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NIHSS Score: National Institutes of Health Stroke Scale Guide
The NIHSS is the global standard for quantifying acute stroke severity across 15 neurological domains. Score 0 = normal; ≥25 = very severe. It predicts 3-month functional outcomes, guides tPA and thrombectomy eligibility, and tracks treatment response. Key limitation: it systematically underestimates posterior circulation strokes — a patient with basilar artery occlusion can score just 2–4 while having a life-threatening deficit. Know the scale. Know its blind spots.
TL;DR: The NIHSS is the global standard for quantifying acute stroke severity across 15 neurological domains. Score 0 = normal; ≥25 = very severe. It predicts 3-month functional outcomes, guides tPA and thrombectomy eligibility, and tracks treatment response. Key limitation: it systematically underestimates posterior circulation strokes — a patient with basilar artery occlusion can score just 2–4 while having a life-threatening deficit. Know the scale. Know its blind spots.
Table of Contents
- What Is the NIHSS?
- Complete NIHSS Scoring Table
- Stroke Severity Classification
- Clinical Applications
- Posterior Circulation Limitations
- Evidence and Guidelines
- Frequently Asked Questions
- References
What Is the NIHSS?
The National Institutes of Health Stroke Scale (NIHSS) was developed in the 1980s and validated across multiple acute stroke trials. It assesses 15 neurological domains, assigning a total score from 0 (no deficit) to 42 (maximum deficit). In practice, very few patients score above 30.
The scale was designed to be quick (takes 5–10 minutes at the bedside), reproducible across providers, and sensitive to deficits that determine tPA and thrombectomy eligibility. It has become the primary language of stroke severity communication in clinical trials, registries, and emergency neurology.
Complete NIHSS Scoring Table
| Domain | Assessment | Score |
|---|---|---|
| 1a. Level of Consciousness | Alert / Not alert, arousable / Not alert, obtunded / Coma | 0–3 |
| 1b. LOC Questions (month, age) | Both correct / One correct / Neither | 0–2 |
| 1c. LOC Commands (eye open, fist) | Both correct / One correct / Neither | 0–2 |
| 2. Best Gaze | Normal / Partial gaze palsy / Forced deviation | 0–2 |
| 3. Visual Fields | No loss / Partial hemianopia / Complete hemianopia / Bilateral | 0–3 |
| 4. Facial Palsy | Normal / Minor / Partial / Complete | 0–3 |
| 5. Motor Arm (left and right, scored separately) | No drift (10s) / Drift before 10s / Falls to bed before 10s / No movement / Amputation | 0–4 each |
| 6. Motor Leg (left and right, scored separately) | No drift (5s) / Drift before 5s / Falls before 5s / No movement / Amputation | 0–4 each |
| 7. Limb Ataxia | Absent / One limb / Two limbs | 0–2 |
| 8. Sensory | Normal / Mild-moderate loss / Severe loss | 0–2 |
| 9. Best Language | No aphasia / Mild-moderate aphasia / Severe aphasia / Mute | 0–3 |
| 10. Dysarthria | Normal / Mild-moderate / Severe / Intubated | 0–2 |
| 11. Extinction/Inattention | No neglect / Partial neglect / Profound neglect | 0–2 |
Maximum score: 42 (items 5 and 6 each scored bilaterally = 8 possible points combined)
Stroke Severity Classification
| NIHSS Score | Severity | Clinical Characteristics |
|---|---|---|
| 0 | No stroke | Normal neurological exam |
| 1–4 | Minor | Subtle deficits; often ambulatory |
| 5–15 | Moderate | Significant impairment; hemiparesis likely |
| 15–20 | Moderate–severe | Major deficit; usually non-ambulatory |
| 21–42 | Severe | Catastrophic neurological injury |
Key correlation with outcomes:
- NIHSS ≤8: ~75% chance of good functional outcome (mRS 0–1) at 3 months
- NIHSS 8–16: ~50% chance good outcome
- NIHSS >16: <10% chance good outcome without recanalization therapy
Clinical Applications
tPA Eligibility
IV alteplase eligibility in ischemic stroke requires NIHSS ≥1 (measurable deficit). There is no upper NIHSS cutoff for tPA per AHA/ASA 2019 guidelines — severe strokes (NIHSS >25) can still receive tPA if otherwise eligible, though hemorrhagic transformation risk increases with severity.
Contraindication for minor, non-disabling strokes: tPA is generally withheld if NIHSS = 0 or symptoms are purely subjective without objective deficit, as the risk-benefit ratio doesn't favor treatment.
Mechanical Thrombectomy (EVT)
NIHSS ≥6 with large vessel occlusion (LVO) on CT/MR angiography → consider thrombectomy up to 24 hours from last known well (DAWN, DEFUSE-3 criteria) if significant salvageable tissue on perfusion imaging. Patients with NIHSS <6 ("mild stroke") with LVO may still benefit from EVT — this is an evolving area.
