A Promising Subject-Level Classification Model for Acute Concussion Based on Cerebrovascular Reactivity Metrics

Assessment of Cerebrovascular Reactivity Using CO2-BOLD MRI: A 15-Year, Single Center Experience

BACKGROUND

Cerebrovascular reactivity (CVR) is a measure of the change in cerebral blood flow (CBF) in response to a vasoactive challenge. It is a useful indicator of the brain's vascular health.

PURPOSE

To evaluate the factors that influence successful and unsuccessful CVR examinations using precise arterial and end-tidal partial pressure of CO2 control during blood oxygen level-dependent (BOLD) MRI.

STUDY TYPE

Retrospective.

SUBJECTS

Patients that underwent a CVR between October 2005 and May 2021 were studied (total of 1162 CVR examinations). The mean (±SD) age was 46.1 (±18.8) years, and 352 patients (43%) were female.

FIELD STRENGTH/SEQUENCE

3 T; T1-weighted images, T2*-weighed two-dimensional gradient-echo sequence with standard echo-planar readout.

ASSESSMENT

Measurements were obtained following precise hypercapnic stimuli using BOLD MRI as a surrogate of CBF. Successful CVR examinations were defined as those where: 1) patients were able to complete CVR testing, and 2) a clinically useful CVR map was generated. Unsuccessful examinations were defined as those where patients were not able to complete the CVR examination or the CVR maps were judged to be unreliable due to, for example, excessive head motion, and poor PETCO2 targeting.

STATISTICAL ANALYSIS

Successful and unsuccessful CVR examinations between hypercapnic stimuli, and between different patterns of stimulus were compared with Chi-Square tests. Interobserver variability was determined by using the intraclass correlation coefficient (P < 0.05 is significant).

RESULTS

In total 1115 CVR tests in 662 patients were included in the final analysis. The success rate of generating CVR maps was 90.8% (1012 of 1115). Among the different hypercapnic stimuli, those containing a step plus a ramp protocol was the most successful (95.18%). Among the unsuccessful examinations (9.23%), most were patient related (89.3%), the most common of which was difficulty breathing.

DATA CONCLUSION

CO2-BOLD MRI CVR studies are well tolerated with a high success rate.

EVIDENCE LEVEL

4 TECHNICAL EFFICACY: Stage 3.

Cerebrovascular Reserve Imaging: Problems and Solutions

The current standard of practice for assessing patients with cerebrovascular steno-occlusive disease is based on measuring resting blood flow metrics using MR imaging and CT perfusion imaging. However, the reliability of these methods decreases as the degree and number of stenoses increase. The reason for this is that measures of adequate baseline blood flow in highly collateralized circulations do not account for possible shortfalls in recruitable blood flow or increased metabolic demand. The following offers a clinically tested solution for this purpose using cerebrovascular reactivity methodology that applies a quantifiable vasodilatory stimulus improving reproducibility and repeatability essential for optimizing patient management.

Determining the effects of elevated partial pressure of oxygen on hypercapnia-induced cerebrovascular reactivity

Evaluation of cerebrovascular reactivity (CVR) to hypo- and hypercapnia is a valuable test for the assessment of vasodilatory reserve. While hypercapnia-induced CVR testing is usually performed at normoxia, mild hyperoxia may increase tolerability of hypercapnia by reducing the ventilatory distress. However, the effects of mild hyperoxia on CVR was unknown. We therefore recruited 21 patients with a range of steno-occlusive diseases and 12 healthy participants who underwent a standardized 13-minute step plus ramp CVR test with a carbon dioxide gas challenge at the subject's resting end-tidal partial pressure of oxygen or at mild hyperoxia (PetO2 = 150 mmHg) depending on to which group they were assigned. In 11 patients, the second CVR test was at normoxia to examine test-retest differences. CVR was defined as % Δ Signal/ΔPetCO2. We found that there was no significant difference between CVR test results conducted at normoxia and at mild hyperoxia for participants in Groups 1 and 2 for the step and ramp portion. We also found no difference between test and retest CVR at normoxia for patients with cerebrovascular pathology (Group 3) for step and ramp portion. We concluded normoxic CVR is repeatable, and that mild hyperoxia does not affect CVR.

