cortex corresponding to the peri-ischemic region compared with that observed at baseline. Conclusions Arterial occlusion led to contraction of capillary pericytes and capillary flow stalling in the peri-ischemic cortex. Capillary dysfunction was associated with neurovascular uncoupling. Neurovascular coupling impairment associated with capillary dysfunction may be a mechanism that contributes to futile recanalization. Hence, the results from this study suggest a novel treatment target to improve neurological outcome after stroke.

Red cell exchange transfusions increase cerebral capillary transit times and may alter oxygen extraction in sickle cell disease

Persons with sickle cell disease (SCD) suffer from chronic hemolytic anemia, reduced blood oxygen content, and lifelong risk of silent and overt stroke. Major conventional stroke risk factors are absent in most individuals with SCD, yet nearly 50% have evidence of brain infarcts by the age of 30 years, indicating alternative etiologies for ischemia. We investigated whether radiological evidence of accelerated blood water transit through capillaries, visible on arterial spin labeling (ASL) magnetic resonance imaging, reduces following transfusion-induced increases in hemoglobin and relates to oxygen extraction fraction (OEF). Neurological evaluation along with anatomical and hemodynamic imaging with cerebral blood flow (CBF)-weighted pseudocontinuous ASL and OEF imaging with T2 -relaxation-under-spin-tagging were applied in sequence before and after blood transfusion therapy (n = 32) and in a comparator cohort of nontransfused SCD participants on hydroxyurea therapy scanned at two time points to assess stability without interim intervention (n = 13). OEF was calculated separately using models derived from human hemoglobin-F, hemoglobin-A, and hemoglobin-S. Gray matter CBF and dural sinus signal, indicative of rapid blood transit, were evaluated at each time point and compared with OEF using paired statistical tests (significance: two-sided p < 0.05). No significant change in sinus signal was observed in nontransfused participants (p = 0.650), but a reduction was observed in transfused participants (p = 0.034), consistent with slower red cell transit following transfusion. The dural sinus signal intensity was inversely associated with OEF pretransfusion (p = 0.011), but not posttransfusion. Study findings suggest that transfusion-induced increases in total hemoglobin may lengthen blood transit times through cerebral capillaries and alter cerebral OEF in SCD.

Capillary transit time heterogeneity inhibits cerebral oxygen metabolism in patients with reduced cerebrovascular reserve capacity from steno-occlusive disease

The healthy cerebral perfusion demonstrates a homogenous distribution of capillary transit times. A disruption of this homogeneity may inhibit the extraction of oxygen. A high degree of capillary transit time heterogeneity (CTH) describes that some capillaries have very low blood flows, while others have excessively high blood flows and consequently short transit times. Very short transit times could hinder the oxygen extraction due to insufficient time for diffusion of oxygen into the tissue. CTH could be a consequence of cerebral vessel disease. We examined whether patients with cerebral steno-occlusive vessel disease demonstrate high CTH and if elevation of cerebral blood flow (CBF) by administration of acetazolamide (ACZ) increases the cerebral metabolic rate of oxygen (CMRO₂), or if some patients demonstrate reduced CMRO₂ related to detrimental CTH. Thirty-four patients and thirty-one healthy controls participated. Global CBF and CMRO₂ were acquired using phase-contrast MRI. Regional brain maps of CTH were acquired using dynamic contrast-enhanced MRI. Patients with impaired cerebrovascular reserve capacity demonstrated elevated CTH and a significant reduction of CMRO₂ after administration of ACZ, which could be related to high CTH. Impaired oxygen extraction from CTH could be a contributing part of the declining brain health observed in patients with cerebral vessel disease.

The interconnections between the microtubules and mitochondrial networks in cardiocerebrovascular diseases: Implications for therapy

Microtubules, a highly dynamic cytoskeleton, participate in many cellular activities including mechanical support, organelles interactions, and intracellular trafficking. Microtubule organization can be regulated by modification of tubulin subunits, microtubule-associated proteins (MAPs) or agents modulating microtubule assembly. Increasing studies demonstrate that microtubule disorganization correlates with various cardiocerebrovascular diseases including heart failure and ischemic stroke. Microtubules also mediate intracellular transport as well as intercellular transfer of mitochondria, a power house in cells which produce ATP for various physiological activities such as cardiac mechanical function. It is known to all that both microtubules and mitochondria participate in the progression of cancer and Parkinson's disease. However, the interconnections between the microtubules and mitochondrial networks in cardiocerebrovascular diseases remain unclear. In this paper, we will focus on the roles of microtubules in cardiocerebrovascular diseases, and discuss the interplay of mitochondria and microtubules in disease development and treatment. Elucidation of these issues might provide significant diagnostic value as well as potential targets for cardiocerebrovascular diseases.

Resting-state BOLD functional connectivity depends on the heterogeneity of capillary transit times in the human brain A combined lesion and simulation study about the influence of blood flow response timing

Functional connectivity (FC) derived from blood oxygenation level dependent (BOLD) functional magnetic resonance imaging at rest (rs-fMRI), is commonly interpreted as indicator of neuronal connectivity. In a number of brain disorders, however, metabolic, vascular, and hemodynamic impairments can be expected to alter BOLD-FC independently from neuronal activity. By means of a neurovascular coupling (NVC) model of BOLD-FC, we recently demonstrated that aberrant timing of cerebral blood flow (CBF) responses may influence BOLD-FC. In the current work, we support and extend this finding by empirically linking BOLD-FC with capillary transit time heterogeneity (CTH), which we consider as an indicator of delayed and broadened CBF responses. We assessed 28 asymptomatic patients with unilateral high-grade internal carotid artery stenosis (ICAS) as a hemodynamic lesion model with largely preserved neurocognitive functioning and 27 age-matched healthy controls. For each participant, we obtained rs-fMRI, arterial spin labeling, and dynamic susceptibility contrast MRI to study the dependence of left-right homotopic BOLD-FC on local perfusion parameters. Additionally, we investigated the dependency of BOLD-FC on CBF response timing by detailed simulations. Homotopic BOLD-FC was negatively associated with increasing CTH differences between homotopic brain areas. This relation was more pronounced in asymptomatic ICAS patients even after controlling for baseline CBF and relative cerebral blood volume influences. These findings match simulation results that predict an influence of delayed and broadened CBF responses on BOLD-FC. Results demonstrate that increasing CTH differences between homotopic brain areas lead to BOLD-FC reductions. Simulations suggest that CTH increases correspond to broadened and delayed CBF responses to fluctuations in ongoing neuronal activity.

Prevalence and Significance of Impaired Microvascular Tissue Reperfusion Despite Macrovascular Angiographic Reperfusion (No-Reflow)

BACKGROUND AND OBJECTIVES

The relevance of impaired microvascular tissue-level reperfusion despite complete upstream macrovascular angiographic reperfusion (no-reflow) in human stroke remains controversial. We investigated the prevalence and clinical-radiologic features of this phenomenon and its associations with outcomes in 3 international randomized controlled thrombectomy trials with prespecified follow-up perfusion imaging.

METHODS

In a pooled analysis of the Extending the Time for Thrombolysis in Emergency Neurological Deficits-Intra-Arterial (EXTEND-IA; ClinicalTrials.gov NCT01492725), Tenecteplase Versus Alteplase Before Endovascular Therapy for Ischemic Stroke (EXTEND-IA TNK; NCT02388061), and Determining the Optimal Dose of Tenecteplase Before Endovascular Therapy for Ischaemic Stroke (EXTEND-IA TNK Part 2; NCT03340493) trials, patients undergoing thrombectomy with final angiographic expanded Treatment in Cerebral Infarction score of 2c to 3 score for anterior circulation large vessel occlusion and 24-hour follow-up CT or MRI perfusion imaging were included. No-reflow was defined as regions of visually demonstrable persistent hypoperfusion on relative cerebral blood volume or flow maps within the infarct and verified quantitatively by >15% asymmetry compared to a mirror homolog in the absence of carotid stenosis or reocclusion.

