The severity of anaemia depletes cerebrovascular dilatory reserve in children with sickle cell disease: a quantitative magnetic resonance imaging study

Age-specific sex-differences in cerebral blood flow velocity in relation to haemoglobin levels

INTRODUCTION

Cerebral blood flow (CBF) declines with age and abnormalities in CBF are associated with age-related cerebrovascular disease and neurodegeneration. Women have higher CBF than men, although this sex-difference diminishes to some extent with age in healthy subjects. The physiological drivers of these age/sex differences are uncertain, but might be secondary to age and sex-differences in haemoglobin (Hb) level. Hb levels are inversely correlated with CBF, are lower in women, and decline with age in men, but the interrelations between these factors have not been explored systematically either in healthy subjects or across the full age-range in patients with vascular risk factors. We aimed to determine the age-specific interrelations between sex, Hb, and CBF velocity in a large cohort of patients with cerebrovascular disease.

PATIENTS AND METHODS

In patients with a recent transient ischaemic attack or minor stroke (Oxford Vascular Study) and no ipsilateral or contralateral stenosis of the carotid or intracranial arteries, we related peak-systolic velocity (PSV) and other parameters on transcranial Doppler ultrasound (TCD) of the middle cerebral artery to sex, age, Hb and vascular risk factors.

RESULTS

Of 958 eligible subjects (mean age/SD = 68.04/14.26, 53.2% male), younger women (age < 55 years) had higher CBF velocities than men (mean sex difference in PSV at age < 55 years = 16.31 cm/s; p < 0.001), but this difference declined with age (interaction p < 0.001), such that it was no longer significant at age 75-84 (∆PSV = 3.26 cm/s; p = 0.12) and was reversed at age ⩾ 85 (∆PSV = -7.42 cm/s; p = 0.05). These changes mirrored trends in levels of Hb, which were higher in men at age < 55 (∆Hb = 1.92 g/dL; p < 0.001), but steadily decreased with age in men but not in women (interaction p < 0.001), with no residual sex-difference at age ⩾ 85 (∆Hb = 0.12 g/dL; p = 0.70). There was an inverse correlation between Hb and PSV in both women and men (both p ⩽ 0.01), and the sex-difference in PSV at age < 55 was substantially diminished after adjustment for Hb (∆PSV = 6.92; p = 0.036; ∆PSV = 5.92, p = 0.13 with further adjustment for end-tidal CO2). In contrast, the sex difference in PSV was unaffected by adjustment for systolic and diastolic blood pressure, heart rate, and vascular risk factors (history of hypertension, diabetes, hyperlipidaemia and smoking).

DISCUSSION

CBF velocity is strongly correlated with Hb level at all ages, and sex-differences in CBF velocity appear to be explained in major part by age-related sex-differences in Hb.

Impaired cerebrovascular reactivity in pediatric sickle cell disease using diffuse correlation spectroscopy

Cerebrovascular reactivity (CVR), defined as the ability of cerebral vasculature to dilate in response to a vasodilatory stimulus, is an integral mechanism in brain homeostasis that is thought to be impaired in sickle cell disease (SCD). This study used diffuse correlation spectroscopy and a simple breath-hold stimulus to quantify CVR non-invasively in a cohort of 12 children with SCD and 14 controls. Median [interquartile range] CVR was significantly decreased in SCD compared to controls (2.03 [1.31, 2.44] versus 3.49 [3.00, 4.11] %/mmHg, p = 0.028). These results suggest DCS may provide a feasible means to routinely monitor CVR impairments in pediatric SCD.

Design and Harmonization Approach for the Multi-Institutional Neurocognitive Discovery Study (MINDS) of Adult Congenital Heart Disease (ACHD) Neuroimaging Ancillary Study: A Technical Note

Dramatic advances in the management of congenital heart disease (CHD) have improved survival to adulthood from less than 10% in the 1960s to over 90% in the current era, such that adult CHD (ACHD) patients now outnumber their pediatric counterparts. ACHD patients demonstrate domain-specific neurocognitive deficits associated with reduced quality of life that include deficits in educational attainment and social interaction. Our hypothesis is that ACHD patients exhibit vascular brain injury and structural/physiological brain alterations that are predictive of specific neurocognitive deficits modified by behavioral and environmental enrichment proxies of cognitive reserve (e.g., level of education and lifestyle/social habits). This technical note describes an ancillary study to the National Heart, Lung, and Blood Institute (NHLBI)-funded Pediatric Heart Network (PHN) "Multi-Institutional Neurocognitive Discovery Study (MINDS) in Adult Congenital Heart Disease (ACHD)". Leveraging clinical, neuropsychological, and biospecimen data from the parent study, our study will provide structural-physiological correlates of neurocognitive outcomes, representing the first multi-center neuroimaging initiative to be performed in ACHD patients. Limitations of the study include recruitment challenges inherent to an ancillary study, implantable cardiac devices, and harmonization of neuroimaging biomarkers. Results from this research will help shape the care of ACHD patients and further our understanding of the interplay between brain injury and cognitive reserve.

Cerebral O2 and CO2 transport in isovolumic haemodilution: Compensation of cerebral delivery of O2 and maintenance of cerebrovascular reactivity to CO2

This study investigated the influence of acute reductions in arterial O₂ content (CaO₂) via isovolumic haemodilution on global cerebral blood flow (gCBF) and cerebrovascular CO₂ reactivity (CVR) in 11 healthy males (age; 28 ± 7 years: body mass index; 23 ± 2 kg/m²). Radial artery and internal jugular vein catheters provided measurement of blood pressure and gases, quantification of cerebral metabolism, cerebral CO₂ washout, and trans-cerebral nitrite exchange (ozone based chemiluminescence). Prior to and following haemodilution, the partial pressure of arterial CO₂ (PaCO₂) was elevated with dynamic end-tidal forcing while gCBF was measured with duplex ultrasound. CVR was determined as the slope of the gCBF response and PaCO₂. Replacement of ∼20% of blood volume with an equal volume of 5% human serum albumin (Alburex® 5%) reduced haemoglobin (13.8 ± 0.8 vs. 11.3 ± 0.6 g/dL; P < 0.001) and CaO₂ (18.9 ± 1.0 vs 15.0 ± 0.8 mL/dL P < 0.001), elevated gCBF (+18 ± 11%; P = 0.002), preserved cerebral oxygen delivery (P = 0.49), and elevated CO₂ washout (+11%; P = 0.01). The net cerebral uptake of nitrite (11.6 ± 14.0 nmol/min; P = 0.027) at baseline was abolished following haemodilution (-3.6 ± 17.9 nmol/min; P = 0.54), perhaps underpinning the conservation of CVR (61.7 ± 19.0 vs. 69.0 ± 19.2 mL/min/mmHg; P = 0.23). These findings demonstrate that the cerebrovascular responses to acute anaemia in healthy humans are sufficient to support the maintenance of CVR.

The Development of Neuroimaging Biomarkers for Cognitive Decline in Sickle Cell Disease

Sickle cell disease (SCD) is complicated by neurologic complications including vasculopathy, hemorrhagic or ischemic overt stroke, silent cerebral infarcts and cognitive dysfunction. Patients with SCD, even in the absence of vasculopathy or stroke, have experience cognitive dysfunction that progresses with age. Transcranial Doppler ultrasound and structural brain MRI are currently used for primary and secondary stroke prevention, but laboratory or imaging biomarkers do not currently exist that are specific to the risk of cognitive dysfunction in patients with SCD. Recent investigations have used advanced MR sequences assessing cerebral hemodynamics, white matter microstructure and functional connectivity to better understand the pathophysiology of cognitive decline in SCD, with the long-term goal of developing neuroimaging biomarkers to be used in risk prediction algorithms and to assess the efficacy of treatment options for patients with SCD.

