What is the NIH Stroke Scale (NIHSS)?

The NIH Stroke Scale (NIHSS) is a standardized 15-item neurological examination developed by the National Institutes of Health to quantify stroke severity in acute ischemic stroke. It scores 11 domains — level of consciousness, gaze, visual fields, facial palsy, motor arm (bilateral), motor leg (bilateral), limb ataxia, sensory, language/aphasia, dysarthria, and extinction/inattention — on a 0 to 42 scale. Higher scores indicate greater neurological deficit. The NIHSS is the universal language for stroke severity across emergency medicine, neurology, interventional neuroradiology, and clinical research. It is required for tPA eligibility assessment, mechanical thrombectomy selection, and enrollment in virtually every acute stroke clinical trial.

What NIHSS score is needed for tPA eligibility?

Current AHA/ASA guidelines (2019) do not specify a hard minimum NIHSS threshold for IV tPA eligibility in ischemic stroke within 3–4.5 hours of onset. However, clinical practice typically requires an NIHSS ≥ 4 (or any disabling deficit) to justify the risk-benefit ratio of thrombolysis. For very low NIHSS scores (0–4), treatment decisions depend on whether the deficit is disabling — isolated aphasia, hemianopia, or ataxia at NIHSS 2–4 may warrant tPA. An NIHSS > 25 (very severe stroke) historically prompted caution due to hemorrhagic transformation risk, though guidelines no longer list this as an absolute contraindication. Baseline NIHSS before tPA is mandatory for monitoring post-treatment response.

What does an NIHSS score of 0 mean?

An NIHSS score of 0 means no neurological deficits detectable by the 15-item scale examination — the patient is alert, answers questions correctly, follows commands, has full gaze, full visual fields, no facial palsy, full motor strength in all four limbs, no ataxia, intact sensation, normal language, normal articulation, and no inattention. However, NIHSS 0 does not exclude stroke. Posterior circulation strokes (cerebellar, brainstem, occipital), small lacunar infarcts, and pure sensory strokes can produce genuine neurological deficits not captured by the NIHSS. A patient with acute stroke symptoms and NIHSS 0 still warrants urgent MRI DWI to exclude ischemic injury.

What is the difference between NIHSS and GCS?

NIHSS and GCS measure different things and serve different clinical purposes. The Glasgow Coma Scale (GCS) assesses level of consciousness and arousal — eye opening, verbal response, and motor response — and is used in TBI, septic encephalopathy, overdose, and any cause of altered consciousness. It ranges 3–15. The NIHSS is stroke-specific and measures the neurological deficit caused by focal brain ischemia — gaze deviation, hemiplegia, aphasia, hemianopia, and related deficits — on a 0–42 scale. A patient with a massive MCA stroke may have an NIHSS of 20 but a GCS of 10T; a patient with mild aphasia and arm weakness post-TIA may have NIHSS 5 but GCS 15. The two scales are complementary, not interchangeable.

NIH Stroke Scale (NIHSS) Calculator — Acute Stroke Severity Assessment

NIHSS Score Interpretation

NIHSS Score	Severity	Interpretation	Clinical Guidance
0	No Stroke Symptoms	No detectable neurological deficit on 15-item exam	Does not exclude stroke — posterior circulation and lacunar infarcts can score 0. Urgent MRI DWI if symptoms present.
1–4	Minor	Minimal deficit; patient likely able to walk and communicate	Consider IV tPA if deficit is disabling (isolated aphasia, hemianopia); thrombectomy generally not indicated unless large vessel occlusion on CTA. Close monitoring for early deterioration.
5–15	Moderate	Significant neurological deficit; may impair ambulation and ADLs	IV tPA within window (3–4.5 hrs); CTA head/neck for LVO assessment. Thrombectomy candidacy if LVO identified and ASPECTS ≥ 6. Stroke unit admission.
16–20	Moderate to Severe	Large cortical or multi-territory deficit; significant disability expected	Emergent CTA for LVO. Strong thrombectomy candidacy if LVO present and within extended window. IV tPA bridge if eligible. ICU or stroke unit. Neurosurgery consult for hemorrhagic transformation risk.
21–42	Severe	Major hemispheric or brainstem stroke; altered consciousness likely	Emergent thrombectomy for LVO regardless of IV tPA eligibility. Airway management. Consider malignant MCA infarct protocol (decompressive hemicraniectomy criteria). Family counseling re: prognosis.

About the NIH Stroke Scale

The NIH Stroke Scale (NIHSS) was developed by Thomas Brott and colleagues at the University of Cincinnati and National Institutes of Health and published in Stroke in 1989. It was designed to provide a standardized, reproducible, and rapid quantitative measure of neurological deficit in acute ischemic stroke — enabling consistent communication among clinicians, objective monitoring of stroke progression and response to treatment, and valid enrollment criteria for clinical trials.

Since its validation in the NINDS tPA trial (1995) — which demonstrated that IV alteplase improved outcomes in patients with ischemic stroke at 3 months — the NIHSS has been embedded in the eligibility criteria, outcome definitions, and treatment algorithms of virtually every acute stroke intervention. It is now mandated in Joint Commission-certified Primary and Comprehensive Stroke Centers.

The 11 NIHSS Domains

Items 1a–1c: Level of Consciousness

LOC items assess the reticular activating system and its cortical projections. Item 1a scores arousal (0=alert, 3=unresponsive); Items 1b and 1c test specific cognitive responses to verbal commands. These early items catch large hemispheric strokes and posterior fossa events with brainstem involvement. Notably, 1b instructions specify that the examiner scores the first answer only — repeated attempts after self-correction do not improve the score.

