Does the National Institutes of Health Stroke Scale (NIHSS) underestimate stroke severity in patients with posterior circulation strokes, particularly in older adults with vascular risk factors?

Does the NIHSS Underestimate Stroke Severity in Posterior Circulation?

Yes, the NIHSS significantly underestimates posterior circulation stroke severity because it focuses primarily on limb and speech impairments while inadequately assessing cranial nerve lesions, ataxia, vertigo, and dysphagia—symptoms that are hallmark features of posterior circulation strokes. 1, 2

Why the NIHSS Fails in Posterior Circulation Strokes

The fundamental problem is structural design bias:

• The NIHSS was designed with anterior circulation strokes in mind, emphasizing cortical deficits like limb weakness, language dysfunction, and neglect while minimizing brainstem and cerebellar signs. 1

• Patients with devastating posterior circulation large vessel occlusions can present with NIHSS scores of 0, showing only headache, vertigo, and nausea—yet these patients may progress to locked-in syndrome or death. 1

• The most common neurological sign in NIHSS 0 posterior circulation strokes is truncal ataxia, which receives minimal weight in the standard scale. 1

The Clinical Impact: Lower Scores, Worse Outcomes

The prognostic thresholds differ dramatically between circulation territories:

• The optimal NIHSS cutoff for outcome prediction is 8 in anterior circulation but only 4 in posterior circulation strokes—a 4-point difference that has major treatment implications. 3

• To achieve >80% sensitivity for detecting poor outcomes, the NIHSS cutoff must be 4 in anterior circulation but only 2 in posterior circulation. 3

• 71% of posterior circulation stroke patients present with NIHSS ≤4, yet 15% of these "minor" strokes result in poor outcomes (mRS >2) at 3 months. 2, 3

• The baseline NIHSS cutoff for favorable 3-month outcome (mRS ≤2) is significantly lower in posterior circulation compared to anterior circulation strokes. 1

Quantifying the Underestimation

Research demonstrates measurable score discrepancies:

• Posterior circulation patients evaluated with expanded NIHSS versions score an average of 2 points higher than with classical NIHSS, indicating systematic underscoring. 2, 4

• The Posterior NIHSS (POST-NIHSS), which adds points for abnormal cough (5 points), dysphagia (4 points), and gait/truncal ataxia (3 points), shows significantly better prognostic accuracy (AUC 0.80 vs 0.73, p=0.03) in patients with NIHSS <10. 5

Critical Clinical Pitfalls to Avoid

Do not withhold thrombolytic therapy or thrombectomy from posterior circulation stroke patients based solely on low NIHSS scores. 2, 3 This is the most dangerous consequence of NIHSS underestimation.

Key warning signs that demand aggressive intervention despite low NIHSS:

• Dysphagia and abnormal cough reflex are powerful independent predictors of poor outcome in posterior circulation strokes. 5
• Truncal or gait ataxia indicates significant cerebellar or brainstem involvement. 1, 5
• Decreased level of consciousness (OR 10.54 for moderate-to-severe stroke). 6
• Diplopia, visual field defects, or vertigo suggest basilar territory involvement. 1, 6

Practical Assessment Strategy

When evaluating suspected posterior circulation stroke:

1. Maintain high clinical suspicion even with NIHSS 0-4, as these scores do not exclude devastating posterior circulation occlusions. 1

2. Specifically assess for posterior circulation signs not adequately captured by NIHSS: dysphagia (have speech therapist evaluate within 48 hours), truncal ataxia, cough reflex, diplopia, vertigo, and nystagmus. 1, 5, 6

3. Recognize that posterior circulation strokes present with non-specific symptoms including headache, nausea, vomiting, dizziness, and altered consciousness—leading to delays in neurology evaluation and longer door-to-needle times. 1

4. Obtain vascular imaging (CTA or MRA) liberally when posterior circulation stroke is suspected, regardless of NIHSS score, as basilar artery occlusion can be clinically subtle. 1

Current Treatment Guidelines Despite NIHSS Limitations

Recent evidence supports aggressive intervention:

• Thrombectomy is indicated within 12 hours for basilar artery occlusion with NIHSS ≥6, PC-ASPECTS ≥6, and age 18-89 years (Class I, Level B-R). 1

• Thrombectomy is reasonable within 12-24 hours from last known well (Class IIa, Level B-R). 1

• Mortality from basilar artery occlusion ranges from 45-86%, with good outcomes occurring in only approximately 20% despite advanced care—far worse than anterior circulation strokes. 1

The NIHSS remains useful for posterior circulation strokes but requires clinical judgment overlay, specific attention to brainstem/cerebellar signs, and recognition that lower scores carry different prognostic weight than in anterior circulation strokes. 1, 2, 3