Stroke in sickle cell disease and the promise of recent disease modifying agents

Sickle cell disease (SCD) is an inherited hemoglobinopathy affecting approximately 100,000 individuals in the United States. Cerebrovascular disease is among the most common and debilitating complications of SCA, with 53% experiencing silent cerebral infarct by age 30 and 3.8% experiencing overt stroke by age 40 years. This review highlights the burden of cerebrovascular disease in SCD, including both stroke and silent cerebral infarct (SCI). We then discuss the pathophysiology of stroke and cerebral fat embolism in the absence of a patent foramen ovale. This review also reveals that options for primary and secondary stroke prevention in SCD are still limited to hydroxyurea and blood transfusion, and that the role of aspirin and anticoagulation in SCD stroke has not been adequately studied. Limited data suggest that the novel disease-modifying agents for SCD management may improve renal dysfunction, leg ulcers, and lower the abnormally high TCD flow velocity. Further research is urgently needed to investigate their role in stroke prevention in SCD, as these novel agents target the main stroke contributors in SCD - hemolysis and vaso-occlusion. This literature review also explores the role of healthcare disparities in slowing progress in SCD management and research in the United States, highlighting the need for more investment in patient and clinician education, SCD management, and research.

Assessment of cerebrovascular function in patients with sickle cell disease using transfer function analysis

In patients with sickle cell disease (SCD), the delivery of oxygen to the brain is compromised by anemia, abnormal rheology, and steno-occlusive vascular disease. Successful compensation depends on an increase in oxygen supply such as that provided by an increase in cerebral blood flow (CBF). We used magnetic resonance imaging to provide a high-resolution assessment of the ability of SCD patients to respond to a vasoactive stimulus in middle, anterior, and posterior cerebral artery territories for both white and gray matter. Cerebrovascular reactivity (CVR) was measured as the blood oxygen level dependent signal (a surrogate for CBF) response to an increase in the end tidal partial pressure of CO2 (PET CO2 ). The dynamic aspect of the response was measured as the time constant of the first order response kinetics (tau). To confirm and support these findings we used an alternative examination of the response, transfer function analysis (TFA), to measure the responsiveness (gain), the speed of response (phase), and the consistency of the response over time (coherence). We tested 34 patients with SCD and compared the results to those of 24 healthy controls participants. The results from a three-way ANOVA showed that patients with SCD have reduced CVR (p < 0.001) and lower coherence (p < 0.001) in gray matter and white matter and reduced gain in gray matter only (p < 0.001). In terms of the speed of the response to CO2 , tau (p < 0.001) and TFA phase (p < 0.001) were increased in SCD patients compared to healthy control subjects. These findings show that the cerebrovascular responsiveness to CO2 in patients with SCD is both decreased and slowed compared to healthy controls.

Stroke propensity in the Th3+/ mouse model of β-thalassemia intermedia

β-thalassemia is associated with multiple hematological and cerebrovascular symptoms linked to a hypercoagulable state that has not been fully replicated in animal models for the development of stroke treatments. Herein we compared the physiological properties and responses to transient cerebral hypoxia-ischemia (tHI) between six-month-old wildtype and heterozygous Th3/+ mice, a model of non-transfusion-dependent β-thalassemia intermedia (β-TI). We found that Th3/+ mice developed microcytic anemia, splenomegaly, higher platelet counts, and increased platelet-erythrocyte plus erythrocyte-leukocyte aggregates. Furthermore, Th3/+ mice showed diminished cerebrovascular reactivity (CVR) and cortical oxygen saturation under repetitive hypercapnic challenges. When subjected to a sub-threshold tHI insult, platelets and leukocytes in Th3/+ mice adhered to the cerebrovascular wall or formed aggregates, while their counterparts flew through smoothly in wildtype mice. Subsequently, Th3/+ mice showed increased fibrin deposition around cerebral blood vessels and larger infarction than wildtype mice, especially in female Th3/+ mice. Collectively these results showed that Th3/+ mice mimic key clinical features and a propensity to thromboembolism in β-TI patients. The hypercoagulable state in Th3/+ mice is likely caused by multiple hematological and CVR anomalies that are similar, but are not identical to those in the mouse model of sickle cell anemia. As such, we suggest that Th3/+ mice are a useful model to study the pathological mechanisms and prophylactic stroke treatments in thalassemia patients.

Sickle cell cerebrovascular reactivity to a CO2 stimulus: Too little, too slow

Background: Despite increased cerebral blood flow (CBF), cerebral infarcts occur in patients with sickle cell disease (SCD). This suggests increased CBF does not meet metabolic demand possibly due to compromised cerebral vasodilatory response. Hypothesis: In adult SCD patients, cerebrovascular reactivity (CVR) and speed of vasodilatory response (tau) to a standardized vasodilatory stimulus, are reduced compared to normal subjects.

Methods: Functional brain imaging performed as part of routine care in adult SCD patients without known large vessel cerebral vasculopathy was reviewed retrospectively. CVR was calculated as the change in CBF measured as the blood-oxygenation-level-dependent (BOLD)-magnetic resonance imaging signal, in response to a standard vasoactive stimulus of carbon dioxide (CO₂). The tau corresponding to the best fit between the convolved end-tidal partial pressures of CO₂ and BOLD signal was defined as the speed of vascular response. CVR and tau were normalized using a previously generated atlas of 42 healthy controls.

Results: Fifteen patients were included. CVR was reduced in grey and white matter (mean Z-score for CVR −0.5 [−1.8 to 0.3] and −0.6 [−2.3 to 0.7], respectively). Tau Z-scores were lengthened in grey and white matter (+0.9 [−0.5 to 3.3] and +0.8 [−0.7 to 2.7], respectively). Hematocrit was the only significant independent predictor of CVR on multivariable regression.

Conclusion: Both measures of cerebrovascular health (CVR and tau) in SCD patients were attenuated compared to normal controls. These findings show that CVR represents a promising tool to assess disease state, stroke risk, and therapeutic efficacy of treatments in SCD and merits further investigation.

Chronic anemia: The effects on the connectivity of white matter

Chronic anemia is commonly observed in patients with hemoglobinopathies, mainly represented by disorders of altered hemoglobin (Hb) structure (sickle cell disease, SCD) and impaired Hb synthesis (e.g. thalassemia syndromes, non-SCD anemia). Both hemoglobinopathies have been associated with white matter (WM) alterations. Novel structural MRI research in our laboratory demonstrated that WM volume was diffusely lower in deep, watershed areas proportional to anemia severity. Furthermore, diffusion tensor imaging analysis has provided evidence that WM microstructure is disrupted proportionally to Hb level and oxygen saturation. SCD patients have been widely studied and demonstrate lower fractional anisotropy (FA) in the corticospinal tract and cerebellum across the internal capsule and corpus callosum. In the present study, we compared 19 SCD and 15 non-SCD anemia patients with a wide range of Hb values allowing the characterization of the effects of chronic anemia in isolation of sickle Hb. We performed a tensor analysis to quantify FA changes in WM connectivity in chronic anemic patients. We calculated the volumetric mean of FA along the pathway of tracks connecting two regions of interest defined by BrainSuite's BCI-DNI atlas. In general, we found lower FA values in anemic patients; indicating the loss of coherence in the main diffusion direction that potentially indicates WM injury. We saw a positive correlation between FA and hemoglobin in these same regions, suggesting that decreased WM microstructural integrity FA is highly driven by chronic hypoxia. The only connection that did not follow this pattern was the connectivity within the left middle-inferior temporal gyrus. Interestingly, more reductions in FA were observed in non-SCD patients (mainly along with intrahemispheric WM bundles and watershed areas) than the SCD patients (mainly interhemispheric).

Anemia Increases Oxygen Extraction Fraction in Deep Brain Structures but Not in the Cerebral Cortex

Sickle cell disease (SCD) is caused by a single amino acid mutation in hemoglobin, causing chronic anemia and neurovascular complications. However, the effects of chronic anemia on oxygen extraction fraction (OEF), especially in deep brain structures, are less well understood. Conflicting OEF values have been reported in SCD patients, but have largely attributed to different measurement techniques, faulty calibration, and different locations of measurement. Thus, in this study, we investigated the reliability and agreement of two susceptibility-based methods, quantitative susceptibility mapping (QSM) and complex image summation around a spherical or a cylindrical object (CISSCO), for OEF measurements in internal cerebral vein (ICV), reflecting oxygen saturation in deep brain structures. Both methods revealed that SCD patients and non-sickle anemia patients (ACTL) have increased OEF in ICV (42.6% ± 5.6% and 30.5% ± 3.6% in SCD by CISSCO and QSM respectively, 37.0% ± 4.1% and 28.5% ± 2.3% in ACTL) compared with controls (33.0% ± 2.3% and 26.8% ± 1.8%). OEF in ICV varied reciprocally with hematocrit (r ² = 0.92, 0.53) and oxygen content (r ² = 0.86, 0.53) respectively. However, an opposite relationship was observed for OEF measurements in sagittal sinus (SS) with the widely used T₂-based oximetry, T₂-Relaxation-Under-Spin-Tagging (TRUST), in the same cohorts (31.2% ± 6.6% in SCD, 33.3% ± 5.9% in ACTL and 36.8% ± 5.6% in CTL). Importantly, we demonstrated that hemoglobin F and other fast moving hemoglobins decreased OEF by TRUST and explained group differences in sagittal sinus OEF between anemic and control subjects. These data demonstrate that anemia causes deep brain hypoxia in anemia subjects with concomitant preservation of cortical oxygenation, as well as the key interaction of the hemoglobin dissociation curve and cortical oxygen extraction.