RESULTS

Regions of no-reflow were identified in 33 of 130 patients (25.3%), encompassed a median of 60.2% (interquartile range 47.8%-70.7%) of the infarct volume, and involved both subcortical (n = 26 of 33, 78.8%) and cortical (n = 10 of 33, 30.3%) regions. Patients with no-reflow had a median 25.2% (interquartile range 16.4%-32.2%, p < 0.00001) relative cerebral blood volume interside reduction and 19.1% (interquartile range 3.9%-28.3%, p = 0.00011) relative cerebral blood flow reduction but similar mean transit time (median -3.3%, interquartile range -11.9% to 24.4%, p = 0.24) within the infarcted region. Baseline characteristics were similar between patients with and those without no-reflow. The presence of no-reflow was associated with hemorrhagic transformation (adjusted odds ratio [aOR] 1.79, 95% confidence interval [CI] 2.32-15.57, p = 0.0002), greater infarct growth (β = 11.00, 95% CI 5.22-16.78, p = 0.00027), reduced NIH Stroke Scale score improvement at 24 hours (β = -4.06, 95% CI 6.78-1.34, p = 0.004) and being dependent or dead at 90 days as assessed by the modified Rankin Scale (aOR 3.72, 95% CI 1.35-10.20, p = 0.011) in multivariable analysis.

DISCUSSION

Cerebral no-reflow in humans is common, can be detected by its characteristic perfusion imaging profile using readily available sequences in the clinical setting, and is associated with posttreatment complications and being dependent or dead. Further studies evaluating the role of no-reflow in secondary injury after angiographic reperfusion are warranted.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that cerebral no-reflow on CT/MRI perfusion imaging at 24 hours is associated with posttreatment complications and poor 3-month functional outcome.

Laminar perfusion imaging with zoomed arterial spin labeling at 7 Tesla

Laminar fMRI based on BOLD and CBV contrast at ultrahigh magnetic fields has been applied for studying the dynamics of mesoscopic brain networks. However, the quantitative interpretations of BOLD/CBV fMRI results are confounded by different baseline physiology across cortical layers. Here we introduce a novel 3D zoomed pseudo-continuous arterial spin labeling (pCASL) technique at 7T that offers the capability for quantitative measurements of laminar cerebral blood flow (CBF) both at rest and during task activation with high spatial specificity and sensitivity. We found arterial transit time in superficial layers is ∼100 ms shorter than in middle/deep layers revealing the time course of labeled blood flowing from pial arteries to downstream microvasculature. Resting state CBF peaked in the middle layers which is highly consistent with microvascular density measured from human cortex specimens. Finger tapping induced a robust two-peak laminar profile of CBF increases in the superficial (somatosensory and premotor input) and deep (spinal output) layers of M1, while finger brushing task induced a weaker CBF increase in superficial layers (somatosensory input). This observation is highly consistent with reported laminar profiles of CBV activation on M1. We further demonstrated that visuospatial attention induced a predominant CBF increase in deep layers and a smaller CBF increase on top of the lower baseline CBF in superficial layers of V1 (feedback cortical input), while stimulus driven activity peaked in the middle layers (feedforward thalamic input). With the capability for quantitative CBF measurements both at baseline and during task activation, high-resolution ASL perfusion fMRI at 7T provides an important tool for in vivo assessment of neurovascular function and metabolic activities of neural circuits across cortical layers.

Cerebral hemodynamics and capillary dysfunction in late-onset major depressive disorder

In major depressive disorder (MDD), perfusion changes in cortico-limbic pathways are interpreted as altered neuronal activity, but they could also signify changes in neurovascular coupling due to altered capillary function. To examine capillary function in late-onset MDD, 22 patients and 22 age- and gender-matched controls underwent perfusion MRI. We measured normalized cerebral blood flow (nCBF), cerebral blood volume (nCBV), and relative transit-time heterogeneity (RTH). Resulting brain oxygenation was estimated in terms of oxygen tension and normalized metabolic rate of oxygen (nCMRO₂). Patients revealed signs of capillary dysfunction (elevated RTH) in the anterior prefrontal cortex and ventral anterior cingulate cortex bilaterally and in the left insulate cortex compared to controls, bilateral hypometabolism (parallel reductions of nCBV, nCBF, and CMRO₂) but preserved capillary function in the subthalamic nucleus and globus pallidus bilaterally, and hyperactivity with preserved capillary function (increased nCBF) in the cerebellum and brainstem. Our data support that perfusion changes in deep nuclei and cerebellum reflect abnormally low and high activity, respectively, in MDD patients, but suggest that microvascular pathology affects neurovascular coupling in ventral circuits. We speculate that microvascular pathology is important for our understanding of etiology of late-onset MDD as well as infererences about resulting brain activity changes.

Metabolic MRI with hyperpolarized [1-13C]pyruvate separates benign oligemia from infarcting penumbra in porcine stroke

Acute ischemic stroke patients benefit from reperfusion in a short time-window after debut. Later treatment may be indicated if viable brain tissue is demonstrated and this outweighs the inherent risks of late reperfusion. Magnetic resonance imaging (MRI) with hyperpolarized [1-13C]pyruvate is an emerging technology that directly images metabolism. Here, we investigated its potential to detect viable tissue in ischemic stroke. Stroke was induced in pigs by intracerebral injection of endothelin 1. During ischemia, the rate constant of pyruvate-to-lactate conversion, kPL, was 52% larger in penumbra and 85% larger in the infarct compared to the contralateral hemisphere (P = 0.0001). Within the penumbra, the kPL was 50% higher in the regions that later infarcted compared to non-progressing regions (P = 0.026). After reperfusion, measures of pyruvate-to-lactate conversion were slightly decreased in the infarct compared to contralateral. In addition to metabolic imaging, we used hyperpolarized pyruvate for perfusion-weighted imaging. This was consistent with conventional imaging for assessment of infarct size and blood flow. Lastly, we confirmed the translatability of simultaneous assessment of metabolism and perfusion with hyperpolarized MRI in healthy volunteers. In conclusion, hyperpolarized [1-13C]pyruvate may aid penumbral characterization and increase access to reperfusion therapy for late presenting patients.

A line through the brain: implementation of human line-scanning at 7T for ultra-high spatiotemporal resolution fMRI

Functional magnetic resonance imaging (fMRI) is a widely used tool in neuroscience to detect neurally evoked responses, e.g. the blood oxygenation level-dependent (BOLD) signal. Typically, BOLD fMRI has millimeter spatial resolution and temporal resolution of one to few seconds. To study the sub-millimeter structures and activity of the cortical gray matter, the field needs an fMRI method with high spatial and temporal resolution. Line-scanning fMRI achieves very high spatial resolution and high sampling rate, at the cost of a sacrifice in volume coverage. Here, we present a human line-scanning implementation on a 7T MRI system. First, we investigate the quality of the saturation pulses that suppress MR signal outside the line. Second, we established the best coil combination for reconstruction. Finally, we applied the line-scanning method in the occipital lobe during a visual stimulation task, showing BOLD responses along cortical depth, every 250 µm with a 200 ms repetition time (TR). We found a good correspondence of t-statistics values with 2D gradient-echo echo planar imaging (GE-EPI) BOLD fMRI data with the same temporal resolution and voxel volume (R = 0.6 ± 0.2). In summary, we demonstrate the feasibility of line-scanning in humans and this opens line-scanning fMRI for applications in cognitive and clinical neuroscience.

