Left Hemisphere Bias of NIH Stroke Scale is Most Severe for Middle Cerebral Artery Strokes

Abstract

Background and Aim: NIHSS score is higher for left versus right hemisphere strokes of equal volumes. However, differences in each vascular territory have not been evaluated yet. We hypothesized that left versus right differences are driven by the middle cerebral artery (MCA) territory, and there is no difference between hemispheres for other vascular territories. Methods: This study is based on data from 802 patients with evidence of acute or early subacute ischemic stroke. These patients had infarct restricted to one major arterial territory (MCA, n=437; PCA, n=209; ACA, n=21; vertebrobasilar, n=46) and received NIHSS and MRI at hospital admission. We examined differences in patients with left or right strokes regarding to lesion volume, NIHSS, and other covariates (age, sex, race). We used linear models to test the effects of these covariates on NIHSS. We looked at the whole sample as well as in the sample stratified by NIHSS (<=5 or >5) and by lesion location (MCA or PCA). Results: Patients with left MCA strokes had significantly higher NIHSS than those with right strokes. Only patients with MCA strokes showed NIHSS score affected by the hemisphere when controlling for stroke volume and patient's age. This difference was driven by the more severe strokes (NIHSS>5). In addition, stroke volume and patient's age significantly correlated with NIHSS. Conclusion: Right MCA infarcts are larger than left MCA infarcts associated with a given NIHSS score, after accounting for other significant associations, such as patient's age. It is important to consider this systematic bias in the NIHSS when using the score for inclusion criteria for treatment or trials. Patients with right MCA stroke may be under-treated and left with disabling deficits that are not captured by the NIHSS.