Lag-Optimized Blood Oxygenation Level Dependent Cerebrovascular Reactivity Estimates Derived From Breathing Task Data Have a Stronger Relationship With Baseline Cerebral Blood Flow

Cerebrovascular reactivity (CVR), an important indicator of cerebrovascular health, is commonly studied with the Blood Oxygenation Level Dependent functional MRI (BOLD-fMRI) response to a vasoactive stimulus. Theoretical and empirical evidence suggests that baseline cerebral blood flow (CBF) modulates BOLD signal amplitude and may influence BOLD-CVR estimates. We address how acquisition and modeling choices affect the relationship between baseline cerebral blood flow (bCBF) and BOLD-CVR: whether BOLD-CVR is modeled with the inclusion of a breathing task, and whether BOLD-CVR amplitudes are optimized for hemodynamic lag effects. We assessed between-subject correlations of average GM values and within-subject spatial correlations across cortical regions. Our results suggest that a breathing task addition to a resting-state acquisition, alongside lag-optimization within BOLD-CVR modeling, can improve BOLD-CVR correlations with bCBF, both between- and within-subjects, likely because these CVR estimates are more physiologically accurate. We report positive correlations between bCBF and BOLD-CVR, both between- and within-subjects. The physiological explanation of this positive correlation is unclear; research with larger samples and tightly controlled vasoactive stimuli is needed. Insights into what drives variability in BOLD-CVR measurements and related measurements of cerebrovascular function are particularly relevant when interpreting results in populations with altered vascular and/or metabolic baselines or impaired cerebrovascular reserve.

Venous cerebral blood flow quantification and cognition in patients with sickle cell anemia

Prior studies have described high venous signal qualitatively using arterial spin labelling (ASL) in patients with sickle cell anemia (SCA), consistent with arteriovenous shunting. We aimed to quantify the effect and explored cross-sectional associations with arterial oxygen content (CaO2), disease-modifying treatments, silent cerebral infarction (SCI), and cognitive performance. 94 patients with SCA and 42 controls underwent cognitive assessment and MRI with single- and multi- inflow time (TI) ASL sequences. Cerebral blood flow (CBF) and bolus arrival time (BAT) were examined across gray and white matter and high-signal regions of the sagittal sinus. Across gray and white matter, increases in CBF and reductions in BAT were observed in association with reduced CaO2 in patients, irrespective of sequence. Across high-signal sagittal sinus regions, CBF was also increased in association with reduced CaO2 using both sequences. However, BAT was increased rather than reduced in patients across these regions, with no association with CaO2. Using the multiTI sequence in patients, increases in CBF across white matter and high-signal sagittal sinus regions were associated with poorer cognitive performance. These novel findings highlight the utility of multiTI ASL in illuminating, and identifying objectively quantifiable and functionally significant markers of, regional hemodynamic stress in patients with SCA.

Cerebral oxygen metabolic stress is increased in children with sickle cell anemia compared to anemic controls

Patients with sickle cell anemia (SCA) experience cerebral metabolic stress with an increase in oxygen extraction fraction (OEF) to compensate for reduced oxygen carrying capacity due to anemia. It remains unclear if anemia alone drives this metabolic stress. Using MRI, we collected voxel-wise OEF measurements to test our hypothesis that OEF would be elevated in anemic controls without SCA (AC) compared to healthy controls (HC), but OEF would be even higher in SCA compared to AC. Brain MRIs (N = 159) were obtained in 120 participants (34 HC, 27 AC, 59 SCA). While hemoglobin was lower in AC versus HC (p < 0.001), hemoglobin was not different between AC and SCA cohorts (p = 0.459). Whole brain OEF was higher in AC compared to HC (p < 0.001), but lower compared to SCA (p = 0.001). Whole brain OEF remained significantly higher in SCA compared to HC (p = 0.001) while there was no longer a difference between AC versus HC (p = 0.935) in a multivariate model controlling for age and hemoglobin. OEF peaked within the border zone regions of the brain in both SCA and AC cohorts, but the volume of white matter with regionally elevated OEF in AC was smaller (1.8%) than SCA (58.0%). While infarcts colocalized within regions of elevated OEF, more SCA participants had infarcts than AC (p < 0.001). We conclude that children with SCA experience elevated OEF compared to AC and HC after controlling for the impact of anemia, suggesting that there are other pathophysiologic factors besides anemia contributing to cerebral metabolic stress in children with SCA.

Revisiting anemia in sickle cell disease and finding the balance with therapeutic approaches

Chronic hemolytic anemia and intermittent acute pain episodes are the 2 hallmark characteristics of sickle cell disease (SCD). Anemia in SCD not only signals a reduction of red cell mass and oxygen delivery, but also ongoing red cell breakdown and release of cell-free hemoglobin, which together contribute to a number of pathophysiological responses and play a key role in the pathogenesis of cumulative multiorgan damage. However, although anemia is clearly associated with many detrimental outcomes, it may also have an advantage in SCD in lowering risks of potential viscosity-related complications. Until recently, clinical drug development for SCD has predominantly targeted a reduction in the frequency of vaso-occlusive crises as an endpoint, but increasingly, more attention is being directed toward addressing the contribution of chronic anemia to poor outcomes in SCD. This article aims to explore the complex pathophysiology and mechanisms of anemia in SCD, as well as the need to balance the benefits of raising hemoglobin levels with the potential risks of increasing blood viscosity, in the context of the current therapeutic landscape for anemia in SCD.

Individual Watershed Areas in Sickle Cell Anemia: An Arterial Spin Labeling Study

Previous studies have pointed to a role for regional cerebral hemodynamic stress in neurological complications in patients with sickle cell anemia (SCA), with watershed regions identified as particularly at risk of ischemic tissue injury. Using single- and multi-inflow time (TI) arterial spin labeling sequences (ASL) in 94 patients with SCA and 42 controls, the present study sought to investigate cerebral blood flow (CBF) and bolus arrival times (BAT) across gray matter, white matter with early arrival times, and in individual watershed areas (iWSAs). In iWSAs, associations between hemodynamic parameters, lesion burden, white matter integrity, and general cognitive performance were also explored. In patients, increases in CBF and reductions in BAT were observed in association with reduced arterial oxygen content across gray matter and white matter with early arrival times using both sequences (all p < 0.001, d = -1.55--2.21). Across iWSAs, there was a discrepancy between sequences, with estimates based on the single-TI sequence indicating higher CBF in association with reduced arterial oxygen content in SCA patients, and estimates based on the multi-TI sequence indicating no significant between-group differences or associations with arterial oxygen content. Lesion burden was similar between white matter with early arrival times and iWSAs in both patients and controls, and using both sequences, only trend-level associations between iWSA CBF and iWSA lesion burden were observed in patients. Further, using the multi-TI sequence in patients, increased iWSA CBF was associated with reduced iWSA microstructural tissue integrity and slower processing speed. Taken together, the results highlight the need for researchers to consider BAT when estimating CBF using single-TI sequences. Moreover, the findings demonstrate the feasibility of multi-TI ASL for objective delineation of iWSAs and for detection of regional hemodynamic stress that is associated with reduced microstructural tissue integrity and slower processing speed. This technique may hold promise for future studies and treatment trials.

Association of laboratory markers and cerebral blood flow among sickle cell anemia children

BACKGROUND

Stroke is one of the highest complications of sickle-cell anemia (SCA). The Transcranial Doppler (TCD) has been adopted worldwide as a gold standard method for detecting alterations in the blood velocity in cerebral arteries. In this study, we investigated the association between laboratory parameters and increased cerebral blood flow velocity in Brazilian SCA pediatric patients.

