The NIH Stroke Scale (NIHSS) was developed by the National Institute of Neurological Disorders and Stroke to provide a standardised, reproducible assessment of neurological deficit severity in acute stroke patients. Originally published in 1989, it has become the global standard for stroke severity quantification, used in clinical trials, treatment eligibility determinations (particularly for IV thrombolysis and mechanical thrombectomy), and outcome prediction. The scale consists of 15 items assessing consciousness, gaze, visual fields, facial palsy, limb motor function, ataxia, sensation, language, dysarthria, and extinction.