Cerebrovascular Reactivity and Concussion

Cerebrovascular reactivity (CVR) reflects the change in cerebral blood flow in response to vasodilatory stimuli enabling assessment of the health of the cerebral vasculature. Recent advances in the quantitative delivery of CO2 stimuli with computer-controlled sequential gas delivery have enabled mapping of the speed and magnitude of response to flow stimuli. These CVR advances when applied to patients with acute concussion have unexpectedly shown faster speed and greater magnitude of responses unseen in other diseases that typically show the opposite effects. The strength of the CVR alterations have diagnostic potential in single subjects with AUC values in the 0.90-0.94 range.

Differential regional cerebrovascular reactivity to end-tidal gas combinations commonly seen during anaesthesia: A blood oxygenation level-dependent MRI observational study in awake adult subjects

BACKGROUND

Regional cerebrovascular reactivity (rCVR) is highly variable in the human brain as measured by blood oxygenation level-dependent (BOLD) MRI to changes in both end-tidal CO 2 and O 2 .

OBJECTIVES

We examined awake participants under carefully controlled end-tidal gas concentrations to assess how regional CVR changes may present with end-tidal gas changes seen commonly with anaesthesia.

DESIGN

Observational study.

SETTING

Tertiary care centre, Winnipeg, Canada. The imaging for the study occurred in 2019.

SUBJECTS

Twelve healthy adult subjects.

INTERVENTIONS

Cerebral BOLD response was studied under two end-tidal gas paradigms. First end-tidal oxygen (ETO 2 ) maintained stable whereas ETCO 2 increased incrementally from hypocapnia to hypercapnia (CO 2 ramp); second ETCO 2 maintained stable whereas ETO 2 increased from normoxia to hyperoxia (O 2 ramp). BOLD images were modeled with end-tidal gas sequences split into two equal segments to examine regional CVR.

MAIN OUTCOME MEASURES

The voxel distribution comparing hypocapnia to mild hypercapnia and mild hyperoxia (mean F I O 2  = 0.3) to marked hyperoxia (mean F I O 2  = 0.7) were compared in a paired fashion ( P  < 0.005 to reach threshold for voxel display). Additionally, type analysis was conducted on CO 2 ramp data. This stratifies the BOLD response to the CO 2 ramp into four categories of CVR slope based on segmentation (type A; +/+slope: normal response, type B +/-, type C -/-: intracranial steal, type D -/+.) Types B to D represent altered responses to the CO 2 stimulus.

RESULTS

Differential regional responsiveness was seen for both end-tidal gases. Hypocapnic regional CVR was more marked than hypercapnic CVR in 0.3% of voxels examined ( P  < 0.005, paired comparison); the converse occurred in 2.3% of voxels. For O 2 , mild hyperoxia had more marked CVR in 0.2% of voxels compared with greater hyperoxia; the converse occurred in 0.5% of voxels. All subjects had altered regional CO 2 response based on Type Analysis ranging from 4 ± 2 to 7 ± 3% of voxels.

CONCLUSION

In awake subjects, regional differences and abnormalities in CVR were observed with changes in end-tidal gases common during the conduct of anaesthesia. On the basis of these findings, consideration could be given to minimising regional CVR fluctuations in patients-at-risk of neurological complications by tighter control of end-tidal gases near the individual's resting values.

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.

Physical Activity Following Sport-Related Concussion in Adolescents: A Systematic Review

OBJECTIVE

To evaluate the evidence related to how physical activity (PA) affects recovery following a sport-related concussion (SRC) in adolescents.

DESIGN

Systematic review Data Source: PUBMED, MEDLINE, and SPORTDiscus Eligibility criteria for selecting study:(1) original research article (e.g., randomized controlled trials (RCT), quasi-experimental designs, cohort, case-control studies), (2) 66% or greater of the sample has to have a SRC diagnosed by a clinician, (4) human research, (5) evaluate the effect of a SRC on PA in adolescents only (<18 years). Participants were seen within 1-2 weeks post-SRC for acute studies and 4 weeks post-SRC for studies focused on prolonged recoveries.

RESULTS

Twenty-two studies met the inclusion criteria (i.e., 8 regarding PA (PA-daily aerobic activity including light-moderate intensities), 8 evaluating active rehabilitation/exercise programs (20 minutes of daily aerobic exercise below symptom threshold), 6 examining a single bout of exertion). The methodological quality of the literature was assessed using the Downs and Black risk of bias (ROB) checklist. The ROB scores ranged from 7-24, with only two RCTs included. Studies demonstrated single bouts of exertion testing were safe and feasible. Daily PA or active rehabilitation/exercise programs led to a reduction in symptoms present and a decrease in number of days to medical clearance.