METHODS

The study included 159 pediatric patients with SCA, submitted to TCD velocity screening, and the time-averaged maximum mean velocity (TAMMV) was determined in the middle cerebral artery (MCA), anterior cerebral artery (ACA), and distal intracranial internal carotid artery (ICA). We compared cerebral blood flow in patients stratified by the following: TCD1-defined as normal, with TAMMV inferior to 170 cm/s; TCD2-conditional, with TAMMV above 170 cm/s, but less than 199 cm/s; TCD3-altered, with TAMMV greater than or equal to 200 cm/s.

RESULTS

TAMMV was negatively correlated with age and weight (p < 0.05). Moreover, TAMMV was associated or correlated with reductions in HbF, RBC, hemoglobin, hematocrit, HDL, and haptoglobin and, increases in MCV, MCH, RDW, reticulocytes, WBC, lymphocytes, monocytes, eosinophils, total and indirect bilirubin, LDH, AST, ALT, glucose, ferritin, and AAT (p < 0.05).

CONCLUSION

The current study highlights the importance of the investigation of hemolytic and inflammatory biomarkers for monitoring the clinical outcome of SCA pediatric patients, to avoid acute or chronic stroke. Moreover, glucose and HDL-C appear useful for predicting higher TAMMV.

A case of Sβ+ sickle cell disease diagnosed in adulthood following acute stroke: it's 2021, are we there yet?

In this report, we present a 29-year-old African American female who was brought to a local emergency department after being found unresponsive by her mother. The etiology of her stroke and severe hemolysis remained unknown, despite her mother reporting the patient's history of co-inheritance of sickle cell trait and beta-thalassemia trait, and extensive workup during her prolonged hospitalization. She was diagnosed with sickle cell disease (Sβ+ type) two years after discharge when she was referred to a sickle cell specialist for persistent anemia. Here, we also briefly review the challenges to diagnose rarer subtypes of sickle cell disease (SCD), in this case Sβ+ type, as well as the pathophysiology and current management of stroke in SCD.

Stroke and stroke prevention in sickle cell anemia in developed and selected developing countries

This comprehensive review provides an insight into the pathophysiology, epidemiology, evaluation, and treatment of sickle cell anemia (SCA)-related stroke in developed and developing countries. Vascular injury, hypercoagulability and vaso-occlusion play a role in the pathophysiology of stroke in SCA. Transcranial Doppler ultrasound (TCD) has lowered the incidence of ischemic stroke from 11% to 1% as TCD identifies children who are at risk for stroke, providing opportunities for interventions to reduce this risk. Whereas blood exchange is indicated in acute stroke, chronic transfusions (either simple or exchange on a monthly basis) are used for primary as well as secondary stroke prevention in developed countries. Children with abnormally high TCD velocities (≥ 200 cm/s) are at high risk of stroke and might benefit from hydroxyurea or hydroxycarbamide (HU) after a period of a successful transition from chronic transfusions. Hematopoietic stem cell transplant presents a cure for SCA. Gene therapy is currently investigated and may be offered to patients with SCA who had a stroke or who are at high risk of stroke if proven efficacious and safe. However, gene therapy is not likely to be implemented in low-income countries due to cost. Alternatively, HU is utilized for primary and secondary stroke prevention in developing countries. Further expansion of TCD implementation should be a priority in those settings.

Advances in neuroimaging to improve care in sickle cell disease

Sickle cell disease is associated with progressive and increased neurological morbidity throughout the lifespan. In people with sickle cell anaemia (the most common and severe type of sickle cell disease), silent cerebral infarcts are found in more than a third of adolescents by age 18 years and roughly half of young adults by age 30 years, many of whom have cognitive impairment despite having few or no conventional stroke risk factors. Common anatomical neuroimaging in individuals with sickle disease can assess structural brain injury, such as stroke and silent cerebral infarcts; however, emerging advanced neuroimaging methods can provide novel insights into the pathophysiology of sickle cell disease, including insights into the cerebral haemodynamic and metabolic contributors of neurological injury. Advanced neuroimaging methods, particularly methods that report on aberrant cerebral blood flow and oxygen delivery, have potential for triaging patients for appropriate disease-modifying or curative therapies before they have irreversible neurological injury, and for confirming the benefit of new therapies on brain health in clinical trials.

Screening for Cognitive Dysfunction Using the Rowland Universal Dementia Assessment Scale in Adults With Sickle Cell Disease

IMPORTANCE

Adults with sickle cell disease (SCD) disproportionally experience early cognitive decline; however, guidance on the optimal screening strategy for cognitive dysfunction is lacking, and several available tools are biased by language, educational level, socioeconomic status, and race/ethnicity. The Rowland Universal Dementia Assessment Scale (RUDAS) was specifically designed for cognitive screening in multicultural populations.

OBJECTIVE

To ascertain the prevalence of suspected dementia in adults with SCD using the RUDAS, and to identify whether age, sex, educational level, several biological variables, and SCD complications were associated with RUDAS scores.

DESIGN, SETTING, AND PARTICIPANTS

This multicenter, bilingual, cross-sectional study was conducted in 2 SCD comprehensive care centers in Canada (Centre Hospitalier de l'Université Montréal in Montréal and University Health Network in Toronto). Participants were adults aged 18 years or older and were enrolled in the study between July 1, 2018, and July 30, 2019. All outpatients were eligible and offered study participation, unless they had an acute medical condition that required inpatient care or they were unable to follow study instructions.

INTERVENTIONS

The RUDAS was administered by trained personnel in either French or English, according to the patient's language preference. A questionnaire on social determinants of health was also administered, and participants underwent screening for anxiety and depression.

MAIN OUTCOMES AND MEASURES

Proportion of participants with RUDAS scores that were suggestive of dementia and the RUDAS score. Any score lower than 23 points was suggestive of dementia, a score between 23 and 27 points indicated a possible association with mild neurocognitive disorder, and a score higher than 27 points was normal.

RESULTS

A total of 252 adult patients with SCD were included (136 women [54.0%]; mean [range] age, 34.8 [18-75] years). Overall, 29 patients (11.5%) had RUDAS scores that were suggestive of dementia, and this proportion increased with age (15 [8.7%] in the 18-39 years age group, 10 [14.5%] in the 40-59 years age group, and 4 [36.4%] in the ≥60 years age group). The RUDAS scores were not associated with sex, language, SCD genotype, and SCD complications. The highest level of education was significantly associated with the RUDAS score; however, the association was small (η2 = 0.02; 95% CI, 0.00-0.07; P = .02). In a multivariable analysis, lower glomerular filtration rate (r = 0.40; 95% CI, 0.29-0.50; P < .001) and increasing age (r = -0.37; 95% CI, -0.47 to -0.26; P < .001), but not SCD genotype or disease severity, were associated with lower RUDAS scores.

CONCLUSIONS AND RELEVANCE

This study found that using the RUDAS revealed a high prevalence of suspected dementia in adult patients with SCD that was associated with worsening kidney function and age. Cognition should be screened in all adult patients with SCD, regardless of age, disease severity, and SCD genotype; further validation of the RUDAS is ongoing.

MRI detection of brain abnormality in sickle cell disease

Introduction: Over the past decades, neuroimaging studies have clarified that a significant proportion of patients with sickle cell disease (SCD) have functionally significant brain abnormalities. Clinically, structural magnetic resonance imaging (MRI) sequences (T2, FLAIR, diffusion-weighted imaging) have been used by radiologists to diagnose chronic and acute cerebral infarction (both overt and clinically silent), while magnetic resonance angiography and venography have been used to diagnose arteriopathy and venous thrombosis. In research settings, imaging scientists are increasingly applying quantitative techniques to shine further light on underlying mechanisms.Areas covered: From a June 2020 PubMed search of 'magnetic' or 'MRI' and 'sickle' over the previous 5 years, we selected manuscripts on T1-based morphometric analysis, diffusion tensor imaging, arterial spin labeling, T2-oximetry, quantitative susceptibility, and connectivity.Expert Opinion: Quantitative MRI techniques are identifying structural and hemodynamic biomarkers associated with risk of neurological and neurocognitive complications. A growing body of evidence suggests that these biomarkers are sensitive to change with treatments, such as blood transfusion and hydroxyurea, indicating that they may hold promise as endpoints in future randomized clinical trials of novel approaches including hemoglobin F upregulation, reduction of polymerization, and gene therapy. With further validation, such techniques may eventually also improve neurological and neurocognitive risk stratification in this vulnerable population.

