Cerebrovascular Reactivity Assays Collateral Function in Carotid Stenosis
Olivia Sobczyk1,2, Kevin Sam2, Daniel M Mandell2
1Institute of Medical Science, University of Toronto, Toronto, ON, Canada.
Insights
Carotid artery stenosis (CAS) impacts stroke risk, but collateral blood flow is hard to measure. Cerebrovascular reactivity (CVR) using BOLD MRI may offer a more reliable assessment of blood flow reserve than stenosis degree alone.
Area of Science:
- Neuroimaging
- Vascular Neurology
- Cardiovascular Imaging
Background:
- Collateral blood flow is crucial for managing stroke risk in carotid artery stenosis (CAS).
- Assessing collateral flow with traditional imaging methods shows poor observer agreement.
- Blood-oxygen-level-dependent (BOLD) MRI can measure cerebrovascular reactivity (CVR) as a surrogate for cerebral blood flow reserve.
Purpose of the Study:
- To investigate if functional collateral flow, indicated by normalized ipsilateral CVR, exists in patients with hemodynamically significant CAS.
- To evaluate CVR as a potential marker for assessing collateral blood flow in CAS patients.
Main Methods:
- 55 patients with varying degrees of CAS were analyzed using BOLD MRI during a hypercapnic challenge to measure CVR.
- CVR was calculated as the change in BOLD signal relative to end-tidal CO2 (Δ BOLD/Δ PETCO2).
- CVR values were normalized using z-scores against a healthy control atlas and averaged over gray and white matter in the middle cerebral artery territory.
Main Results:
- CVR varied across all CAS severity groups, generally decreasing with increased stenosis.
- Ipsilateral CVR was reduced in all CAS groups, even those with <50% stenosis.
- A significant portion of patients exhibited normal CVR despite varying degrees of stenosis, including complete occlusion, highlighting individual variability.
Conclusions:
- Carotid artery stenosis generally reduces downstream vascular reserve.
- The extent of collateralization in CAS is highly variable and not solely predictable by the degree of stenosis.
- CVR measured by BOLD MRI appears to be a more reliable indicator of recruitable collateral blood flow than the degree of stenosis itself.
Abstract:
In patients with carotid artery stenosis (CAS), the risk of stroke, its severity, and response to revascularization are strongly related to the availability of collateral blood flow. Unfortunately, there is poor agreement between observers in assessing collateral flow using flow-based imaging. We used changes in blood-oxygen-level-dependent (BOLD) MRI as a surrogate of changes in regional cerebral blood flow in response to a hypercapnic stimulus [i.e., cerebrovascular reactivity (CVR)] as indicating flow reserve ipsilateral to CAS. We hypothesized that some patients with hemodynamically significant CAS develop functional collateral flow as indicated by normalization of ipsilateral CVR. We identified 55 patients in our CVR database with various degrees of CAS assessed by angiography and classed them as <50% stenosis, 50-69% stenosis, 70-90% stenosis, >90% stenosis, and full occlusion. CVR was measured as the change in BOLD signal in response to changes in end-tidal partial pressure of CO2 (Δ BOLD/Δ PETCO2) and normalized voxel-wise relative to the mean and standard deviation of the CVR in the corresponding voxels of an atlas of 46 healthy controls (CVR z scores). CVR and z scores were then averaged over gray matter (GM) and white matter (WM) on each side of the middle cerebral artery (MCA) territory. As hypothesized, CVR varied for each severity of CAS. Ipsilateral MCA territory CVR was less than normal in each class, including that with <50% stenosis (Student t-test, two-tailed; p = 0.0014 for GM and p = 0.030 for WM), with a trend of decreasing average CVR with increasing stenosis. Remarkably, the considerable individual variability in MCA CVR included some patients with normal CVR in each class - including that with complete occlusion. We conclude that, in general, CAS depresses downstream vascular reserve, but the extent of collateralization is highly variable and not predictable from the degree of stenosis, including both <50% stenosis and complete occlusion. CVR may be the more reliable marker for recruitable collateral blood flow than degree of CAS.
