Infarction volume modulates and corrects left hemisphere bias in the NIHSS for middle cerebral artery stroke.

BACKGROUND AND PURPOSE: The National Institutes of Health Stroke Scale (NIHSS) exhibits a left hemisphere bias in middle cerebral artery (MCA) stroke. This study aimed to quantify this bias, investigate its modulation by infarction volume, and develop a correction model. METHODS: This retrospective study included 263 first-ever MCA stroke patients, with 173 in the training set and 90 in the test set. Baseline NIHSS scores were compared between left and right hemispheric stroke before and after propensity score matching. Negative binomial regression tested the interaction between infarction side and volume on NIHSS scores in the training set. A correction was then applied to NIHSS scores for left hemisphere strokes. Correction effectiveness was verified by comparing interhemispheric difference in before and after corrected scores, correlation with infarction volume and predictive value for hemorrhagic transformation (HT). RESULTS: Left hemisphere stroke had higher NIHSS scores than right before and after matching, both in the training and test sets (p < 0.05). A significant negative interaction between infarction side and volume (p < 0.05) indicated that larger infarction volume attenuates the left hemisphere bias in the NIHSS scores. After correction, no significant interhemispheric difference in NIHSS scores remained (p > 0.05). Corrected scores showed stronger correlation with infarction volume (training set: r = 0.509 for NIHSS1 and 0.483 for NIHSS2 vs. 0.293 pre-correction; test set: r = 0.715 and 0.675 vs. 0.562) and better HT prediction (training set: AUC = 0.688 for NIHSS1 and 0.703 for NIHSS2 vs. 0.653 pre-correction; test set: AUC = 0.747 and 0.757 vs. 0.723) than original NIHSS. CONCLUSION: Infarction volume modulates the left hemisphere bias of the NIHSS scores in MCA stroke. The infarction volume-based correction eliminates this bias, strengthens biological validity, and improves the prediction of clinically relevant outcomes like HT.