Impairment of Cerebrovascular Hemodynamics in Patients With Severe and Milder Forms of Sickle Cell Disease

In patients with sickle cell disease (SCD), cerebral blood flow (CBF) is elevated to counteract anemia and maintain oxygen supply to the brain. This may exhaust the vasodilating capacity of the vessels, possibly increasing the risk of silent cerebral infarctions (SCI). To further investigate cerebrovascular hemodynamics in SCD patients, we assessed CBF, arterial transit time (ATT), cerebrovascular reactivity of CBF and ATT (CVR CBF and CVR ATT ) and oxygen delivery in patients with different forms of SCD and matched healthy controls. We analyzed data of 52 patients with severe SCD (HbSS and HbSβ0-thal), 20 patients with mild SCD (HbSC and HbSβ+-thal) and 10 healthy matched controls (HbAA and HbAS). Time-encoded arterial spin labeling (ASL) scans were performed before and after a vasodilatory challenge using acetazolamide (ACZ). To identify predictors of CBF and ATT after vasodilation, regression analyses were performed. Oxygen delivery was calculated and associated with hemoglobin and fetal hemoglobin (HbF) levels. At baseline, severe SCD patients showed significantly higher CBF and lower ATT compared to both the mild SCD patients and healthy controls. As CBF postACZ was linearly related to CBF preACZ , CVR CBF decreased with disease severity. CVR ATT was also significantly affected in severe SCD patients compared to mild SCD patients and healthy controls. Considering all groups, women showed higher CBF postACZ than men (p < 0.01) independent of baseline CBF. Subsequently, post ACZ oxygen delivery was also higher in women (p < 0.05). Baseline, but not post ACZ, GM oxygen delivery increased with HbF levels. Our data showed that baseline CBF and ATT and CVR CBF and CVR ATT are most affected in severe SCD patients and to a lesser extent in patients with milder forms of SCD compared to healthy controls. Cerebrovascular vasoreactivity was mainly determined by baseline CBF, sex and HbF levels. The higher vascular reactivity observed in women could be related to their lower SCI prevalence, which remains an area of future work. Beneficial effects of HbF on oxygen delivery reflect changes in oxygen dissociation affinity from hemoglobin and were limited to baseline conditions suggesting that high HbF levels do not protect the brain upon a hemodynamic challenge, despite its positive effect on hemolysis.

Reduced oxygen extraction efficiency in sickle cell anemia patients with evidence of cerebral capillary shunting

Arterial spin labeling (ASL) magnetic resonance imaging (MRI) utilizes arterial blood water as an endogenous contrast agent to provide a quantitative measure of cerebral blood flow (CBF). Recently, hyperintense signal within dural venous sinuses in ASL images of sickle cell anemia (SCA) patients has been shown to be consistent with elevated flow velocities and may indicate capillary shunting and reduced oxygen extraction. Here, we performed oxygen extraction fraction (OEF) and CBF measurements in adults (cumulative n = 114) with (n = 69) and without (n = 45) SCA to test the hypothesis that hyperintense venous ASL signal is associated with reduced OEF. Higher categorical scores of shunting on ASL MRI were associated with lower OEF in participants with silent cerebral infarcts or white matter hyperintensities (p = 0.003), but not in those without lesions (p = 0.551). These findings indicate that venous hyperintense signal in ASL images in SCA patients may represent a marker of capillary-level disturbances in oxygen exchange efficiency and small vessel pathology.

Cerebrovascular Reactivity Measurement Using Magnetic Resonance Imaging: A Systematic Review

Cerebrovascular reactivity (CVR) magnetic resonance imaging (MRI) probes cerebral haemodynamic changes in response to a vasodilatory stimulus. CVR closely relates to the health of the vasculature and is therefore a key parameter for studying cerebrovascular diseases such as stroke, small vessel disease and dementias. MRI allows in vivo measurement of CVR but several different methods have been presented in the literature, differing in pulse sequence, hardware requirements, stimulus and image processing technique. We systematically reviewed publications measuring CVR using MRI up to June 2020, identifying 235 relevant papers. We summarised the acquisition methods, experimental parameters, hardware and CVR quantification approaches used, clinical populations investigated, and corresponding summary CVR measures. CVR was investigated in many pathologies such as steno-occlusive diseases, dementia and small vessel disease and is generally lower in patients than in healthy controls. Blood oxygen level dependent (BOLD) acquisitions with fixed inspired CO₂ gas or end-tidal CO₂ forcing stimulus are the most commonly used methods. General linear modelling of the MRI signal with end-tidal CO₂ as the regressor is the most frequently used method to compute CVR. Our survey of CVR measurement approaches and applications will help researchers to identify good practice and provide objective information to inform the development of future consensus recommendations.

A cross-sectional, case-control study of intracranial arterial wall thickness and complete blood count measures in sickle cell disease

In sickle cell disease (SCD), cerebral oxygen delivery is dependent on the cerebral vasculature's ability to increase blood flow and volume through relaxation of the smooth muscle that lines intracranial arteries. We hypothesised that anaemia extent and/or circulating markers of inflammation lead to concentric macrovascular arterial wall thickening, visible on intracranial vessel wall magnetic resonance imaging (VW-MRI). Adult and pediatric SCD (n = 69; age = 19.9 ± 8.6 years) participants and age- and sex-matched control participants (n = 38; age = 22.2 ± 8.9 years) underwent 3-Tesla VW-MRI; two raters measured basilar and bilateral supraclinoid internal carotid artery (ICA) wall thickness independently. Mean wall thickness was compared with demographic, cerebrovascular and haematological variables. Mean vessel wall thickness was elevated (P < 0·001) in SCD (1·07 ± 0·19 mm) compared to controls (0·97 ± 0·07 mm) after controlling for age and sex. Vessel wall thickness was higher in participants on chronic transfusions (P = 0·013). No significant relationship between vessel wall thickness and flow velocity, haematocrit, white blood cell count or platelet count was observed; however, trends (P < 0·10) for wall thickness increasing with decreasing haematocrit and increasing white blood cell count were noted. Findings are discussed in the context of how anaemia and circulating inflammatory markers may impact arterial wall morphology.

Age-dependent characterization of carotid and cerebral artery geometries in a transgenic mouse model of sickle cell anemia using ultrasound and microcomputed tomography

To define morphological changes in carotid and cerebral arteries in sickle cell transgenic mice (SS) as they age, a combination of ultrasound and microcomputed tomography of plastinated arteries was used to quantify arterial dimensions and changes in mice 4, 12, and 24 weeks of age. 12-week SS mice had significantly larger common carotid artery diameters than AS mice, which continued through to the extracranial and intracranial portions of the internal carotid artery (ICA). There were also side specific differences in diameters between the left and right vessels. Significant ICA tapering along its length occurred by 12- and 24-weeks in SS mice, decreasing by as much as 70%. Significant narrowing along the length was also measured in SS anterior cerebral arteries at 12- and 24-weeks, but not AS. Collectively, these findings indicate that sickle cell anemia induces arterial remodeling in 12- and 24-weeks old mice. Catalog of measurements are also provided for the common carotid, internal carotid, anterior cerebral, and middle cerebral arteries for AS and SS genotypes, as a reference for other investigators using mathematical and computational models of age-dependent arterial complications caused by sickle cell anemia.

Transient Hypoxia Model Revealed Cerebrovascular Impairment in Anemia Using BOLD MRI and Near-Infrared Spectroscopy

BACKGROUND

Obstructive sleep apnea and nocturnal oxygen desaturations, which are prevalent in sickle cell disease (SCD) and chronic anemia disorders, have been linked to risks of stroke and silent cerebral infarcts (SCI). Cerebrovascular response to intermittent desaturations has not been well studied and may identify patients at greatest risk.

PURPOSE

To investigate the cerebral dynamic response to induced desaturation in SCD patients with and without SCI, chronic anemia, and healthy subjects.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-six SCD patients (age = 21 ± 8.2, female 46.2%), including 15 subjects without SCI and nine subjects with SCI, 15 nonsickle anemic patients (age = 22 ± 5.8, female 66.7%), and 31 controls (age = 28 ± 12.3, female 77.4%).

