What is the National Institutes of Health Stroke Scale (NIHSS) and how are its scores used to assess stroke severity and determine eligibility for intravenous thrombolysis?

National Institutes of Health Stroke Scale (NIHSS)

What the NIHSS Is

The NIHSS is a standardized 42-point neurological examination scale that quantifies stroke severity through 11 core items, takes 5-10 minutes to administer, and serves as the primary tool for determining eligibility for thrombolytic therapy while predicting patient outcomes. 1, 2, 3

• The scale was developed specifically by the National Institutes of Health for use in clinical stroke trials and has been extensively validated to reduce interobserver error 1
• Higher scores indicate more severe neurological deficits, with the scale designed so virtually any stroke will register some abnormality 1
• The examination is based solely on objective findings and requires no historical information or contributions from family members 1, 3
• An additional item examining finger extension is commonly added to assess distal upper extremity weakness, though it does not contribute to the total score 1, 3

How NIHSS Scores Determine Thrombolytic Eligibility

A baseline NIHSS score establishes initial stroke severity, and a change of ±4 points from baseline may alter a patient's eligibility for thrombolytic therapy, requiring reassessment before proceeding with treatment. 2, 4

• The screening NIHSS must be performed by an examiner experienced in acute stroke treatment who has achieved certification in administering the scale 1
• If the NIHSS score significantly improves from baseline (such as a 4-point improvement) or crosses defined thresholds before angiography, the patient might not be eligible for randomization in treatment protocols 1
• The scale is used on admission to determine patient eligibility for thrombolytic therapy and guides all acute stroke therapy decisions 1, 5

Prognostic Value of NIHSS Scores

Scores greater than 16 forecast a high probability of death or severe disability, while scores less than 6 forecast good recovery. 1, 2, 3

• Initial NIHSS scores are highly correlated with outcome in retrospective analyses of randomized clinical trials 1
• The score strongly predicts the likelihood of patient recovery and correlates with initial infarct volume, cerebral perfusion, and functional outcome 5
• During the first week after acute ischemic stroke, the NIHSS can identify patients highly likely to have poor outcomes 1, 3
• An initial NIHSS of ≤7 is associated with a 14.8% worsening rate and 45% chance of functional normality at 48 hours, whereas scores >7 show a 65.9% worsening rate and only 2.4% chance of recovery within this period 6

When to Perform the NIHSS

Perform the NIHSS at three mandatory timepoints: at presentation/hospital admission (or within the first 24 hours), immediately before any intervention, and at acute care discharge. 1, 2, 3

• A second assessment just before diagnostic cerebral angiography serves as a recheck and may be more accurate because the patient will have stabilized and can better cooperate with the examiner 1
• If a patient is transferred to rehabilitation without NIHSS scores in the record, the rehabilitation team should complete an NIHSS assessment 1, 3
• Serial assessments throughout the acute hospital stay and at 3 months are used to assess neurological recovery 5

Training Requirements

All professionals involved in any aspect of stroke care must be trained and certified to assess stroke severity using the NIHSS by watching a training videotape and passing an examination. 1, 2, 3

• Certification ensures accurate assessment of stroke severity and reduces interobserver variability 1
• The scale has high inter-rater reliability between examiners, making it highly reproducible across different healthcare settings 3

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. 1, 3

• The scale was designed primarily for anterior-circulation ischemic stroke evaluation 1
• Posterior circulation patients evaluated with expanded NIHSS versions score an average of 2 points higher than with classical NIHSS 3
• 71% of posterior circulation stroke patients present with NIHSS ≤4, yet 15% of these "minor" strokes result in poor outcomes at 3 months 3
• Thrombolytic treatment should not be withheld based solely on low NIHSS scores in suspected posterior circulation strokes 3

Items with Lower Reliability

• Facial palsy and dysarthria demonstrate lower inter-rater reliability compared to other NIHSS components 2, 3
• Limb ataxia shows poor reliability, especially when assessment is performed via telemedicine 2, 4
• These items require extra attention during assessment to ensure accuracy 2

Telemedicine Considerations

• The NIHSS can be administered remotely with high-quality video conferencing, yielding strong correlation with bedside scores (Pearson r = 0.97) 2
• Remote assessments take slightly longer (mean 9.7 minutes) than in-person examinations (mean 6.55 minutes) but maintain scoring accuracy 2
• Rearrange the NIHSS order to reduce camera manipulations, performing close-up items before zoomed-out views 2