Comprehensive Evaluation of Cerebral Hemodynamics and Oxygen Metabolism in Revascularization of Asymptomatic High-Grade Carotid Stenosis

Introduction

Revascularization procedures in carotid artery stenosis have shown a positive effect in the restoration of cerebral oxygen metabolism as assessed by T2’ (T2 prime) imaging as well as capillary homeostasis by measurement of capillary transit time heterogeneity (CTH); however, data in patients with asymptomatic carotid stenosis without manifest brain lesions are scarce.

Patients and Methods

The effect of revascularization on the hemodynamic profile and capillary homeostasis was evaluated in 13 patients with asymptomatic high-grade carotid stenosis without ischemic brain lesions using dynamic susceptibility contrast perfusion imaging and oxygenation-sensitive T2’ mapping before and 6–8 weeks after revascularization by endarterectomy or stenting. The cognitive performance at both timepoints was further assessed.

Results

Perfusion impairment at baseline was accompanied by an increased CTH (p = 0.008) in areas with a time to peak delay ≥ 2 s in the affected hemisphere compared to contralateral regions. Carotid intervention improved the overall moderate hemodynamic impairment at baseline by leading to an increase in normalized cerebral blood flow (p = 0.017) and a decrease in mean transit time (p = 0.027), oxygen extraction capacity (OEC) (p = 0.033) and CTH (p = 0.048). The T2’ values remained unchanged.

Conclusion

This study presents novel evidence of a state of altered microvascular function in patients with high-grade carotid artery stenosis in the absence of ischemic brain lesions, which shows sustained normalization after revascularization procedures.

Supplementary Information

The online version of this article (10.1007/s00062-021-01077-3) contains supplementary material, which is available to authorized users.

Cerebral Macro- and Microcirculation during Ephedrine versus Phenylephrine Treatment in Anesthetized Brain Tumor Patients: A Randomized Clinical Trial Using Magnetic Resonance Imaging

BACKGROUND

This study compared ephedrine versus phenylephrine treatment on cerebral macro- and microcirculation, measured by cerebral blood flow, and capillary transit time heterogeneity, in anesthetized brain tumor patients. The hypothesis was that capillary transit time heterogeneity in selected brain regions is greater during phenylephrine than during ephedrine, thus reducing cerebral oxygen tension.

METHODS

In this single-center, double-blinded, randomized clinical trial, 24 anesthetized brain tumor patients were randomly assigned to ephedrine or phenylephrine. Magnetic resonance imaging of peritumoral and contralateral hemispheres was performed before and during vasopressor infusion. The primary endpoint was between-group difference in capillary transit time heterogeneity. Secondary endpoints included changes in cerebral blood flow, estimated oxygen extraction fraction, and brain tissue oxygen tension.

RESULTS

Data from 20 patients showed that mean (± SD) capillary transit time heterogeneity in the contralateral hemisphere increased during phenylephrine from 3.0 ± 0.5 to 3.2 ± 0.7 s and decreased during ephedrine from 3.1 ± 0.8 to 2.7 ± 0.7 s (difference phenylephrine versus difference ephedrine [95% CI], -0.6 [-0.9 to -0.2] s; P = 0.004). In the peritumoral region, the mean capillary transit time heterogeneity increased during phenylephrine from 4.1 ± 0.7 to 4.3 ± 0.8 s and decreased during ephedrine from 3.5 ± 0.9 to 3.3 ± 0.9 s (difference phenylephrine versus difference ephedrine [95%CI], -0.4[-0.9 to 0.1] s; P = 0.130). Cerebral blood flow (contralateral hemisphere ratio difference [95% CI], 0.3 [0.06 to 0.54]; P = 0.018; and peritumoral ratio difference [95% CI], 0.3 [0.06 to 0.54; P = 0.018) and estimated brain tissue oxygen tension (contralateral hemisphere ratio difference [95% CI], 0.34 [0.09 to 0.59]; P = 0.001; and peritumoral ratio difference [95% CI], 0.33 [0.09 to 0.57]; P = 0.010) were greater during ephedrine than phenylephrine in both regions.

CONCLUSIONS

Phenylephrine caused microcirculation in contralateral tissue, measured by the change in capillary transit time heterogeneity, to deteriorate compared with ephedrine, despite reaching similar mean arterial pressure endpoints. Ephedrine improved cerebral blood flow and tissue oxygenation in both brain regions and may be superior to phenylephrine in improving cerebral macro- and microscopic hemodynamics and oxygenation.

EDITOR’S PERSPECTIVE

Mathematical modeling of cerebral capillary blood flow heterogeneity and its effect on brain tissue oxygen levels

Maintaining cerebral blood flow is critical for adequate neuronal function. Previous computational models of brain capillary networks have predicted that heterogeneous cerebral capillary flow patterns result in lower brain tissue partial oxygen pressures PO2). However, these previous models have often considered simple capillary networks in terms of their geometric properties. In this current work, we developed and analyzed computational models of brain capillary networks to determine how perturbations of network properties impact tissue oxygen levels. The models include variabilities in both their geometric (segment lengths and diameters) and three-dimensional, topological structure. Two classes of capillary network models are considered. The first consists of equations for the oxygen partial pressure, PO₂, in both a capillary network and the surrounding tissue. In order to gain insight into the behavior of this detailed model, we also consider a reduced model for changes in PO₂ in just the capillary network. The main result is that for a general class of networks, random perturbations of either segment diameters or conductances will always, on average, decrease the average tissue oxygen levels. This result is supported through both simulations of the models and mathematical analysis. Our results promise to expand our understanding of cerebral capillary blood flow and its impact on the brain function in health and disease.

Does Src Kinase Mediated Vasoconstriction Impair Penumbral Reperfusion?

Despite successful recanalization, a significant number of patients with ischemic stroke experience impaired local brain tissue reperfusion with adverse clinical outcome. The cause and mechanism of this multifactorial complication are yet to be understood. At the current moment, major attention is given to dysfunction in blood-brain barrier and capillary blood flow but contribution of exaggerated constriction of cerebral arterioles has also been suggested. In the brain, arterioles significantly contribute to vascular resistance and thus control of perfusion. Accordingly, pathological changes in arteriolar wall function can, therefore, limit sufficient reperfusion in ischemic stroke, but this has not yet received sufficient attention. Although an increased vascular tone after reperfusion has been demonstrated in several studies, the mechanism behind it remains to be characterized. Importantly, the majority of conventional mechanisms controlling vascular contraction failed to explain elevated cerebrovascular tone after reperfusion. We propose here that the Na,K-ATPase-dependent Src kinase activation are the key mechanisms responsible for elevation of cerebrovascular tone after reperfusion. The Na,K-ATPase, which is essential to control intracellular ion homeostasis, also executes numerous signaling functions. Under hypoxic conditions, the Na,K-ATPase is endocytosed from the membrane of vascular smooth muscle cells. This initiates the Src kinase signaling pathway that sensitizes the contractile machinery to intracellular Ca²⁺ resulting in hypercontractility of vascular smooth muscle cells and, thus, elevated cerebrovascular tone that can contribute to impaired reperfusion after stroke. This mechanism integrates with cerebral edema that was suggested to underlie impaired reperfusion and is further supported by several studies, which are discussed in this article. However, final demonstration of the molecular mechanism behind Src kinase-associated arteriolar hypercontractility in stroke remains to be done.

