Piyush Ojha, MBBS, MD, DM
In patients with acute ischemic stroke (AIS) due to large vessel occlusion (LVO), infarct grows over time after arterial occlusion, the progression of which may be non-linear across individuals depending on the variations in the collateral blood flow capacity and the cerebral ischemic tolerance.
The pial collateral status, which can be assessed by conventional angiography, single-phase or multiphase CT angiography (mCTA), CT perfusion, and contrast-enhanced MRI, is a key determinant of the infarct volume and progression in patients with AIS due to LVO. In addition to a small core (ASPECTS ≥6 points on NCCT), pial collateral status can be used for guiding patient selection for EVT.
Inwu Yu et al. in this study hypothesized that the pial collateral status at the time of presentation could predict the infarct size on MRI in patients with similar degrees of early ischemic changes on CT and hence tested the association between serial changes in collateral status and infarct volume defined as DWI lesions in patients with LVO and small core. They also tested whether mCTA- and MRI-based collaterals are congruent over time during the hyperacute phase of stroke.
The study was an observational, single-center, retrospective cohort study of prospectively acquired data, which evaluated consecutive patients with acute ischemic stroke having ASPECTS > 6 who were candidates for endovascular treatment within the anterior circulation territory and underwent both pretreatment multiphasic CTA (mCTA) and multimodal MRI to determine baseline early ischemic changes and collateral status.
The inclusion criteria were as follows: (1) presentation within 24 hours from symptom onset/last-known well time; (2) both pre-treatment multiphase CTA and multimodal MRI, including DWI, MR perfusion (MRP), and MR angiography; (3) NIHSS score ≥4 points at admission; (4) ASPECTS ≥6 points; and (5) internal carotid artery and proximal middle cerebral artery (M1 or M1 equivalent [2 or more M2 segment]) occlusion relevant to the acute symptoms. For analysis, all the patients were divided into three collateral grade by each modality: (1) poor; (2) intermediate; and (3) good.
A total of 65 patients were used for analysis. The median ASPECTS was 8.0 points, and the median time from the onset of the symptoms to the acquisition of the images was 138.0 minutes for mCTA and 204.0 minutes for multimodal MRI.
Multiphase CTA showed poor collaterals in 5.4%, intermediate in 12.3%, and good collaterals in 72.3% patients. Age, sex, risk factors, and stroke subtype were not different between the groups stratified by the mCTA collateral status. In spite of similar CT ASPECTS between the groups, the DWI lesion volume on the MRI performed a median of 59 minutes after CT significantly differed, correlating with the mCTA collateral grades (P<0.0001). Lower admission NIHSS score was associated with good collateral status.
The DWI lesion volumes were inversely associated with the collateral grades on mCTA, even though with similar ASPECTS. After adjusting for other variables, the mCTA collateral grade was the only factor independently associated with the DWI lesion volume. Both the mCTA- and MRP-based collateral assessments reported a good inter-rater correlation in classifying the collateral status into different groups. The collateral status remained at the same grade in most participants during the hyperacute phase on correlating the mCTA and MRP collateral grades.
The authors, hence, observed that in patients with a similar degree of early ischemic changes (CT ASPECTS), the mCTA-based collateral status at the time of presentation was the key predictor of DWI lesion volume. The collaterals measured using the 2 modalities (mCTA and MRP) were congruent over time, suggesting that collateral status is stable for at least 1 hour during the hyperacute phase of stroke.
The study concluded that the mCTA assessed collateral adequacy is the sole predictor of eventual DWI lesion volume before EVT in patients with small ischemic changes and large vessel occlusion.
Limitations of the study include being a retrospective analysis of a prospective cohort at a single medical center, selection bias in enrolment, a longer time interval between CT and multimodal MRI, and that the study did not analyze the clinical outcomes and results of the EVT.
Future prospective studies are needed for the assessment of the collateral status in patients eligible for EVT and to determine whether the collateral status, along with other known parameters, could affect the decision-making and prognosis in patients undergoing EVT for AIS.