Item 2: Best Gaze

Horizontal gaze testing identifies frontal eye field or brainstem pathways involvement. A "forced deviation" toward one side (score 2) indicates a large frontal lobe lesion driving gaze toward the lesion side, or a pontine lesion driving gaze away. Internuclear ophthalmoplegia (INO) is typically scored as 1. Only horizontal gaze is tested; vertical gaze palsy, nystagmus, and monocular changes are not captured.

Item 3: Visual Fields

Confrontation testing using finger-counting or visual threat detects hemianopia from occipital cortex or optic tract lesions. Monocular visual loss is scored as 1 if due to retinal infarction (central retinal artery occlusion). Bilateral hemianopia (cortical blindness) scores 3 — these patients may be unaware of their visual loss (Anton syndrome).

Item 4: Facial Palsy

Lower motor neuron (Bell's palsy) vs. upper motor neuron (stroke) distinction matters here: central facial palsy (stroke) spares the forehead due to bilateral cortical innervation. Score 1–3 for central UMN palsy. Complete ipsilateral facial paralysis including forehead suggests peripheral (LMN) etiology — not typical stroke, score carefully.

Items 5–6: Motor Arm and Leg (Bilateral)

Each limb is scored independently (0–4). Arms are tested at 90° (sitting) or 45° (supine) for 10 seconds; legs at 30° supine for 5 seconds. Score 4 (no movement) is distinguished from score 3 (some movement but cannot overcome gravity). Amputees or prior pre-existing deficits: score as 9 (untestable) and document. Maximum motor contribution to NIHSS = 16 (4 limbs × 4 points).

Item 7: Limb Ataxia

Tests cerebellar function via finger-nose-finger (upper) and heel-to-shin (lower). Ataxia is scored only when clearly disproportionate to weakness. If the patient cannot move the limb at all (score 3–4 on motor), ataxia = 0 (untestable, score 0). Ataxia in stroke typically indicates ipsilateral cerebellar hemisphere or contralateral cerebellar pathways involvement.

Item 8: Sensory

Pinprick sensation tested in all extremities and face. Hemibody sensory loss (including face) = 1; bilateral or complete loss = 2. Obtunded patients can be tested by grimace to noxious stimulus. Pure sensory stroke (lacunar) from thalamic or thalamocortical tract infarcts can score 1–2 with otherwise minimal NIHSS — clinically disabling despite low total score.

Item 9: Best Language (Aphasia)

Assessed via picture description, object naming (pictured items), and reading standard sentences. Global aphasia (score 3) means no usable expressive or receptive language. Broca's (expressive) aphasia typically scores 1–2; Wernicke's (receptive) aphasia tends to score higher due to comprehension failure affecting multiple items. Aphasia is the most prognostically significant language impairment — even mild aphasia (score 1) significantly impairs quality of life.

Items 10–11: Dysarthria and Neglect

Dysarthria is motor speech production (articulation) separate from aphasia (language). A patient can be dysarthric without aphasia (cerebellar or bulbar dysarthria) or aphasic without dysarthria. Extinction/inattention (item 11) captures hemispatial neglect — the patient may have intact sensation but extinguishes one side when both are stimulated simultaneously. Right MCA strokes produce neglect far more frequently than left (typically producing aphasia instead).

NIHSS in tPA and Thrombectomy Decisions

The 1995 NINDS tPA trial established IV alteplase benefit for ischemic stroke within 3 hours regardless of stroke severity, but excluded patients with rapidly improving or very minor stroke (NIHSS 0–1). Current AHA/ASA guidelines (2019) allow tPA for minor but disabling deficits (e.g., isolated aphasia NIHSS 2). The upper NIHSS limit (previously 25) is no longer an absolute contraindication. For mechanical thrombectomy, landmark trials (MR CLEAN, SWIFT PRIME, DAWN, DEFUSE 3) demonstrated benefit across a wide NIHSS range (typically 6–30) when LVO is confirmed by CTA/MRA and tissue viability criteria are met.

Limitations of the NIHSS

Anterior circulation bias: NIHSS is heavily weighted toward large MCA territory strokes; posterior circulation deficits (diplopia, dysphagia, ataxia, isolated field cuts) are incompletely captured — a major basilar artery occlusion can score < 10
Intubation and sedation: Pharmacological sedation and neuromuscular blockade invalidate multiple items; document as untestable
Training dependence: Reliable scoring requires NIHSS certification (free at strokeassociation.org) — interrater reliability improves substantially with training
Right hemisphere stroke underscoring: Neglect (item 11) partially captures right hemisphere deficits, but executive dysfunction, visuospatial impairment, and anosognosia are poorly measured
Aphasia scoring inconsistency: Language items (1b, 1c, 9) can compound scoring complexity in multilingual or pre-existing aphasic patients
Not a prognostic tool in isolation: Outcome depends on infarct location and volume, collateral flow, time to treatment, age, comorbidities, and reperfusion success — NIHSS alone is insufficient

References

Brott T, Adams HP Jr, Olinger CP, et al. Measurements of acute cerebral infarction: a clinical examination scale. Stroke. 1989;20(7):864–870.

The National Institute of Neurological Disorders and Stroke rt-PA Stroke Study Group. Tissue plasminogen activator for acute ischemic stroke. N Engl J Med. 1995;333:1581–1587.

Powers WJ, Rabinstein AA, Ackerson T, et al. Guidelines for the Early Management of Patients With Acute Ischemic Stroke: 2019 Update to the 2018 Guidelines. Stroke. 2019;50(12):e344–e418.

Lyden P, Brott T, Tilley B, et al. Improved reliability of the NIH Stroke Scale using video training. Stroke. 1994;25(11):2220–2226.

NIH Stroke Scale (NIHSS) Calculator