Dynamics of cerebral perfusion and oxygenation parameters following endovascular treatment of acute ischemic stroke

BACKGROUND

We studied the effects of endovascular treatment (EVT) and the impact of the extent of recanalization on cerebral perfusion and oxygenation parameters in patients with acute ischemic stroke (AIS) and large vessel occlusion (LVO).

METHODS

Forty-seven patients with anterior LVO underwent computed tomography perfusion (CTP) before and immediately after EVT. The entire ischemic region (T_max >6 s) was segmented before intervention, and tissue perfusion (time-to-maximum (T_max), time-to-peak (TTP), mean transit time (MTT), cerebral blood volume (CBV), cerebral blood flow (CBF)) and oxygenation (coefficient of variation (COV), capillary transit time heterogeneity (CTH), metabolic rate of oxygen (CMRO₂), oxygen extraction fraction (OEF)) parameters were quantified from the segmented area at baseline and the corresponding area immediately after intervention, as well as within the ischemic core and penumbra. The impact of the extent of recanalization (modified Treatment in Cerebral Infarction (mTICI)) on CTP parameters was assessed with the Wilcoxon test and Pearson's correlation coefficients.

RESULTS

The T_max, MTT, OEF and CTH values immediately after EVT were lower in patients with complete (as compared with incomplete) recanalization, whereas CBF and COV values were higher (P<0.05) and no differences were found in other parameters. The ischemic penumbra immediately after EVT was lower in patients with complete recanalization as compared with those with incomplete recanalization (P=0.002), whereas no difference was found for the ischemic core (P=0.12). Specifically, higher mTICI scores were associated with a greater reduction of ischemic penumbra volumes (R²=-0.48 (95% CI -0.67 to -0.22), P=0.001) but not of ischemic core volumes (P=0.098).

CONCLUSIONS

Our study demonstrates that the ischemic penumbra is the key target of successful EVT in patients with AIS and largely determines its efficacy on a tissue level. Furthermore, we confirm the validity of the mTICI score as a surrogate parameter of interventional success on a tissue perfusion level.

Dynamic capillary stalls in reperfused ischemic penumbra contribute to injury: A hyperacute role for neutrophils in persistent traffic jams

Ever since the introduction of thrombolysis and the subsequent expansion of endovascular treatments for acute ischemic stroke, it remains to be identified why the actual outcomes are less favorable despite recanalization. Here, by high spatio-temporal resolution imaging of capillary circulation in mice, we introduce the pathological phenomenon of dynamic flow stalls in cerebral capillaries, occurring persistently in salvageable penumbra after reperfusion. These stalls, which are different from permanent cellular plugs of no-reflow, were temporarily and repetitively occurring in the capillary network, impairing the overall circulation like small focal traffic jams. In vivo microscopy in the ischemic penumbra revealed leukocytes traveling slowly through capillary lumen or getting stuck, while red blood cell flow was being disturbed in the neighboring segments under reperfused conditions. Stall dynamics could be modulated, by injection of an anti-Ly6G antibody specifically targeting neutrophils. Decreased number and duration of stalls were associated with improvement in penumbral blood flow within 2-24 h after reperfusion along with increased capillary oxygenation, decreased cellular damage and improved functional outcome. Thereby, dynamic microcirculatory stall phenomenon can be a contributing factor to ongoing penumbral injury and is a potential hyperacute mechanism adding on previous observations of detrimental effects of activated neutrophils in ischemic stroke.

SARS CoV-2 related microvascular damage and symptoms during and after COVID-19: Consequences of capillary transit-time changes, tissue hypoxia and inflammation

Corona virus disease 2019 (COVID-19) causes symptoms from multiple organs after infection by severe acute respiratory syndrome corona virus 2 (SARS CoV-2). They range from early, low blood oxygen levels (hypoxemia) without breathlessness ("silent hypoxia"), delirium, rashes, and loss of smell (anosmia), to persisting chest pain, muscle weakness and -pain, fatigue, confusion, memory problems and difficulty to concentrate ("brain fog"), mood changes, and unexpected onset of hypertension or diabetes. SARS CoV-2 affects the microcirculation, causing endothelial cell swelling and damage (endotheliitis), microscopic blood clots (microthrombosis), capillary congestion, and damage to pericytes that are integral to capillary integrity and barrier function, tissue repair (angiogenesis), and scar formation. Similar to other instances of critical illness, COVID-19 is also associated with elevated cytokine levels in the systemic circulation. This review examines how capillary damage and inflammation may contribute to these acute and persisting COVID-19 symptoms by interfering with blood and tissue oxygenation and with brain function. Undetectable by current diagnostic methods, capillary flow disturbances limit oxygen diffusion exchange in lungs and tissue and may therefore cause hypoxemia and tissue hypoxia. The review analyzes the combined effects of COVID-19-related capillary damage, pre-existing microvascular changes, and upstream vascular tone on tissue oxygenation in key organs. It identifies a vicious cycle, as infection- and hypoxia-related inflammation cause capillary function to deteriorate, which in turn accelerates hypoxia-related inflammation and tissue damage. Finally, the review addresses the effects of low oxygen and high cytokine levels in brain tissue on neurotransmitter synthesis and mood. Methods to assess capillary functions in human organs and therapeutic means to protect capillary functions and stimulate capillary bed repair may prove important for the individualized management of COVID-19 patients and targeted rehabilitation strategies.

The impact of native leptomeningeal collateralization on rapid blood flow recruitment following ischemic stroke

The leptomeningeal collateral status is an independent predictor of stroke outcome. By means of optical coherent tomography angiography to compare two mouse strains with different extent of native leptomeningeal collateralization, we determined the spatiotemporal dynamics of collateral flow and downstream hemodynamics following ischemic stroke. A robust recruitment of leptomeningeal collateral flow was detected immediately after middle cerebral artery (MCA) occlusion in C57BL/6 mice, with continued expansion over the course of seven days. In contrast, little collateral recruitment was seen in Balb/C mice during- and one day after MCAO, which coincided with a greater infarct size and worse functional outcome compared to C57BL/6, despite a slight improvement of cortical perfusion seven days after MCAO. Both strains of mice experienced a reduction of blood flow in the penetrating arterioles (PA) by more than 90% 30-min after dMCAO, although the decrease of PA flow was greater and the recovery was less in the Balb/C mice. Further, Balb/C mice also displayed a prolonged greater heterogeneity of capillary transit time after dMCAO in the MCA territory compared to C57BL/6 mice. Our data suggest that the extent of native leptomeningeal collaterals affects downstream hemodynamics with a long lasting impact in the microvascular bed after cortical stroke.

Blood flow, capillary transit times, and tissue oxygenation: the centennial of capillary recruitment

The transport of oxygen between blood and tissue is limited by blood's capillary transit time, understood as the time available for diffusion exchange before blood returns to the heart. If all capillaries contribute equally to tissue oxygenation at all times, this physical limitation would render vasodilation and increased blood flow insufficient means to meet increased metabolic demands in the heart, muscle, and other organs. In 1920, Danish physiologist August Krogh was awarded the Nobel Prize in Physiology or Medicine for his mathematical and quantitative, experimental demonstration of a solution to this conceptual problem: capillary recruitment, the active opening of previously closed capillaries to meet metabolic demands. Today, capillary recruitment is still mentioned in textbooks. When we suspect symptoms might represent hypoxia of a vascular origin, however, we search for relevant, flow-limiting conditions in our patients and rarely ascribe hypoxia or hypoxemia to short capillary transit times. This review describes how natural changes in capillary transit-time heterogeneity (CTH) and capillary hematocrit (HCT) across open capillaries during blood flow increases can account for a match of oxygen availability to metabolic demands in normal tissue. CTH and HCT depend on a number of factors: on blood properties, including plasma viscosity, the number, size, and deformability of blood cells, and blood cell interactions with capillary endothelium; on anatomical factors including glycocalyx, endothelial cells, basement membrane, and pericytes that affect the capillary diameter; and on any external compression. The review describes how risk factor- and disease-related changes in CTH and HCT interfere with flow-metabolism coupling and tissue oxygenation and discusses whether such capillary dysfunction contributes to vascular disease pathology.

