NIH Stroke Scale (NIHSS) Calculator
Calculate the NIHSS score to quantify stroke severity and guide treatment decisions. Score 15 neurological parameters to assess acute stroke patients.
Understanding the NIH Stroke Scale Components
The NIHSS systematically evaluates 15 neurological domains to create a comprehensive picture of stroke-related deficits. Level of consciousness (LOC) comprises three components: alertness (0-3, from alert to coma), orientation questions (0-2, asking month and age), and following commands (0-2, testing eye closure and hand grip). These baseline assessments establish the patient's mental status and ability to cooperate with the remaining exam.
Visual and motor function dominate the scale's predictive power. Best gaze (0-2) tests horizontal eye movement; forced deviation toward one side indicates a large hemispheric stroke. Visual field testing (0-3) identifies hemianopia, common in posterior circulation strokes. Facial palsy (0-3) ranges from normal symmetry to complete paralysis of upper and lower face. Motor function in each limb (0-4 per limb, testing arms and legs separately) grades drift or complete paralysis when the limb is held against gravity. Limb ataxia (0-2) detects cerebellar involvement through finger-to-nose and heel-to-shin testing.
Additional components not shown in this simplified calculator include sensory testing, language assessment (aphasia evaluation through naming, comprehension, and repetition), dysarthria (speech clarity), and extinction/inattention (neglect testing). The full NIHSS captures subtle deficits that influence functional outcomes. A patient with NIHSS 8 due to complete hemiparesis has a different prognosis than one with NIHSS 8 from global aphasia with minimal weakness.
Certification in NIHSS administration requires watching standardized patient videos and scoring within acceptable ranges compared to expert raters. This training ensures consistency across examiners and institutions, making the NIHSS the gold standard for stroke severity assessment worldwide. Telemedicine stroke consults rely on remote NIHSS scoring to determine tPA eligibility when on-site neurology isn't available.
Clinical Application in Acute Stroke Management
The NIHSS serves three critical functions in acute stroke care: triage, treatment eligibility determination, and prognostication. When a patient arrives with suspected stroke, emergency providers calculate the NIHSS within minutes. An NIHSS of 0-3 may represent a transient ischemic attack (TIA) or very mild stroke not requiring intensive monitoring. An NIHSS of 4-15 signals moderate stroke warranting thrombolytic consideration if within the treatment window. An NIHSS above 15 indicates severe stroke; these patients are candidates for mechanical thrombectomy if large vessel occlusion is confirmed on imaging.
Thrombolytic therapy with alteplase (tPA) has strict eligibility criteria, and NIHSS plays a key role. Historically, NIHSS below 4 was considered too mild for tPA's hemorrhage risk to be justified. Current practice is more nuanced—a patient with NIHSS 3 but disabling aphasia or hemianopia may still receive tPA. Conversely, very high NIHSS (>25) increases hemorrhagic transformation risk, though withholding tPA in these cases remains controversial. The DEFUSE-3 and DAWN trials showed benefit from thrombectomy in large core infarcts with high NIHSS even 6-24 hours after onset, revolutionizing stroke care.
Serial NIHSS measurements track treatment response and detect complications. Providers repeat the NIHSS at 2, 6, 12, and 24 hours post-tPA. Early improvement (NIHSS drop ≥4 points within 2 hours) suggests successful reperfusion and predicts good outcomes. Early worsening (NIHSS increase ≥4 points) raises concern for hemorrhagic transformation, reocclusion, or malignant edema. An urgent CT scan is mandated for any neurological deterioration to rule out intracranial bleeding.
Long-term prognosis correlates strongly with admission NIHSS. Patients with NIHSS 0-7 have roughly 60-70% probability of good functional outcome (modified Rankin Scale 0-2) at 90 days. Those with NIHSS 8-13 drop to 40-50% good outcome. NIHSS above 20 predicts less than 20% chance of functional independence. These statistics guide family counseling, goals of care discussions, and discharge planning. However, individual variability is substantial—age, stroke location, and comorbidities modify these probabilities significantly.
Limitations and Special Considerations in NIHSS Scoring
The NIHSS has well-recognized biases that affect its clinical utility. Posterior circulation strokes involving the brainstem or cerebellum often produce devastating symptoms (locked-in syndrome, inability to swallow, severe ataxia) but score deceptively low on the NIHSS because cortical functions like language remain intact. A basilar artery occlusion causing coma may score only 10-15 on NIHSS, underestimating severity. Conversely, left hemisphere strokes with dense aphasia score high even when motor function is minimally affected, potentially overestimating disability in patients who may recover language function.
Inter-rater reliability varies by component. Motor testing and level of consciousness show excellent agreement between trained examiners. Subtle visual field cuts, mild ataxia, and sensory deficits are more subjective and prone to inconsistency. Certification courses emphasize standardized techniques—for example, testing visual fields by wiggling fingers in quadrants, not asking "can you see my hand?" Limb drift is timed for 10 seconds; the limb must start from a specific position (arms extended 90° sitting or 45° supine). These details matter when scores near treatment thresholds.
Certain deficits aren't captured by the NIHSS at all. Cognitive impairment, executive dysfunction, and visuospatial neglect (unless severe enough to cause extinction on bilateral stimulation) can profoundly impact independence but contribute minimally to the score. A patient with right hemisphere stroke causing left-sided neglect, anosognosia (denial of deficit), and poor safety awareness may have NIHSS 4 but require 24-hour supervision. The modified Rankin Scale (mRS) assesses functional outcome more holistically, which is why clinical trials use mRS as the primary endpoint despite using NIHSS for enrollment.
Special populations pose scoring challenges. Patients with baseline deficits from prior strokes should have the NIHSS scored relative to their new deficits, though this introduces subjectivity. Intubated patients cannot be tested for dysarthria or LOC questions/commands; convention is to score these items as if normal (0) if intubation is for airway protection unrelated to stroke, or as abnormal (maximum points) if stroke severity necessitated intubation. Non-English speakers require interpreters or non-verbal assessment techniques to fairly evaluate language function. The NIHSS remains the best available standardized tool despite these limitations, providing a common language for stroke teams worldwide.
Frequently Asked Questions
What is the NIH Stroke Scale?
The NIHSS is a standardized 15-item neurological examination used to quantify stroke severity. It assesses level of consciousness, vision, motor function, sensation, language, and attention. Scores range from 0 (no deficit) to 42 (severe stroke).
How do you interpret NIHSS scores?
NIHSS 0 = no stroke symptoms, 1-4 = minor stroke, 5-15 = moderate stroke, 16-20 = moderate-severe stroke, 21-42 = severe stroke. Scores guide thrombolytic therapy decisions and predict outcomes.
What NIHSS score qualifies for tPA?
Most guidelines recommend tPA (tissue plasminogen activator) for NIHSS ≥4, though some centers treat scores as low as 2-3 if symptoms are disabling. Very high scores (>25) increase hemorrhage risk but may still benefit from treatment in selected cases.
Can NIHSS scores change over time?
Yes. Serial NIHSS assessments track improvement or deterioration. A decrease of ≥4 points or reaching 0-1 indicates significant improvement. An increase of ≥4 points suggests worsening, possibly from hemorrhagic transformation or stroke progression.
Who can administer the NIHSS?
Healthcare providers trained and certified in NIHSS administration, typically physicians, nurse practitioners, physician assistants, and stroke nurses. Proper training ensures inter-rater reliability and accurate scoring.