Time-dependence of NIHSS in predicting functional outcome of patients with acute ischemic stroke treated with intravenous thrombolysis

Objectives: The National Institute of Health Stroke Scale (NIHSS) is a predictor for the prognosis of acute ischaemic stroke (AIS) and its prediction is time-dependent. We examined the performance of NIHSS at different timepoints in predicting functional outcome of patients with thrombolysed AIS.

Methods: This prospective study included 269 patients with AIS treated with recombinant tissue plasminogen activator (rt-PA). Unfavourable functional outcome was defined as modified Rankin Scale score 4-6 at 3 months after rt-PA treatment. Receiver operating characteristic curves were used to examine the predictive power of NIHSS score at admission and 2 hours/24 hours/7 days/10 days after rt-PA treatment. Youden's index was used to select the threshold of NIHSS score. Logistic regression was used to estimate the ORs of unfavourable functional outcome for patients with NIHSS score higher than the selected thresholds.

Results: The threshold of NIHSS score at admission was 12 (sensitivity: 0.51, specificity: 0.84) with an acceptable predictive power (area under curve [AUC] 0.74) for unfavourable functional outcome. The threshold changed to 5 at 24 hours after rt-PA treatment (sensitivity: 0.83, specificity: 0.65) and remained unchanged afterwards. The predictive power and sensitivity sequentially increased over time and peaked at 10 days after rt-PA treatment (AUC 0.92, sensitivity: 0.85, specificity: 0.80). NIHSS scores higher than the thresholds were associated with elevated risk of unfavourable functional outcome at all timepoints (all p<0.001).

Conclusions: NIHSS is time-dependent in predicting AIS prognosis with increasing predictive power over time. Since patients whose NIHSS score ≥ 12 are likely to have unfavourable functional outcome with rt-PA treatment only, mechanical thrombectomy should be largely taken into consideration for these patients.