NIH Stroke Scale Calculator (NIHSS)
The NIH Stroke Scale (NIHSS) is the standard bedside tool for objectively quantifying neurological deficit caused by stroke. Score all 11 assessment items below to get the total NIHSS score, a severity category, and a plain-English interpretation. The scale was developed by Dr. Patrick D. Lyden and is used worldwide for triage, treatment decisions, thrombolysis eligibility, and outcome tracking.
What is the NIH Stroke Scale (NIHSS)?
The NIH Stroke Scale, commonly abbreviated NIHSS, is a standardized neurological examination scoring system developed by Dr. Patrick D. Lyden and colleagues at the National Institutes of Health. It was designed to quantify the neurological impairment caused by an acute stroke in a reproducible, objective way. The scale covers 11 domains of neurological function: level of consciousness, language, neglect, visual-field loss, extraocular movement, motor strength, ataxia, dysarthria, and sensory loss. Each domain is scored from 0 (normal) to a maximum of 2, 3, or 4 points depending on the item, and the scores are summed to give a total between 0 and 42. A score of 0 represents no detectable deficit, while 42 represents the most severe possible impairment. The tool takes less than 10 minutes to administer and is used in emergency departments, stroke units, and clinical trials worldwide.
How to Administer and Score the NIHSS
Three core rules govern NIHSS administration. First, score what you see, not what you think: record the observed response, not your interpretation of the underlying cause. Second, score the first response, not the best response, with the single exception of item 9 (Best Language), where the examiner intentionally elicits the highest possible language performance. Third, do not coach: provide no hints or corrections once the instruction has been given. For patients who cannot follow verbal commands due to language barriers, pantomime the required action. For patients who are obtunded, use a grimace rather than a voluntary smile to assess facial symmetry (item 4). Motor items (5A, 5B, 6A, 6B) are scored 0 for amputation or joint fusion. Ataxia (item 7) is scored 0 when paralysis or inability to understand prevents valid testing. Dysarthria (item 10) is scored 0 when the patient is intubated or has a physical barrier.
Clinical Uses of NIHSS in Stroke Care
The NIHSS serves four major clinical purposes. First, it guides acute treatment decisions: patients with scores between 5 and 25 are generally considered for intravenous alteplase (tPA) if within the time window, while those with large vessel occlusion and higher scores are prioritized for mechanical thrombectomy. Patients with very minor deficits (score 0-4) may still be candidates for thrombolysis if symptoms are disabling. Second, serial scoring tracks neurological change: a drop of 4 or more points from baseline typically signals significant improvement, while an increase of 4 or more points signals deterioration requiring urgent reassessment. Third, the baseline score predicts functional outcome: each 1-point increase in NIHSS at admission corresponds to approximately a 17% reduction in the likelihood of excellent 3-month outcome. Fourth, the NIHSS is used in clinical trials and registries as a standardized outcome measure, making it central to stroke research globally.
Limitations and Important Caveats
The NIHSS is weighted toward left (dominant) hemisphere deficits because it includes three language items (1B, 1C, and 9) but only one neglect item. Right hemisphere and posterior circulation strokes involving the brainstem or cerebellum are systematically underscored. For example, a patient with a lateral medullary (Wallenberg) infarct may have severe dysphagia, vertigo, and ipsilateral facial palsy but score very low on the NIHSS. The scale also does not capture executive function, memory, or mood, which can be severely affected by stroke. Inter-rater reliability is good when examiners are certified, but varies without training. Several NIHSS certification programs are available online through the NIH and American Heart Association. This calculator is an educational and documentation aid and does not replace certified training or clinical judgment.