FIELD STRENGTH/SEQUENCE

3T, gradient-echo echo-planar imaging.

ASSESSMENT

A transient hypoxia challenge of five breaths of 100% nitrogen gas was performed with blood oxygen level-dependent (BOLD) MRI and near-infrared spectroscopy (NIRS) acquisitions. Hypoxia responses were characterized by desaturation depth, time-to-peak, return-to-baseline half-life, and posthypoxia recovery in the BOLD and NIRS time courses. SCI were documented by T₂ fluid-attenuation inversion recovery (FLAIR).

STATISTICAL TESTS

Univariate and multivariate regressions were performed between hypoxic parameters and anemia predictors. Voxelwise two-sample t-statistic maps were used to assess the regional difference in hypoxic responses between anemic and control groups.

RESULTS

Compared to controls, SCD and chronically anemic patients demonstrated significantly higher desaturation depth (P < 0.01) and shorter return-to-baseline timing response (P < 0.01). Patients having SCI had shorter time-to-peak (P < 0.01), return-to-baseline (P < 0.01), and larger desaturation depth (P < 0.01) in both white matter regions at risk and normal-appearing white matter than patients without infarcts. On multivariate analysis, desaturation depth and timing varied with age, sex, blood flow, white blood cells, and cell-free hemoglobin (r² = 0.25 for desaturation depth; r² = 0.18 for time-to-peak; r² = 0.37 for return-to-baseline).

DATA CONCLUSION

Transient hypoxia revealed global and regional response differences between anemic and healthy subjects. SCI was associated with extensive heterogeneity of desaturation dynamics, consistent with extensive underlying microvascular remodeling.

Cerebral oxygen metabolism in adults with sickle cell disease

In sickle cell disease (SCD), oxygen delivery is impaired due to anemia, especially during times of increased metabolic demand, and cerebral blood flow (CBF) must increase to meet changing physiologic needs. But hyperemia limits cerebrovascular reserve (CVR) and ischemic risk prevails despite elevated CBF. The cerebral metabolic rate of oxygen (CMRO₂ ) directly reflects oxygen supply and consumption and may therefore be more insightful than flow-based CVR measures for ischemic risk in SCD. We hypothesized that adults with SCD have impaired CMRO₂ at rest and that a vasodilatory challenge with acetazolamide would improve CMRO₂ . CMRO₂ was calculated from CBF and oxygen extraction fraction (OEF), measured with arterial spin labeling and T₂ -prepared tissue relaxation with inversion recovery (T₂ -TRIR) MRI. We studied 36 adults with SCD without a clinical history of overt stroke, and nine healthy controls. As expected, CBF was higher in patients with SCD versus controls (mean ± SD: 74 ± 16 versus 46 ± 5 mL/100 g/min, P < .001), resulting in similar oxygen delivery (SCD: 377 ± 67 versus controls: 368 ± 42 μmol O₂ /100g/min, P = .69). OEF was lower in patients versus controls (27 ± 4 versus 35 ± 4%, P < .001), resulting in lower CMRO₂ in patients versus controls (102 ± 24 versus 127 ± 20 μmol O₂ /100g/min, P = .002). After acetazolamide, CMRO₂ declined further in patients (P < .01) and did not decline significantly in controls (P = .78), indicating that forcing higher CBF worsened oxygen utilization in SCD patients. This lower CMRO₂ could reflect variation between healthy and unhealthy vascular beds in terms of dilatory capacity and resistance whereby dysfunctional vessels become more oxygen-deprived, hence increasing the risk of localized ischemia.

Ischemic stroke in children and young adults with sickle cell disease in the post-STOP era

The Stroke Prevention Trial in Sickle Cell Anemia (STOP) and Optimizing Primary Stroke Prevention in Sickle Cell Anemia (STOP 2) trials established routine transcranial Doppler ultrasound (TCD) screening, with indefinite chronic red cell transfusions (CRCT) for children with abnormal TCD as standard of care. Implementation failures and limitations to the STOP protocol may contribute to continued ischemic stroke occurrence. In the "Post-STOP" study, we sought to assess the impact of the STOP protocol on the incidence of ischemic stroke in a multicenter cohort of former STOP and/or STOP 2 trial participants. A central team abstracted data for 2851 (74%) of the 3835 children who took part in STOP and/or STOP 2. Data included TCD and neuroimaging results, treatment, laboratory data, and detailed clinical information pertaining to the stroke. Two stroke neurologists independently confirmed each stroke using pre-specified imaging and clinical criteria and came to consensus. Among the 2808 patients who were stroke-free at the start of Post-STOP with available follow-up, the incidence of first ischemic stroke was 0.24 per 100 patient-years (95% CI, 0.18, 0.31), with a mean (SD) duration of follow-up of 9.1 (3.4) [median 10.3, range (0-15.4)] years. Most (63%) strokes occurred in patients in whom the STOP protocol had not been properly implemented, either failure to screen appropriately with TCD (38%) or failure to transfuse adequately patients with abnormal TCD (25%). This study shows that substantial opportunities for ischemic stroke prevention remain by more complete implementation of the STOP Protocol.

Ultrasound Detection of Abnormal Cerebrovascular Morphology in a Mouse Model of Sickle Cell Disease Based on Wave Reflection

Sickle cell disease (SCD) is associated with a high risk of stroke, and affected individuals often have focal brain lesions termed silent cerebral infarcts. The mechanisms leading to these types of injuries are at present poorly understood. Our group has recently demonstrated a non-invasive measurement of cerebrovascular impedance and wave reflection in mice using high-frequency ultrasound in the common carotid artery. To better understand the pathophysiology in SCD, we used this approach in combination with micro-computed tomography to investigate changes in cerebrovascular morphology in the Townes mouse model of SCD. Relative to controls, the SCD mice demonstrated the following: (i) increased carotid artery diameter, blood flow and vessel wall thickness; (ii) elevated pulse wave velocity; (iii) increased reflection coefficient; and (iv) an increase in the total number of vessel segments in the brain. This study highlights the potential for wave reflection to aid the non-invasive clinical assessment of vascular pathology in SCD.

Vascular Instability and Neurological Morbidity in Sickle Cell Disease: An Integrative Framework

It is well-established that patients with sickle cell disease (SCD) are at substantial risk of neurological complications, including overt and silent stroke, microstructural injury, and cognitive difficulties. Yet the underlying mechanisms remain poorly understood, partly because findings have largely been considered in isolation. Here, we review mechanistic pathways for which there is accumulating evidence and propose an integrative systems-biology framework for understanding neurological risk. Drawing upon work from other vascular beds in SCD, as well as the wider stroke literature, we propose that macro-circulatory hyper-perfusion, regions of relative micro-circulatory hypo-perfusion, and an exhaustion of cerebral reserve mechanisms, together lead to a state of cerebral vascular instability. We suggest that in this state, tissue oxygen supply is fragile and easily perturbed by changes in clinical condition, with the potential for stroke and/or microstructural injury if metabolic demand exceeds tissue oxygenation. This framework brings together recent developments in the field, highlights outstanding questions, and offers a first step toward a linking pathophysiological explanation of neurological risk that may help inform future screening and treatment strategies.

Hydroxycarbamide treatment in children with Sickle Cell Anaemia is associated with more intact white matter integrity: a quantitative MRI study

Sickle cell anaemia (SCA) is a devastating genetic blood disorder leading to chronic anaemia, impaired cerebrovascular dilatory capacity and cerebral infarctions. Our aim was to assess the relationship between microstructural properties of the white matter (WM) and both cerebrovascular reactivity (CVR) and cerebral blood flow, as well as the effects of hydroxycarbamide on these relationships. Our results demonstrate that mean CVR was increased in hydroxycarbamide-treated patients compared to untreated patients. Moreover, untreated SCA patients had increased skew and kurtosis of mean diffusivity histograms in the WM compared to hydroxycarbamide-treated patients and healthy age-matched controls, indicating disruption of WM integrity. Regression analysis of CVR and WM mean diffusivity (MD) revealed a significant linear relationship between CVR and MD histogram skew and kurtosis in healthy controls, but not in either of the two SCA groups. These findings suggest that patients treated with hydroxycarbamide possess white matter MD histogram parameters which more closely resemble those of healthy controls.