Ephedrine versus Phenylephrine Effect on Cerebral Blood Flow and Oxygen Consumption in Anesthetized Brain Tumor Patients: A Randomized Clinical Trial

BACKGROUND

Studies in anesthetized patients suggest that phenylephrine reduces regional cerebral oxygen saturation compared with ephedrine. The present study aimed to quantify the effects of phenylephrine and ephedrine on cerebral blood flow and cerebral metabolic rate of oxygen in brain tumor patients. The authors hypothesized that phenylephrine reduces cerebral metabolic rate of oxygen in selected brain regions compared with ephedrine.

METHODS

In this double-blinded, randomized clinical trial, 24 anesthetized patients with brain tumors were randomly assigned to ephedrine or phenylephrine treatment. Positron emission tomography measurements of cerebral blood flow and cerebral metabolic rate of oxygen in peritumoral and normal contralateral regions were performed before and during vasopressor infusion. The primary endpoint was between-group difference in cerebral metabolic rate of oxygen. Secondary endpoints included changes in cerebral blood flow, oxygen extraction fraction, and regional cerebral oxygen saturation.

RESULTS

Peritumoral mean ± SD cerebral metabolic rate of oxygen values before and after vasopressor (ephedrine, 67.0 ± 11.3 and 67.8 ± 25.7 μmol · 100 g · min; phenylephrine, 68.2 ± 15.2 and 67.6 ± 18.0 μmol · 100 g · min) showed no intergroup difference (difference [95% CI], 1.5 [-13.3 to 16.3] μmol · 100 g · min [P = 0.839]). Corresponding contralateral hemisphere cerebral metabolic rate of oxygen values (ephedrine, 90.8 ± 15.9 and 94.6 ± 16.9 μmol · 100 g · min; phenylephrine, 100.8 ± 20.7 and 96.4 ± 17.7 μmol · 100 g · min) showed no intergroup difference (difference [95% CI], 8.2 [-2.0 to 18.5] μmol · 100 g · min [P = 0.118]). Ephedrine significantly increased cerebral blood flow (difference [95% CI], 3.9 [0.7 to 7.0] ml · 100 g · min [P = 0.019]) and regional cerebral oxygen saturation (difference [95% CI], 4 [1 to 8]% [P = 0.024]) in the contralateral hemisphere compared to phenylephrine. The change in oxygen extraction fraction in both regions (peritumoral difference [95% CI], -0.6 [-14.7 to 13.6]% [P = 0.934]; contralateral hemisphere difference [95% CI], -0.1 [- 12.1 to 12.0]% [P = 0.989]) were comparable between groups.

CONCLUSIONS

The cerebral metabolic rate of oxygen changes in peritumoral and normal contralateral regions were similar between ephedrine- and phenylephrine-treated patients. In the normal contralateral region, ephedrine was associated with an increase in cerebral blood flow and regional cerebral oxygen saturation compared with phenylephrine.

Non-Proliferative Diabetic Retinopathy Is Characterized by Nonuniform Alterations of Peripapillary Capillary Networks

Purpose

The purpose of this study was to use three-dimensional confocal microscopy to quantify the spatial patterns of capillary network alterations in nonproliferative diabetic retinopathy (NPDR).

Methods

The retinal microvasculature was perfusion-labelled in seven normal human donor eyes and six age-matched donor eyes with NPDR. The peripapillary microcirculation was studied using confocal scanning laser microscopy. Capillary density and diameters of the radial peripapillary capillary plexus (RPCP), superficial capillary plexus (SCP), intermediate capillary plexus (ICP), and deep capillary plexus (DCP) were quantified and compared. Three-dimensional visualization strategies were also used to compare the communications between capillary beds and precapillary arterioles and postcapillary venules.

Results

Mean capillary diameter was significantly increased in the NPDR group (P < 0.001). Intercapillary distance was significantly increased in the DCP (P = 0.004) and RPCP (P = 0.022) of the NPDR group (P = 0.010) but not the SCP (P = 0.155) or ICP (P = 0.103). The NPDR group was associated with an increased frequency of inflow communication between the SCP and ICP/DCP and a decreased frequency of communication between the SCP and RPCP (P = 0.023). There was no difference in the patterns of outflow communications between the two groups (P = 0.771).

Conclusions

This study demonstrates that capillary plexuses are nonuniformly perturbed in NPDR. These structural changes may be indicative of perturbations to blood flow patterns between different retinal layers. Our findings may aid the interpretation of previous clinical observations made using optical coherence tomography angiography as well as improve our understanding of the pathogenesis of NPDR.

Preventing dementia by preventing stroke: The Berlin Manifesto

The incidence of stroke and dementia are diverging across the world, rising for those in low- and middle-income countries and falling in those in high-income countries. This suggests that whatever factors cause these trends are potentially modifiable. At the population level, neurological disorders as a group account for the largest proportion of disability-adjusted life years globally (10%). Among neurological disorders, stroke (42%) and dementia (10%) dominate. Stroke and dementia confer risks for each other and share some of the same, largely modifiable, risk and protective factors. In principle, 90% of strokes and 35% of dementias have been estimated to be preventable. Because a stroke doubles the chance of developing dementia and stroke is more common than dementia, more than a third of dementias could be prevented by preventing stroke. Developments at the pathological, pathophysiological, and clinical level also point to new directions. Growing understanding of brain pathophysiology has unveiled the reciprocal interaction of cerebrovascular disease and neurodegeneration identifying new therapeutic targets to include protection of the endothelium, the blood-brain barrier, and other components of the neurovascular unit. In addition, targeting amyloid angiopathy aspects of inflammation and genetic manipulation hold new testable promise. In the meantime, accumulating evidence suggests that whole populations experiencing improved education, and lower vascular risk factor profiles (e.g., reduced prevalence of smoking) and vascular disease, including stroke, have better cognitive function and lower dementia rates. At the individual levels, trials have demonstrated that anticoagulation of atrial fibrillation can reduce the risk of dementia by 48% and that systolic blood pressure lower than 140 mmHg may be better for the brain. Based on these considerations, the World Stroke Organization has issued a proclamation, endorsed by all the major international organizations focused on global brain and cardiovascular health, calling for the joint prevention of stroke and dementia. This article summarizes the evidence for translation into action.