Serial Assessment and Treatment Response
Score NIHSS at arrival, 24 hours, 7 days, and at discharge. Early neurological improvement (ENI) = NIHSS decrease ≥4 points or reaching 0 within 24 hours — one of the strongest predictors of good 90-day outcome.
Neurological deterioration = NIHSS increase ≥4 points → triggers urgent workup (hemorrhagic transformation, malignant edema, re-occlusion, metabolic derangement).
Case Vignettes
Case 1: 68-year-old presents with sudden right arm weakness and expressive aphasia. NIHSS: gaze 0, visual 0, facial palsy 1, right arm motor 3, right leg motor 1, sensory 1, language (aphasia) 2, dysarthria 1. Total = 9. Moderate stroke. Imaging shows left MCA M1 occlusion. tPA within window + EVT.
Case 2: 55-year-old with acute dizziness, diplopia, gait ataxia. NIHSS: LOC 0, gaze 0, visual 0, ataxia 2, all other items 0. Total = 2. Radiologically severe — vertebrobasilar TIA vs posterior circulation stroke. NIHSS massively underestimates risk here. Proceed to MRI DWI urgently.
Posterior Circulation Limitations
This is the most clinically dangerous limitation of the NIHSS. The scale was designed with anterior circulation (MCA territory) stroke in mind. Posterior circulation deficits — dysphagia, truncal ataxia, abnormal cough, diplopia, crossed sensorimotor findings, Horner syndrome — are either not scored or underweighted.
A patient with basilar artery occlusion may present with:
- NIHSS of 2–4 (ataxia = 2, partial gaze palsy = 1)
- But face catastrophic brainstem herniation and 80–90% mortality without treatment
The POST-NIHSS (Posterior NIHSS), adding points for abnormal cough (+5), dysphagia (+4), and gait/truncal ataxia (+3), was developed to improve prognostic accuracy in this population.
Clinical rule: Any patient with brainstem/cerebellar symptoms should receive urgent MRI DWI and neurology consultation regardless of NIHSS score. Never use NIHSS alone to triage posterior circulation stroke.
Evidence and Guidelines
Based on articles retrieved from PubMed:
Alemseged F et al. (2022) developed and validated the Posterior NIHSS (POST-NIHSS) in 202 derivation and 65 validation cohort patients with posterior circulation stroke and NIHSS <10. POST-NIHSS (adding points for abnormal cough, dysphagia, and gait/truncal ataxia to baseline NIHSS) demonstrated significantly higher AUC for predicting poor 3-month functional outcome (mRS ≥3) compared to standard NIHSS — validation AUC 0.82 vs 0.73 (p=0.04). Stroke. 2022;53(4):1247–1255. PMID: 34905944.
Mistry EA, Yeatts SD, Khatri P et al. (2022) examined methodological issues with using NIHSS as a trial outcome measure. Stroke. 2022;53(4):e150–e155. PMID: 35012328.
AHA/ASA 2019 Acute Ischemic Stroke Guidelines (Powers WJ et al.) establish NIHSS as the required severity assessment tool for all acute stroke interventions.
Frequently Asked Questions
Q: Does a low NIHSS mean the stroke is minor? Not always. Posterior circulation strokes, isolated visual field deficits, and pure aphasia without motor deficits can all produce NIHSS <5 while representing significant lesions with serious prognosis.
Q: Does NIHSS need to be done by a neurologist? No — the NIHSS was designed for standardized use by nurses, NPs, PAs, and physicians of any specialty after formal training.
Q: Is NIHSS useful for hemorrhagic stroke? The NIHSS was developed for ischemic stroke but is routinely used for intracerebral hemorrhage (ICH) to quantify deficit severity.
Q: What is mRS and how does it relate to NIHSS? Modified Rankin Scale (mRS) measures functional disability 0–6 (0=no symptoms, 6=death). NIHSS predicts mRS at 90 days but is not equivalent.
Q: What counts as clinically significant change in serial NIHSS? Generally: ≥4-point change is considered clinically significant in major guidelines.
Related Calculators
References
- Alemseged F, et al. Posterior NIHSS Improves Prognostic Accuracy in Posterior Circulation Stroke. Stroke. 2022;53(4):1247–1255. PMID: 34905944.
- Mistry EA, et al. NIHSS as Outcome in Stroke Research. Stroke. 2022;53(4):e150–e155. PMID: 35012328.
- Brott T, et al. Measurements of acute cerebral infarction. Stroke. 1989;20(7):864–870. PMID: 2749846.
- Powers WJ, et al. 2019 AHA/ASA Guidelines for Acute Ischemic Stroke. Stroke. 2019;50(12):e344–e418. PMID: 31662037.