Hemodynamic provocation with acetazolamide shows impaired cerebrovascular reserve in adults with sickle cell disease

Sickle cell disease is characterized by chronic hemolytic anemia and vascular inflammation, which can diminish the vasodilatory capacity of the small resistance arteries, making them less adept at regulating cerebral blood flow. Autoregulation maintains adequate oxygen delivery, but when vasodilation is maximized, the low arterial oxygen content can lead to ischemia and silent cerebral infarcts. We used magnetic resonance imaging of cerebral blood flow to quantify whole-brain cerebrovascular reserve in 36 adult patients with sickle cell disease (mean age, 31.9±11.3 years) and 11 healthy controls (mean age, 37.4±15.4 years), and we used high-resolution 3D FLAIR magnetic resonance imaging to determine the prevalence of silent cerebral infarcts. Cerebrovascular reserve was calculated as the percentage change in cerebral blood flow after a hemodynamic challenge with acetazolamide. Co-registered lesion maps were used to demonstrate prevalent locations for silent cerebral infarcts. Cerebral blood flow was elevated in patients with sickle cell disease compared to controls (median [interquartile range]: 82.8 [20.1] vs 51.3 [4.8] mL/100g/min, P<0.001). Cerebral blood flow was inversely associated with age, hemoglobin, and fetal hemoglobin, and correlated positively with bilirubin, and LDH, indicating that cerebral blood flow may reflect surrogates of hemolytic rate. Cerebrovascular reserve in sickle cell disease was decreased by half compared to controls (34.1 [33.4] vs 69.5 [32.4] %, P<0.001) and was associated with hemoglobin and erythrocyte count indicating anemia-induced hemodynamic adaptations. In total, 29/36 patients (81%) and 5/11 controls (45%) had silent cerebral infarcts (median volume of 0.34 vs 0.02 mL, P=0.03). Lesions were preferentially located in the borderzone. In conclusion, patients with sickle cell disease have a globally reduced cerebrovascular reserve as determined by arterial spin labeling with acetazolamide and reflects anemia-induced impaired vascular function in sickle cell disease. This study was registered at clinicaltrials.gov identifier 02824406.

Hydroxyurea reduces cerebral metabolic stress in patients with sickle cell anemia

Chronic transfusion therapy (CTT) prevents stroke in selected patients with sickle cell anemia (SCA). We have shown that CTT mitigates signatures of cerebral metabolic stress, reflected by elevated oxygen extraction fraction (OEF), which likely drives stroke risk reduction. The region of highest OEF falls within the border zone, where cerebral blood flow (CBF) nadirs; OEF in this region was reduced after CTT. The neuroprotective efficacy of hydroxyurea (HU) remains unclear. To test our hypothesis that patients receiving HU therapy have lower cerebral metabolic stress compared with patients not receiving disease-modifying therapy, we prospectively obtained brain magnetic resonance imaging scans with voxel-wise measurements of CBF and OEF in 84 participants with SCA who were grouped by therapy: no disease-modifying therapy, HU, or CTT. There was no difference in whole-brain CBF among the 3 cohorts (P = .148). However, whole-brain OEF was significantly different (P < .001): participants without disease-modifying therapy had the highest OEF (median 42.9% [interquartile range (IQR) 39.1%-49.1%]), followed by HU treatment (median 40.7% [IQR 34.9%-43.6%]), whereas CTT treatment had the lowest values (median 35.3% [IQR 32.2%-38.9%]). Moreover, the percentage of white matter at highest risk for ischemia, defined by OEF greater than 40% and 42.5%, was lower in the HU cohort compared with the untreated cohort (P = .025 and P = .034 respectively), but higher compared with the CTT cohort (P = .018 and P = .029 respectively). We conclude that HU may offer neuroprotection by mitigating cerebral metabolic stress in patients with SCA, but not to the same degree as CTT.

Reduced cerebrovascular reactivity among adolescents with bipolar disorder

OBJECTIVES

Cardiovascular disease (CVD) is excessive and premature among individuals with bipolar disorder (BD). Cerebrovascular reactivity (CVR), reflecting vasodilatory capacity of cerebral blood vessels in response to vasoactive substances, is a marker of cerebrovascular health. Despite informative findings in other diseases, CVR has not previously been examined in BD.

METHODS

Twenty-five adolescents with BD and 25 age and sex-matched psychiatrically healthy controls (HCs) completed six 15-second breath-holds (BHs) during functional magnetic resonance imaging (fMRI) at 3-Tesla. CVR was determined by comparing blood-oxygenation-level dependent (BOLD) signal changes. Voxel-wise contrasts were analyzed. Body mass index (BMI) was examined as a potential confound.

RESULTS

CVR in the posterior cingulate gyrus and periventricular white matter was lower in BD vs HC. After controlling for differences in BMI, additional between-group CVR differences were observed in the temporal poles, supramarginal gyrus, and lingual gyrus. There were no regions in which CVR was significantly greater in BD vs HC. CVR was not associated with mood symptoms.

CONCLUSIONS

This preliminary study provides evidence of cerebrovascular dysfunction in BD, including regions known to be susceptible to cerebrovascular dysfunction and/or disease. These findings warrant additional research on the causes and consequences of cerebrovascular dysfunction in early-onset BD.

Neuroimaging of vascular reserve in patients with cerebrovascular diseases

Cerebrovascular reactivity, defined broadly as the ability of brain parenchyma to adjust cerebral blood flow in response to altered metabolic demand or a vasoactive stimulus, is being measured with increasing frequency and may have a use for portending new or recurrent stroke risk in patients with cerebrovascular disease. The purpose of this review is to outline (i) the physiological basis of variations in cerebrovascular reactivity, (ii) available approaches for measuring cerebrovascular reactivity in research and clinical settings, and (iii) clinically-relevant cerebrovascular reactivity findings in the context of patients with cerebrovascular disease, including atherosclerotic arterial steno-occlusion, non-atherosclerotic arterial steno-occlusion, anemia, and aging. Literature references summarizing safety considerations for these procedures and future directions for standardizing protocols and post-processing procedures across centers are presented in the specific context of major unmet needs in the setting of cerebrovascular disease.

Quantification of pathophysiological alterations in venous oxygen saturation: A comparison of global MR susceptometry techniques

The purpose of this study was to compare the Infinite Cylinder and Forward Field methods of quantifying global venous oxygen saturation (Y_v) in the superior sagittal sinus (SSS) from MRI phase data, and assess their applicability in systemic cerebrovascular disease.15 children with sickle cell disease (SCD) and 10 healthy age-matched controls were imaged on a 3.0 T MRI system. Anatomical and phase data around the superior sagittal sinus were acquired from a clinically available susceptibility weighted imaging sequence and converted to Y_v using the Infinite Cylinder and Forward Field methods. Y_v was significantly higher when calculated using the Infinite Cylinder method compared to the Forward Field method in both patients (p = 0.003) and controls (p < 0.001). A significant difference in Y_v was observed between patients and controls for the Forward Field method only (p = 0.006). While various implementations of Y_v quantification can be used in practice, the results can differ significantly. Simplistic models such as the Infinite Cylinder method may be easier to implement, but their dependence on broad assumptions can lead to an overestimation of Y_v, and may reduce the sensitivity to pathophysiological changes in Y_v.

Differential cerebral hemometabolic responses to blood transfusions in adults and children with sickle cell anemia

BACKGROUND

Blood transfusions are administered to children and adults with sickle cell anemia (SCA) for secondary stroke prevention, or as treatment for recurrent pain crises or acute anemia, but transfusion effects on cerebral hemodynamics and metabolism are not well-characterized.

PURPOSE

To compare blood transfusion-induced changes in hemometabolic parameters, including oxygen extraction fraction (OEF) and cerebral blood flow (CBF), within and between adults and children with SCA.

STUDY TYPE

Prospective, longitudinal study.

SUBJECTS

Adults with SCA (n = 16) receiving simple (n = 7) or exchange (n = 9) transfusions and children with SCA (n = 11) receiving exchange transfusions were scanned once when hematocrit was near nadir and again within 7 days of transfusion. Adult controls without SCA or sickle trait (n = 7) were scanned twice on separate days.

FIELD STRENGTH/SEQUENCE

3.0T T₁ -weighted, T₂ -weighted, and T₂ -relaxation-under-spin-tagging (TRUST) imaging, and phase contrast angiography.