Physiologic predictors of collateral circulation and infarct growth during anesthesia - Detailed analyses of the GOLIATH trial

Collateral circulation plays a pivotal role in acute ischemic stroke due to large vessel occlusion (LVO) and may be affected by multiple variables during sedation for endovascular therapy (EVT). We conducted detailed analyses of the GOLIATH trial to identify predictors of collateral circulation grade and infarct growth. We also modified the ASITN collateral grading scale and sought to determine its impact on clinical outcome and infarct growth. Multivariable analysis was used to identify predictors of collaterals and infarct growth. Ordinal analysis demonstrated nominal, but non-significant association between modified ASITN scale and infarct growth. Among all analyzed baseline clinical and procedural variables, the most significant predictors of infarct growth at 24 h were phenylephrine dose (estimate 6.78; p = 0.014) and baseline infarct volume (estimate 0.93; p = 0.03). The most significant predictors of worse collateral grade were mean arterial pressure (MAP) <70 mmHg (OR 0.35; p = 0.048) and baseline infarct volume (OR 0.96; p = 0.003). Hypotension during sedation for EVT for LVO negatively impacts collateral circulation, while higher pressor dose is a strong predictor of infarct growth. Avoidance of anesthesia-induced hypotension and consequent need for pressor therapy may prevent collateral failure and minimize infarct growth.

Impaired perfusion and capillary dysfunction in prodromal Alzheimer's disease

INTRODUCTION

Cardiovascular disease increases the risk of developing Alzheimer's disease (AD), and growing evidence suggests an involvement of cerebrovascular pathology in AD. Capillary dysfunction, a condition in which capillary flow disturbances rather than arterial blood supply limit brain oxygen extraction, could represent an overlooked vascular contributor to neurodegeneration. We examined whether cortical capillary transit-time heterogeneity (CTH), an index of capillary dysfunction, is elevated in amyloid-positive patients with mild cognitive impairment (prodromal AD [pAD]).

METHODS

We performed structural and perfusion weighted MRI in 22 pAD patients and 21 healthy controls.

RESULTS

We found hypoperfusion, reduced blood volume, and elevated CTH in the parietal and frontal cortices of pAD-patients compared to controls, while only the precuneus showed focal cortical atrophy.

DISCUSSION

We propose that microvascular flow disturbances antedate cortical atrophy and may limit local tissue oxygenation in pAD. We speculate that capillary dysfunction contributes to the development of neurodegeneration in AD.

Anesthesia practice for endovascular therapy of acute ischemic stroke in Europe

PURPOSE OF REVIEW

Anesthetic assistance is often required during endovascular therapy (EVT) of large vessel occlusion in patients with acute ischemic stroke. It is currently debated whether EVT should be performed under general anesthesia or conscious sedation. This review will summarize the recent literature with emphasis on the influence of anesthesia method on neurological outcome.

RECENT FINDINGS

Recent randomized trials have reported no difference in outcome after EVT performed under either conscious sedation or general anesthesia. This is in contrast to a substantial number of retrospective studies, which found that EVT performed under general anesthesia was associated with a worse neurologic outcome compared with conscious sedation. Anesthetic drugs affect vessel tone and the level of blood pressure may influence outcome. The most favorable choice of anesthetic agents and ventilatory strategy is still debated.

SUMMARY

The optimal anesthetic practice for EVT remains to be identified. Currently, conscious sedation is often an easy first-line strategy, but general anesthesia can be considered an equal and safe alternative to conscious sedation when there is a carefully administered anesthetic that maintains strict hemodynamic control. Attention to ventilation is advocated. The presence of a specialized neuroanesthesiologist or otherwise dedicated anesthesia personnel is highly recommended.

Special topic section: linkages among cerebrovascular, cardiovascular, and cognitive disorders: Preventing dementia by preventing stroke: The Berlin Manifesto

The incidence of stroke and dementia are diverging across the world, rising for those in low-and middle-income countries and falling in those in high-income countries. This suggests that whatever factors cause these trends are potentially modifiable. At the population level, neurological disorders as a group account for the largest proportion of disability-adjusted life years globally (10%). Among neurological disorders, stroke (42%) and dementia (10%) dominate. Stroke and dementia confer risks for each other and share some of the same, largely modifiable, risk and protective factors. In principle, 90% of strokes and 35% of dementias have been estimated to be preventable. Because a stroke doubles the chance of developing dementia and stroke is more common than dementia, more than a third of dementias could be prevented by preventing stroke. Developments at the pathological, pathophysiological, and clinical level also point to new directions. Growing understanding of brain pathophysiology has unveiled the reciprocal interaction of cerebrovascular disease and neurodegeneration identifying new therapeutic targets to include protection of the endothelium, the blood-brain barrier, and other components of the neurovascular unit. In addition, targeting amyloid angiopathy aspects of inflammation and genetic manipulation hold new testable promise. In the meantime, accumulating evidence suggests that whole populations experiencing improved education, and lower vascular risk factor profiles (e.g., reduced prevalence of smoking) and vascular disease, including stroke, have better cognitive function and lower dementia rates. At the individual levels, trials have demonstrated that anticoagulation of atrial fibrillation can reduce the risk of dementia by 48% and that systolic blood pressure lower than 140 mmHg may be better for the brain. Based on these considerations, the World Stroke Organization has issued a proclamation, endorsed by all the major international organizations focused on global brain and cardiovascular health, calling for the joint prevention of stroke and dementia. This article summarizes the evidence for translation into action. © 2019 the Alzheimer's Association and the World Stroke Organisation. Published by Elsevier Inc. All rights reserved.

Small Vessels Are a Big Problem in Neurodegeneration and Neuroprotection

The cerebral microcirculation holds a critical position to match the high metabolic demand by neuronal activity. Functionally, microcirculation is virtually inseparable from other nervous system cells under both physiological and pathological conditions. For successful bench-to-bedside translation of neuroprotection research, the role of microcirculation in acute and chronic neurodegenerative disorders appears to be under-recognized, which may have contributed to clinical trial failures with some neuroprotectants. Increasing data over the last decade suggest that microcirculatory impairments such as endothelial or pericyte dysfunction, morphological irregularities in capillaries or frequent dynamic stalls in blood cell flux resulting in excessive heterogeneity in capillary transit may significantly compromise tissue oxygen availability. We now know that ischemia-induced persistent abnormalities in capillary flow negatively impact restoration of reperfusion after recanalization of occluded cerebral arteries. Similarly, microcirculatory impairments can accompany or even precede neural loss in animal models of several neurodegenerative disorders including Alzheimer's disease. Macrovessels are relatively easy to evaluate with radiological or experimental imaging methods but they cannot faithfully reflect the downstream microcirculatory disturbances, which may be quite heterogeneous across the tissue at microscopic scale and/or happen fast and transiently. The complexity and size of the elements of microcirculation, therefore, require utilization of cutting-edge imaging techniques with high spatiotemporal resolution as well as multidisciplinary team effort to disclose microvascular-neurodegenerative connection and to test treatment approaches to advance the field. Developments in two photon microscopy, ultrafast ultrasound, and optical coherence tomography provide valuable experimental tools to reveal those microscopic events with high resolution. Here, we review the up-to-date advances in understanding of the primary microcirculatory abnormalities that can result in neurodegenerative processes and the combined neurovascular protection approaches that can prevent acute as well as chronic neurodegeneration.

Leukoaraiosis May Confound the Interpretation of CT Perfusion in Patients Treated with Mechanical Thrombectomy for Acute Ischemic Stroke

BACKGROUND AND PURPOSE

Leukoaraiosis frequently coexists in patients with acute stroke. We studied whether leukoaraiosis could confound the interpretation of CTP findings in patients treated with mechanical thrombectomy.

MATERIALS AND METHODS

We analyzed 236 patients with stroke treated with mechanical thrombectomy and studied with CTP, of whom 127 (53.8%) achieved complete reperfusion. Periventricular white matter hyperintensities on MR imaging and hypodensities on NCCT were assessed through the Fazekas score. CTP-predicted nonviable tissue was defined as relative CBF <30%, and final infarct volume was quantified in DWI. We estimated mean MTT, CBV, and CBF in the asymptomatic hemisphere. In patients achieving complete reperfusion, we assessed the accuracy of nonviable tissue to predict final infarct volume using the intraclass correlation coefficient across periventricular hyperintensity/hypodensity Fazekas scores and variable relative CBF cutoffs.

