National Institutes of Health Stroke Scale (Brott 1989). Acute stroke severity.

What it is and when to use it

The NIHSS (National Institutes of Health Stroke Scale) quantifies the severity of neurological deficit in patients with acute ischemic stroke through 11 items assessing level of consciousness, gaze, visual fields, facial palsy, limb strength, ataxia, sensation, language, dysarthria, and extinction/inattention. It is used in initial assessment, to guide thrombolysis and thrombectomy decisions, and to monitor progression. It is the scale recommended by the AHA/ASA guidelines for the management of acute ischemic stroke.

How to interpret it

The total score ranges from 0 to 42; higher scores indicate more severe deficit. A common stratification is: 0 no symptoms; 1-4 minor stroke; 5-15 moderate; 16-20 moderate-to-severe; 21-42 severe. Higher scores correlate with worse prognosis and larger infarct volume. A high score (often indicatively around 6 or more) together with a large-vessel occlusion supports consideration of mechanical thrombectomy, always integrated with neuroimaging and the time window.

Limitations and when not to use it

Validated in adults with acute ischemic stroke; it should be administered by trained and certified personnel to ensure interrater reliability. It underestimates posterior-circulation and non-dominant-hemisphere strokes, where clinically relevant deficits may yield low scores. It is not a diagnostic tool: it does not distinguish ischemia from hemorrhage (neuroimaging is required) or assess etiology, and it does not replace a functional disability scale (such as the modified Rankin Scale). Sedation, pre-existing aphasia, intubation, or coma can confound the score.

Frequently asked questions

What NIHSS score is considered a severe stroke?

In general, scores of 16 or more indicate high-severity stroke, and 21-42 are considered severe, with worse functional prognosis.

What NIHSS score is needed for thrombectomy?

There is no absolute threshold, but an NIHSS of approximately 6 or more with an imaging-confirmed large-vessel occlusion typically supports thrombectomy, within the appropriate time window.

Does the NIHSS detect all strokes?

No; it particularly underestimates posterior-circulation and non-dominant-hemisphere strokes, so a low score does not rule out a clinically relevant stroke.