Ammad Mahmood, MBChB, BMedSci

Lopez-Rivera V, Abdelkhaleq R, Yamal J-M, Singh N, Savitz SI, Czap AL, Alderazi Y, Chen PR, Grotta JC, Blackburn S, et al. Impact of Initial Imaging Protocol on Likelihood of Endovascular Stroke Therapy. Stroke. 2020;51:3055–3063.

The optimum imaging to evaluate patients presenting with acute ischemic stroke and determine suitability for endovascular thrombectomy (EVT) remains contentious. Non-contrast CT brain (NCCT) is universal, and CT angiogram (CTA) is necessary if EVT is planned. The added value of CT perfusion (CTP) is the subject of ongoing research and debate.

Lopez-Rivera et al. conducted a retrospective analysis of data collected in a study of implementation of EVT in the Houston area. They compared data from one center where CTP was carried out in all patients without contraindications (CTP-H – high usage) and three centers where CTP was carried out optionally at the discretion of the clinical team (CTP-L – low usage). Baseline populations differed significantly with CTP-H sites having higher proportions of patients with history of AF (24% vs 16%), diabetes (34% vs 21%), hypertension (77% vs 49%), hyperlipidaemia (45% vs 23%) and higher median NIHSS (14 vs 11), but better baseline modified Rankin scale (mRS) score (78% mRS 0-2 vs 66%). CTP-H sites also had proportionately fewer direct presentations but more early time window (0-6 hours) presentations and more patients treated with IV thrombolysis. The proportion of patients undergoing EVT in both CTP-H and CTP-L centers showed no significant difference, including amongst patients who had undergone CTP (47% vs 51%) and those who had not (41% vs 49%); those with large (>50ml) predicted ischemic core (53% vs 37%); and those with ASPECTS score <6 (32% vs 23%).

In multivariate logistic regression analysis, patients presenting to the CTP-H center were 41% less likely to undergo EVT (odds ratio 0.59 [0.41-0.85]). This effect was significant in late window patients (OR 0.61 (0.40-0.93)) but not in early window patients (OR 0.62 (0.27-1.41)). The effect was maintained in subgroup analysis, such as anterior circulation occlusion only (OR 0.49 (0.32-0.75)) or direct presentation (OR 0.57 (0.36-0.90)). In patients with large vessel occlusion (LVO), there was no significant difference in rate of significant hemorrhage (1% in both), 90-day mRS (OR 1.04 (0.70-1.54)), or mortality (OR 0.9 (0.53-1.52)) between CTP-H and CTP-L, though functional outcome at the time of discharge was worse in the CTP-H group (OR for good outcome 0.62 (0.43-0.90)).

This analysis provides several points of discussion about the value added to patient selection for EVT by CTP. Are patients who stand to benefit from EVT being unnecessarily excluded on the basis of CTP, particularly those with large predicted ischemic cores? Conversely, could the risk and cost of additional EVT procedures which are producing no benefit to the whole population be avoided by CTP selection? It should be noted that the late window population in which the reduction of EVT likelihood was found requires advanced imaging for selection of eligible patients for EVT according to current guidelines.

The extrapolation and generalization of data from 4 centers within one healthcare set-up to other healthcare settings is difficult. The authors highlighted that clinician selection bias may have been minimized through the rotation of staff around each site; however, other sources of bias inherent in each site are difficult to account for, such as post-EVT management or length of hospital stay. The decision to proceed with EVT is multi-faceted and nuanced, which can be difficult to capture within a logistic regression model. The baseline characteristics demonstrated that the population presenting to the CTP-H facility had more co-morbidities and was presenting earlier (when the effect of facility type of likelihood of EVT was non-significant) with more severe strokes.

Other studies have not been conclusive about whether CTP is informative in EVT selection. Bouslama et al. found that patients selected by CTP for EVT rather than NCCT alone had better functional outcome and increased chance of reperfusion.1 Conversely, Campbell et al., in a meta-analysis of EVT trials in the HERMES collaboration, concluded that mismatch volume on CTP did not modify treatment effect of EVT in terms of functional outcome.2

The added value of CTP to evaluation of patients with acute ischemic stroke remains a topic of debate. Clinical trials randomizing patients to different imaging protocols would be informative.

References:

  1. Bouslama M, Haussen DC, Nogueira RG. Computed Tomographic Perfusion Selection and Clinical Outcomes After Endovascular Therapy in Large Vessel Occlusion Stroke. Stroke. 2017;48:1271–1277.
  2. Campbell BCV et al; HERMES collaborators. Penumbral imaging and functional outcome in patients with anterior circulation ischaemic stroke treated with endovascular thrombectomy versus medical therapy: a meta-analysis of individual patient-level data. Lancet Neurol. 2019;18:46-55.