RESULTS

MTT was longer (Spearman ρ = 0.279, P < .001) and CBF was lower (ρ = -0.263, P < .001) as the periventricular hyperintensity Fazekas score increased, while CBV was similar across groups (ρ = -0.043, P = .513). In the subgroup of patients achieving complete reperfusion, nonviable tissue-final infarct volume reliability was excellent in patients with periventricular hyperintensity Fazekas score grade 0 (intraclass correlation coefficient, 0.900; 95% CI, 0.805-0.950), fair in patients with periventricular hyperintensity Fazekas scores 1 (intraclass correlation coefficient, 0.569; 95% CI, 0.327-0.741) and 2 (intraclass correlation coefficient, 0.444; 95% CI, 0.165-0.657), and poor in patients with periventricular hyperintensity Fazekas score 3 (intraclass correlation coefficient, 0.310; 95% CI, -0.359-0.769). The most accurate cutoffs were relative CBF <30% for periventricular hyperintensity Fazekas score grades 0 and 1, relative CBF <25% for periventricular hyperintensity Fazekas score 2, and relative CBF <20% for periventricular hyperintensity Fazekas score 3. The reliability analysis according to periventricular hypodensity Fazekas score grades on NCCT was similar to that in follow-up MR imaging.

CONCLUSIONS

In patients with stroke, the presence of leukoaraiosis confounds the interpretation of CTP despite proper adjustment of CBF thresholds.

Intracranial pressure monitoring: Gold standard and recent innovations

Intracranial pressure monitoring (ICP) is based on the doctrine proposed by Monroe and Kellie centuries ago. With the advancement of technology and science, various invasive and non-invasive modalities of monitoring ICP continue to be developed. An ideal monitor to track ICP should be easy to use, accurate, reliable, reproducible, inexpensive and should not be associated with infection or haemorrhagic complications. Although the transducers connected to the extra ventricular drainage continue to be Gold Standard, its association with the likelihood of infection and haemorrhage have led to the search for alternate non-invasive methods of monitoring ICP. While Camino transducers, Strain gauge micro transducer based ICP monitoring devices and the Spiegelberg ICP monitor are the emerging technology in invasive ICP monitoring, optic nerve sheath diameter measurement, venous opthalmodynamometry, tympanic membrane displacement, tissue resonance analysis, tonometry, acoustoelasticity, distortion-product oto-acoustic emissions, trans cranial doppler, electro encephalogram, near infra-red spectroscopy, pupillometry, anterior fontanelle pressure monitoring, skull elasticity, jugular bulb monitoring, visual evoked response and radiological based assessment of ICP are the non-invasive methods which are assessed against the gold standard.

Mapping flow velocity in the human retinal capillary network with pixel intensity cross correlation

We present a new method for determining cellular velocity in the smallest retinal vascular networks as visualized with adaptive optics. The method operates by comparing the intensity profile of each movie pixel with that of every other pixel, after shifting in time by one frame. The time-shifted pixel which most resembles the reference pixel is deemed to be a 'source' or 'destination' of flow information for that pixel. Velocity in the transverse direction is then calculated by dividing the spatial displacement between the two pixels by the inter-frame period. We call this method pixel intensity cross-correlation, or "PIX". Here we compare measurements derived from PIX to two other state-of-the-art algorithms (particle image velocimetry and the spatiotemporal kymograph), as well as to manually tracked cell data. The examples chosen highlight the potential of the new algorithm to substantially improve spatial and temporal resolution, resilience to noise and aliasing, and assessment of network flow properties compared with existing methods.

Cerebrovascular function and mitochondrial bioenergetics after ischemia-reperfusion in male rats

The underlying factors promoting increased mitochondrial proteins, mtDNA, and dilation to mitochondrial-specific agents in male rats following tMCAO are not fully elucidated. Our goal was to determine the morphological and functional effects of ischemia/reperfusion (I/R) on mitochondria using electron microscopy, Western blot, mitochondrial oxygen consumption rate (OCR), and Ca2+ sparks activity measurements in middle cerebral arteries (MCAs) from male Sprague Dawley rats (Naïve, tMCAO, Sham). We found a greatly increased OCR in ipsilateral MCAs (IPSI) compared with contralateral (CONTRA), Sham, and Naïve MCAs. Consistent with our earlier findings, the expression of Mitofusin-2 and OPA-1 was significantly decreased in IPSI arteries compared with Sham and Naïve. Mitochondrial morphology was disrupted in vascular smooth muscle, but morphology with normal and perhaps greater numbers of mitochondria were observed in IPSI compared with CONTRA MCAs. Consistently, there were significantly fewer baseline Ca2+ events in IPSI MCAs compared with CONTRA, Sham, and Naïve. Mitochondrial depolarization significantly increased Ca2+ sparks activity in the IPSI, Sham, Naïve, but not in the CONTRA group. Our data indicate that altered mitochondrial structure and function occur in MCAs exposed to I/R and that these changes impact not only OCR but Ca2+ sparks activity in both IPSI and CONTRA MCAs.

[18F]-Flutemetamol Uptake in Cortex and White Matter: Comparison with Cerebrospinal Fluid Biomarkers and [18F]-Fludeoxyglucose

Flutemetamol (¹⁸F-Flut) is an [¹⁸F]-labelled amyloid PET tracer with increasing availability. The main objectives of this study were to investigate 1) cerebrospinal fluid (CSF) Aβ 1-42 (Aβ₄₂) concentrations associated with regional ¹⁸F-Flut uptake, 2) associations between cortical ¹⁸F-Flut and [¹⁸F]-fludeoxyglucose (¹⁸F-FDG)-PET, and 3) the potential use of ¹⁸F-Flut in WM pathology. Cognitively impaired, nondemented subjects were recruited (n = 44). CSF was drawn, and ¹⁸F-Flut-PET, ¹⁸F-FDG-PET, and MRI performed. Our main findings were: 1) Different Alzheimer’s disease predilection areas showed increased ¹⁸F-Flut retention at different CSF Aβ₄₂ concentrations (posterior regions were involved at higher concentrations). 2) There were strong negative correlations between regional cortical ¹⁸F-Flut and ¹⁸F-FDG uptake. 3) Increased ¹⁸F-Flut uptake were observed in multiple subcortical regions in amyloid positive subjects, including investigated reference regions. However, WM hyperintensity ¹⁸F-Flut standardized uptake value ratios (SUVr) were not significantly different, thus we cannot definitely conclude that the higher uptake in ¹⁸F-Flut(+) is due to amyloid deposition. In conclusion, our findings support clinical use of CSF Aβ₄₂, putatively relate decreasing CSF Aβ₄₂ concentrations to a sequence of regional amyloid deposition, and associate amyloid pathology to cortical hypometabolism. However, we cannot conclude that ¹⁸F-Flut-PET is a suitable marker for WM pathology due to high aberrant WM uptake.