CONCLUSION

Following a brief period of rest (24-48 hours), individuals can gradually and safely return to PA below their physical symptom exacerbation thresholds. Further research is warranted to delineate how to optimize the timing, intensity, duration, and modality of PA impacts symptom resolution and physiological recovery following SRC.

Sex and Gender Science: The World Writes on the Body

Sex and Gender Science seeks to better acknowledge that the body cannot be removed from the world it inhabits. We believe that to best answer any neuroscience question, the biological and the social need to be addressed through both objective means to learn, "how it is like" and subjective means to learn, "what it is like." We call bringing the biological and social together, "Situated Neuroscience" and the mixing of approaches to do so, Very Mixed Methods. Taken together, they constitute an approach to Sex and Gender Science. In this chapter, we describe neural phenomena for which considering sex and gender together produces a fuller knowledge base: sleep, pain, memory, and concussion. For these brain phenomena examples, studying only quantitative measures does not reveal the full impact of these lived experiences on the brain but studying only the qualitative would not reveal how the brain responds. We discuss how Sex and Gender Science allows us to begin to bring together biology and its social context and acknowledge where context can contribute to resolving ignorance to offer more expansive, complementary, and interrelating pictures of an intricate neuro-landscape.

Cerebrovascular reactivity changes in acute concussion: a controlled cohort study

Background

Evidence suggests that cerebrovascular reactivity (CVR) increases within the first week after the incidence of concussion, indicating a disruption of normal autoregulation. We sought to extend these findings by investigating the effects of acute concussion on the speed of CVR response and by visualizing global and regional impairments in individual patients with acute concussion.

Methods

Twelve patients aged 18-40 years who experienced concussion less than a week before this prospective study were included. Twelve age and sex-matched healthy subjects constituted the control group. In all subjects, CVR was assessed using blood oxygenation level-dependent (BOLD) echo-planar imaging with a 3.0T MRI scanner, in combination with changes in end-tidal partial pressure of CO₂ (P_ETCO₂). In each subject, we calculated the CVR amplitude and CVR response time in the gray and white matter using a step and ramp P_ETCO₂ challenge. In addition, a separate group of 39 healthy controls who underwent the same evaluation was used to create atlases with voxel-wise mean and standard deviation of CVR amplitude and CVR response time. This allowed us to convert each metric of the 12 patients with concussion and the 12 healthy controls into z-score maps. These maps were then used to generate and compare z-scores for each of the two groups. Group differences were calculated using an unpaired t-test.

Results

All studies were well tolerated without any serious adverse events. Anatomical MRI was normal in all study subjects. No differences in CO₂ stimulus and O₂ targeting were observed between the two participant groups during BOLD MRI. With regard to the gray matter, the CVR magnitude step (P=0.117) and ramp + 10 (P=0.085) were not significantly different between patients with concussion and healthy controls. However, the tau value was significantly lower in patients with concussion than in the healthy controls (P=0.04). With regard to the white matter, the CVR magnitude step (P=0.003) and ramp + 10 (P=0.031) were significantly higher and the tau value (P=0.024) was significantly shorter in patients with concussion than in healthy controls. After z-score transformation, the z tau value was significantly lower in patients with concussion than in healthy controls (Grey matter P=0.021, White matter P=0.003). Comparison of the three parameters, z ramp + 10, z step, and z tau, between the two groups showed that z step (Grey matter P=0.035, White matter P=0.005) was the most sensitive parameter and that z ramp + 10 (Grey matter P=0.073, White matter P=0.126) was the least sensitive parameter.

Conclusions

Concussion is associated with patient-specific abnormalities in BOLD cerebrovascular responsiveness that occur in the setting of normal global CVR. This study demonstrates that the measurement of CVR using BOLD MRI and precise CO₂ control is a safe, reliable, reproducible, and clinically useful method for evaluating the state of patients with concussion. It has the potential to be an important tool for assessing the severity and duration of symptoms after concussion.

Differential regional cerebral blood flow reactivity to alterations in end-tidal gases in healthy volunteers

PURPOSE

Anesthesia is associated with alterations in end-tidal (ET) respiratory gases from the awake state. These alterations result in marked vasoactive changes in regional cerebral blood flow (rCBF). Altered regional cerebrovascular reactivity (rCVR) is linked to neurologic dysfunction. We examined these differences in reactivity from prior work by focusing on the ratio of vasoconstriction with hyperoxia/hypocapnia (HO/hc):vasodilation with hypercapnia (HC) using magnetic resonance imaging pseudo-continuous arterial spin labelling (pCASL) to measure rCBF and compare rCVR The distribution and magnitude of these ratios could provide insights into rCBF during clinical anesthesia and inform future research into the origins of postoperative delirium (POD).