NIHSS Severity Categories
| Score Range | Severity | Clinical Implication |
|---|---|---|
| 0 | No stroke symptoms | No detectable deficit; consider TIA if symptoms resolved |
| 1-4 | Minor stroke | High likelihood of good outcome; consider thrombolysis window |
| 5-15 | Moderate stroke | Typically eligible for IV tPA and thrombectomy evaluation |
| 16-20 | Moderate to severe stroke | Urgent thrombectomy evaluation; ICU-level care |
| 21-42 | Severe stroke | High mortality risk; intensive intervention required |
Standard clinical interpretation of NIHSS total scores. Thresholds are used for triage, treatment eligibility, and outcome prediction.
Frequently asked questions
What is the maximum NIHSS score and what does it mean?
The maximum possible NIHSS score is 42, representing the most severe possible neurological impairment across all 11 assessed domains. In practice, scores above 20-22 are considered severe strokes with high early mortality and poor functional outcome. Most published clinical trials and treatment protocols define major or severe stroke as an NIHSS score of 22 or above, though the threshold varies by context. A score of 42 would require maximal deficit in every single category, which is rarely observed in surviving patients.
Is a higher or lower NIHSS score better?
A lower score is better. A score of 0 means no detectable neurological deficit. Scores of 1-4 indicate a minor stroke with high likelihood of a good functional recovery. Scores rise as the severity and number of neurological deficits increase, with scores above 16 reflecting moderate to severe impairment and scores above 21 reflecting severe deficits.
Can someone have a score of 0 and still have had a stroke?
Yes. A patient who had a transient ischemic attack (TIA) with symptom resolution may score 0 at the time of assessment. Posterior circulation strokes affecting the brainstem or cerebellum can also produce severe symptoms (vertigo, dysphagia, ataxia) that are incompletely captured by the NIHSS, resulting in an underestimated score. A score of 0 does not rule out stroke and should not delay brain imaging.
Why does item 9 say "score the best response" while all other items say "score the first response"?
Item 9 measures language ability (aphasia), and the goal is to capture the patient's maximum language capability. Performing multiple tasks - naming objects, reading sentences, describing a picture - gives the examiner a broad sample from which to select the best performance. For all other items, the first response is used because coaching or repeated attempts can overestimate function and reduce the reliability of the measurement.
What NIHSS score qualifies a patient for IV alteplase (tPA)?
There is no absolute score threshold for IV alteplase eligibility. U.S. and European guidelines generally support treatment for patients with measurable neurological deficit who can be treated within the recommended time window (4.5 hours from symptom onset under most guidelines). Very minor or rapidly improving deficits (roughly NIHSS 0-1) may not warrant treatment unless symptoms are disabling, while very severe deficits (NIHSS above 25) were excluded from early pivotal trials. Treatment eligibility always requires individual clinical judgment, CT imaging to rule out hemorrhage, and review of the full contraindication list.
How is the NIHSS different from the modified NIHSS (mNIHSS)?
The modified NIHSS (mNIHSS) removes four items from the original scale (1B LOC questions, 1C LOC commands, 4 facial palsy, and one limb ataxia item) to reduce time and the influence of aphasia on the score, and has a maximum of 31 points. The mNIHSS was developed to improve inter-rater reliability and reduce ceiling effects in aphasic patients. The original 42-point NIHSS remains the standard for most clinical and research use.
Do I need certification to use the NIHSS?
Certification is not legally required in most settings, but it is strongly recommended. Inter-rater reliability improves substantially with standardized training, and many stroke programs, hospitals, and clinical trials require certified examiners. Free online training and certification are available through NIH Professional Education (nihstrokescale.org) and the American Heart Association. This calculator is intended as a documentation aid and educational reference, not as a substitute for proper clinical training.
Sources
- Lyden P, et al. Improved reliability of the NIH Stroke Scale using video training. Stroke. 1994;25(11):2220-2226.
- National Institute of Neurological Disorders and Stroke. NIH Stroke Scale Training and Certification.
- Powers WJ, et al. 2019 AHA/ASA Guidelines for the Early Management of Patients With Acute Ischemic Stroke. Stroke. 2019;50(12):e344-e418.