Spatio-temporal dynamics of cerebral capillary segments with stalling red blood cells

Optical coherence tomography (OCT) allows label-free imaging of red blood cell (RBC) flux within capillaries with high spatio-temporal resolution. In this study, we utilized time-series OCT-angiography to demonstrate interruptions in capillary RBC flux in mouse brain in vivo. We noticed ∼7.5% of ∼200 capillaries had at least one stall in awake mice with chronic windows during a 9-min recording. At any instant, ∼0.45% of capillaries were stalled. Average stall duration was ∼15 s but could last over 1 min. Stalls were more frequent and longer lasting in acute window preparations. Further, isoflurane anesthesia in chronic preparations caused an increase in the number of stalls. In repeated imaging, the same segments had a tendency to stall again over a period of one month. In awake animals, functional stimulation decreased the observance of stalling events. Stalling segments were located distally, away from the first couple of arteriolar-side capillary branches and their average RBC and plasma velocities were lower than nonstalling capillaries within the same region. This first systematic analysis of capillary RBC stalls in the brain, enabled by rapid and continuous volumetric imaging of capillaries with OCT-angiography, will lead to future investigations of the potential role of stalling events in cerebral pathologies.

Methods to label, image, and analyze the complex structural architectures of microvascular networks

Microvascular networks play key roles in oxygen transport and nutrient delivery to meet the varied and dynamic metabolic needs of different tissues throughout the body, and their spatial architectures of interconnected blood vessel segments are highly complex. Moreover, functional adaptations of the microcirculation enabled by structural adaptations in microvascular network architecture are required for development, wound healing, and often invoked in disease conditions, including the top eight causes of death in the Unites States. Effective characterization of microvascular network architectures is not only limited by the available techniques to visualize microvessels but also reliant on the available quantitative metrics that accurately delineate between spatial patterns in altered networks. In this review, we survey models used for studying the microvasculature, methods to label and image microvessels, and the metrics and software packages used to quantify microvascular networks. These programs have provided researchers with invaluable tools, yet we estimate that they have collectively attained low adoption rates, possibly due to limitations with basic validation, segmentation performance, and nonstandard sets of quantification metrics. To address these existing constraints, we discuss opportunities to improve effectiveness, rigor, and reproducibility of microvascular network quantification to better serve the current and future needs of microvascular research.

Time-domain near-infrared spectroscopy in acute ischemic stroke patients

Large vessel occlusion (LVO) stroke might cause different degrees of hemodynamic impairment that affects microcirculation and contributes to metabolic derangement. Time-domain near-infrared spectroscopy (TD-NIRS) estimates the oxygenation of microcirculation of cerebral outer layers. We measure hemoglobin species and tissue oxygen saturation ( StO 2 ) of anterior circulation stroke patients, classified as LVO or lacunar, and assess the differences compared with controls and according to LVO recanalization status. Fiducial markers categorize the brain region below each TD-NIRS probe as ischemic or nonstroke areas. The study includes 47 consecutive acute ischemic stroke patients and 35 controls. The ischemic area has significantly higher deoxy-hemoglobin (HbR) and total hemoglobin (HbT) compared with controls in both recanalized and nonrecanalized patients but lower StO 2 only in recanalized patients. Recanalized patients have significantly lower mean StO 2 in the ipsilateral hemisphere compared with nonrecanalized patients. This is the first study to report TD-NIRS measurements in acute ischemic stroke patients. TD-NIRS is able to detect significant differences in hemoglobin species in LVO stroke compared with controls and according to recanalization status. This preliminary data might suggest that StO 2 can serve as a surrogate functional marker of the metabolic activity of rescued brain tissue.

Transit time homogenization in ischemic stroke - A novel biomarker of penumbral microvascular failure?

Cerebral ischemia causes widespread capillary no-flow in animal studies. The extent of microvascular impairment in human stroke, however, is unclear. We examined how acute intra-voxel transit time characteristics and subsequent recanalization affect tissue outcome on follow-up MRI in a historic cohort of 126 acute ischemic stroke patients. Based on perfusion-weighted MRI data, we characterized voxel-wise transit times in terms of their mean transit time (MTT), standard deviation (capillary transit time heterogeneity - CTH), and the CTH:MTT ratio (relative transit time heterogeneity), which is expected to remain constant during changes in perfusion pressure in a microvasculature consisting of passive, compliant vessels. To aid data interpretation, we also developed a computational model that relates graded microvascular failure to changes in these parameters. In perfusion-diffusion mismatch tissue, prolonged mean transit time (>5 seconds) and very low cerebral blood flow (≤6 mL/100 mL/min) was associated with high risk of infarction, largely independent of recanalization status. In the remaining mismatch region, low relative transit time heterogeneity predicted subsequent infarction if recanalization was not achieved. Our model suggested that transit time homogenization represents capillary no-flow. Consistent with this notion, low relative transit time heterogeneity values were associated with lower cerebral blood volume. We speculate that low RTH may represent a novel biomarker of penumbral microvascular failure.

Consensus statement on current and emerging methods for the diagnosis and evaluation of cerebrovascular disease

Cerebrovascular disease (CVD) remains a leading cause of death and the leading cause of adult disability in most developed countries. This work summarizes state-of-the-art, and possible future, diagnostic and evaluation approaches in multiple stages of CVD, including (i) visualization of sub-clinical disease processes, (ii) acute stroke theranostics, and (iii) characterization of post-stroke recovery mechanisms. Underlying pathophysiology as it relates to large vessel steno-occlusive disease and the impact of this macrovascular disease on tissue-level viability, hemodynamics (cerebral blood flow, cerebral blood volume, and mean transit time), and metabolism (cerebral metabolic rate of oxygen consumption and pH) are also discussed in the context of emerging neuroimaging protocols with sensitivity to these factors. The overall purpose is to highlight advancements in stroke care and diagnostics and to provide a general overview of emerging research topics that have potential for reducing morbidity in multiple areas of CVD.

Prediction of Tissue Outcome and Assessment of Treatment Effect in Acute Ischemic Stroke Using Deep Learning

Background and Purpose—

Treatment options for patients with acute ischemic stroke depend on the volume of salvageable tissue. This volume assessment is currently based on fixed thresholds and single imagine modalities, limiting accuracy. We wish to develop and validate a predictive model capable of automatically identifying and combining acute imaging features to accurately predict final lesion volume.

Methods—

Using acute magnetic resonance imaging, we developed and trained a deep convolutional neural network (CNN deep ) to predict final imaging outcome. A total of 222 patients were included, of which 187 were treated with rtPA (recombinant tissue-type plasminogen activator). The performance of CNN deep was compared with a shallow CNN based on the perfusion-weighted imaging biomarker Tmax (CNN Tmax ), a shallow CNN based on a combination of 9 different biomarkers (CNN shallow ), a generalized linear model, and thresholding of the diffusion-weighted imaging biomarker apparent diffusion coefficient (ADC) at 600×10 −6 mm 2 /s (ADC thres ). To assess whether CNN deep is capable of differentiating outcomes of ±intravenous rtPA, patients not receiving intravenous rtPA were included to train CNN deep, −rtpa to access a treatment effect. The networks’ performances were evaluated using visual inspection, area under the receiver operating characteristic curve (AUC), and contrast.

Results—

CNN deep yields significantly better performance in predicting final outcome (AUC=0.88±0.12) than generalized linear model (AUC=0.78±0.12; P =0.005), CNN Tmax (AUC=0.72±0.14; P <0.003), and ADC thres (AUC=0.66±0.13; P <0.0001) and a substantially better performance than CNN shallow (AUC=0.85±0.11; P =0.063). Measured by contrast, CNN deep improves the predictions significantly, showing superiority to all other methods ( P ≤0.003). CNN deep also seems to be able to differentiate outcomes based on treatment strategy with the volume of final infarct being significantly different ( P =0.048).

Conclusions—

The considerable prediction improvement accuracy over current state of the art increases the potential for automated decision support in providing recommendations for personalized treatment plans.