METHODS

Ten healthy subjects underwent cerebral blood flow (CBF) studies using pCASL with computer-controlled delivery of ET gases to assess flow effects of hyperoxia, hypercapnia, and hyperoxia/hypocapnia as part of a larger study into cerebrovascular reactivity. The vasoconstrictor stimulus was compared with the vasodilator stimulus by the ratio HO/hc:HC.

RESULTS

Hyperoxia minimally decreased whole brain CBF by - 0.6%/100 mm Hg increase in ETO₂. Hypercapnia increased CBF by +4.6%/mm Hg carbon dioxide (CO₂) and with HO/hc CBF decreased by - 5.1%/mm Hg CO₂. The brain exhibited markedly different rCVR-regional HO/hc:HC ratios varied from 7.2:1 (greater response to vasoconstriction) to 0.49:1 (greater response to vasodilation). Many of the ratios greater than 1, where vasoconstriction predominated, were seen in regions associated with memory, cognition, and executive function, including the entorhinal cortex, hippocampus, parahippocampus, and dorsolateral prefrontal cortex.

CONCLUSIONS

In awake humans, marked rCBF changes occurred with alterations in ET respiratory gases common under anesthesia. Such heterogeneous reactivity may be relevant to future studies to identify those at risk of POD.

Cerebrovascular Reactivity Measures Are Associated With Post-traumatic Headache Severity in Chronic TBI; A Retrospective Analysis

OBJECTIVE

To characterize the relationship between persistent post-traumatic headache (pPTH) and traumatic cerebrovascular injury (TCVI) in chronic traumatic brain injury (TBI). Cerebrovascular reactivity (CVR), a measure of the cerebral microvasculature and endothelial cell function, is altered both in individuals with chronic TBI and migraine headache disorder (Amyot et al., 2017; Lee et al., 2019b). The pathophysiologies of pPTH and migraine are believed to be associated with chronic microvascular dysfunction. We therefore hypothesize that TCVI may contribute to the underlying migraine-like mechanism(s) of pPTH.

MATERIALS AND METHODS

22 moderate/severe TBI participants in the chronic stage (>6 months) underwent anatomic and functional magnetic resonance imaging (fMRI) scanning with hypercapnia gas challenge to measure CVR as well as the change in CVR (ΔCVR) after single-dose treatment of a specific phosphodiesterase-5 (PDE-5) inhibitor, sildenafil, which potentiates vasodilation in response to hypercapnia in impaired endothelium, as part of a Phase2a RCT of sildenafil in chronic TBI (NCT01762475). CVR and ΔCVR measures of each participant were compared with the individual's pPTH severity measured by the headache impact test-6 (HIT-6) survey.

RESULTS

There was a moderate correlation between HIT-6 and both CVR and ΔCVR scores [Spearman's correlation = -0.50 (p = 0.018) and = 0.46 (p = 0.03), respectively], indicating that a higher headache burden is associated with decreased endothelial function in our chronic TBI population.

CONCLUSION

There is a correlation between PTH and CVR in chronic moderate-severe TBI. This relationship suggests that chronic TCVI may underlie the pathobiology of pPTH. Further, our results suggest that novel treatment strategies that target endothelial function and vascular health may be beneficial in refractory pPTH.

Cerebrovascular Reactivity: Purpose, Optimizing Methods, and Limitations to Interpretation - A Personal 20-Year Odyssey of (Re)searching

The brain is a neurovascular organ. A stimulus-response approach is effective in interrogating the physiology of its vasculature. Ideally, the stimulus is standardized across patients, and in a single patient over time. We developed a standard stimulus and attempted to measure, classify, and interpret the many forms of responses. Over the past 20 years, our work has delivered nuanced insights into normal cerebral vascular physiology, as well as adaptive physiological responses in the presence of disease. The trajectory of our understanding did not follow a logical linear progression; rather, it emerged as a coalescence of new, old, and previously dismissed, ideas that had accumulated over time. In this essay, we review what we believe were our most valuable - and sometimes controversial insights during our two decades-long journey.