National Institutes of Health Stroke Scale (NIHSS)
Primary Purpose
The NIHSS is a standardized 42-point assessment tool specifically designed to quantify neurological impairment after stroke, with higher scores indicating more severe deficits, and serves as the primary instrument for guiding acute treatment decisions including thrombolytic therapy eligibility. 1, 2
Scale Structure and Administration
The NIHSS consists of 11 core items evaluating level of consciousness, visual fields, gaze, facial palsy, motor function (arms and legs), sensation, language, dysarthria, and extinction/inattention 1, 2
Administration takes only 5-10 minutes and is based solely on examination without requiring historical information or surrogate input 1
All clinicians involved in stroke care must be trained and certified through watching training videos and passing an examination to ensure accurate, reproducible assessments 1, 2
An additional item assessing finger extension is often added to evaluate distal upper extremity weakness, which occurs more commonly than proximal weakness in stroke patients 1
Critical Assessment Timepoints
Perform NIHSS at three mandatory intervals: at presentation/within first 24 hours of admission, at acute care discharge, and upon transfer to rehabilitation if previous scores unavailable 1, 2
A 4-point improvement or worsening from baseline may alter treatment eligibility and should trigger reassessment of therapeutic options 2
Prognostic Interpretation
NIHSS >16 forecasts high probability of death or severe disability 1, 2
The scale demonstrates excellent discrimination for 30-day mortality risk with a c-statistic of 0.82 when used as a continuous variable 3
Mortality rates by NIHSS categories: 0-7 points = 4.2%, 8-13 points = 13.9%, 14-21 points = 31.6%, 22-42 points = 53.5% 3
Clinical Applications
The NIHSS directly determines eligibility for thrombolytic therapy and guides acute stroke management decisions 2
The scale exhibits high inter-rater reliability between trained examiners, making it highly reproducible across different healthcare settings 1
NIHSS strongly predicts functional outcomes, with significant correlation to modified Rankin Scale scores at 30 days (correlation 0.74) and 3 months (correlation 0.66) 4
Critical Limitations and Pitfalls
The NIHSS significantly underestimates posterior circulation stroke severity because symptoms like vertigo, dysphagia, and ataxia are not included in the assessment 5, 2
For posterior circulation strokes, lower NIHSS cutoffs are required: an NIHSS ≥4 optimally predicts poor outcomes in posterior circulation versus ≥8 in anterior circulation 6
71% of posterior circulation stroke patients present with NIHSS ≤4, yet 15% of these "minor" strokes result in poor outcomes at 3 months, indicating that thrombolytic treatment should not be withheld based solely on low NIHSS scores 6
Posterior circulation patients evaluated with expanded NIHSS versions score an average of 2 points higher than with classical NIHSS, revealing the scale's systematic underestimation 7
Two items (facial palsy and dysarthria) demonstrate lower inter-rater reliability compared to other components 2
Distal motor function and balance/gait disorders are inadequately assessed by the standard scale 2
Motor Assessment Scoring (Item 5)
Proper technique requires the patient to extend arms palm down at 90 degrees (sitting) or 45 degrees (supine) and hold for 10 seconds 8
Scoring criteria: 0 = no drift for full 10 seconds, 1 = drift before 10 seconds, 2 = some effort against gravity but cannot maintain position, 3 = no effort against gravity (arm falls), 4 = no movement at all 8
Common pitfall: Do not confuse score 3 (some movement present but no effort against gravity) with score 4 (complete absence of movement) 8