ASSESSMENT

Global OEF was computed as the relative difference between venous oxygenation (from TRUST) and arterial oxygenation (from pulse oximetry). Global CBF was computed as total blood flow to the brain normalized by intracranial tissue volume.

STATISTICAL TESTS

Hemometabolic variables were compared using two-sided Wilcoxon signed-rank tests; associations were analyzed using two-sided Spearman's correlation testing.

RESULTS

In adults with SCA, posttransfusion OEF = 0.38 ± 0.05 was lower (P = 0.001) than pretransfusion OEF = 0.45 ± 0.09. A change in OEF was correlated with increases in hematocrit (P = 0.02; rho = -0.62) and with pretransfusion hematocrit (P = 0.02; rho = 0.65). OEF changes after transfusion were greater (P = 0.002) in adults receiving simple versus exchange transfusions. Posttransfusion CBF = 77.7 ± 26.4 ml/100g/min was not different (P = 0.27) from pretransfusion CBF = 82.3 ± 30.2 ml/100g/min. In children with SCA, both posttransfusion OEF = 0.28 ± 0.04 and CBF = 76.4 ± 26.4 were lower than pretransfusion OEF = 0.36 ± 0.06 (P = 0.004) and CBF = 96.4 ± 16.5 (P = 0.004).

DATA CONCLUSION

Cerebral OEF reduces following transfusions in adults and children with SCA. CBF reduces following transfusions more often in children compared to adults, indicating that vascular reserve capacity may remain near exhaustion posttransfusion in many adults.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage 5 J. Magn. Reson. Imaging 2019;49:466-477.

Cerebral Blood Flow and Marrow Diffusion Alterations in Children with Sickle Cell Anemia after Bone Marrow Transplantation and Transfusion

BACKGROUND AND PURPOSE

Hematopoietic marrow hyperplasia and hyperperfusion are compensatory mechanisms in sickle cell anemia. We have observed marrow diffusion and arterial spin-labeling perfusion changes in sickle cell anemia following bone marrow transplantation. We aimed to compare arterial spin-labeling perfusion and marrow diffusion/ADC values in patients with sickle cell anemia before and after bone marrow transplantation or transfusion.

MATERIALS AND METHODS

We reviewed brain MRIs from patients with sickle cell anemia obtained during 6 consecutive years at a children's hospital. Quantitative marrow diffusion values were procured from the occipital and sphenoid bones. Pseudocontinuous arterial spin-labeling perfusion values (milliliters/100 g of tissue/min) of MCA, anterior cerebral artery, and posterior cerebral artery territories were determined. Territorial CBF, whole-brain average CBF, and marrow ADC values were compared for changes before and after either bone marrow transplantation or transfusion. Bone marrow transplantation and transfusion groups were compared. Two-tailed paired and unpaired Student t tests were used; P < .05 was considered significant.

RESULTS

Fifty-three examinations from 17 patients with bone marrow transplantation and 29 examinations from 9 patients with transfusion were included. ADC values significantly increased in the sphenoid and occipital marrow following bone marrow transplantation in contrast to patients with transfusion (P > .83). Whole-brain mean CBF significantly decreased following bone marrow transplantation (77.39 ± 13.78 to 60.39 ± 13.62 ml/100 g tissue/min; P < .001), without significant change thereafter. CBF did not significantly change following the first (81.11 ± 12.23 to 80.25 ± 8.27 ml/100 g tissue/min; P = .47) or subsequent transfusions. There was no significant difference in mean CBF between groups before intervention (P = .22).

CONCLUSIONS

Improved CBF and marrow diffusion eventuate following bone marrow transplantation in children with sickle cell anemia in contrast to transfusion therapy.

Cerebral hemodynamic assessment and neuroimaging across the lifespan in sickle cell disease

Children and adults with sickle cell anemia (SCA) have a higher risk of strokes compared to age- and race-matched peers. Velocity in the middle cerebral or distal internal carotid artery as measured by transcranial Doppler ultrasound is a recognized method to identify children but not adults with SCA at high-risk for first stroke. For both children and adults with SCA that have had a stroke, no methods clearly identify individuals at highest risk of recurrent strokes or an initial silent stroke, the most common neurological injury. Methods to assess cerebral hemodynamics in SCA have been utilized for decades but often required radiotracers making them not feasible for screening and longitudinal follow-up. MRI approaches that do not require exogenous contrast have been introduced and are appealing in both clinical and research scenarios. Improved neuroimaging strategies hold promise for identifying individuals with SCA at increased risk of initial and recurrent infarcts, justifying more aggressive risk-based therapy. We review the epidemiology of stroke in SCA, the impact of strokes, stroke mechanisms, and potential imaging strategies including regional and global oxygen extraction fraction, cerebral blood flow, and vessel wall imaging to identify individuals at high-risk of stroke.

Breath-Hold Blood Oxygen Level-Dependent MRI: A Tool for the Assessment of Cerebrovascular Reserve in Children with Moyamoya Disease

BACKGROUND AND PURPOSE

There is a critical need for a reliable and clinically feasible imaging technique that can enable prognostication and selection for revascularization surgery in children with Moyamoya disease. Blood oxygen level-dependent MR imaging assessment of cerebrovascular reactivity, using voluntary breath-hold hypercapnic challenge, is one such simple technique. However, its repeatability and reliability in children with Moyamoya disease are unknown. The current study sought to address this limitation.

MATERIALS AND METHODS

Children with Moyamoya disease underwent dual breath-hold hypercapnic challenge blood oxygen level-dependent MR imaging of cerebrovascular reactivity in the same MR imaging session. Within-day, within-subject repeatability of cerebrovascular reactivity estimates, derived from the blood oxygen level-dependent signal, was computed. Estimates were associated with demographics and intellectual function. Interrater reliability of a qualitative and clinically applicable scoring scheme was assessed.

RESULTS

Twenty children (11 males; 12.1 ± 3.3 years) with 30 MR imaging sessions (60 MR imaging scans) were included. Repeatability was "good" on the basis of the intraclass correlation coefficient (0.70 ± 0.19). Agreement of qualitative scores was "substantial" (κ = 0.711), and intrarater reliability of scores was "almost perfect" (κ = 0.83 and 1). Younger participants exhibited lower repeatability (P = .027). Repeatability was not associated with cognitive function (P > .05). However, abnormal cerebrovascular reactivity was associated with slower processing speed (P = .015).

CONCLUSIONS

Breath-hold hypercapnic challenge blood oxygen level-dependent MR imaging is a repeatable technique for the assessment of cerebrovascular reactivity in children with Moyamoya disease and is reliably interpretable for use in clinical practice. Standardization of such protocols will allow further research into its application for the assessment of ischemic risk in childhood cerebrovascular disease.

White matter integrity and processing speed in sickle cell anemia

Objective

The purpose of this retrospective cross-sectional study was to investigate whether changes in white matter integrity are related to slower processing speed in sickle cell anemia.

Methods

Thirty-seven patients with silent cerebral infarction, 46 patients with normal MRI, and 32 sibling controls (age range 8–37 years) underwent cognitive assessment using the Wechsler scales and 3-tesla MRI. Tract-based spatial statistics analyses of diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI) parameters were performed.

Results

Processing speed index (PSI) was lower in patients than controls by 9.34 points (95% confidence interval: 4.635–14.855, p = 0.0003). Full Scale IQ was lower by 4.14 scaled points (95% confidence interval: −1.066 to 9.551, p = 0.1), but this difference was abolished when PSI was included as a covariate (p = 0.18). There were no differences in cognition between patients with and without silent cerebral infarction, and both groups had lower PSI than controls (both p < 0.001). In patients, arterial oxygen content, socioeconomic status, age, and male sex were identified as predictors of PSI, and correlations were found between PSI and DTI scalars (fractional anisotropy r = 0.614, p < 0.00001; r = −0.457, p < 0.00001; mean diffusivity r = −0.341, p = 0.0016; radial diffusivity r = −0.457, p < 0.00001) and NODDI parameters (intracellular volume fraction r = 0.364, p = 0.0007) in widespread regions.

Conclusion

Our results extend previous reports of impairment that is independent of presence of infarction and may worsen with age. We identify processing speed as a vulnerable domain, with deficits potentially mediating difficulties across other domains, and provide evidence that reduced processing speed is related to the integrity of normal-appearing white matter using microstructure parameters from DTI and NODDI.