Defining Mild Ischemic Stroke by NIHSS Score
A mild ischemic stroke is most commonly defined as an NIHSS score of 0-5, though the specific threshold varies by clinical context, with NIHSS 0-4 representing the most widely accepted definition in contemporary guidelines. 1
Primary Definition Framework
The standard definition of mild stroke is NIHSS score 0-4, as consistently referenced in the NINDS trials and subsequent clinical policy statements from the American College of Emergency Physicians 1
An alternative threshold of NIHSS 0-5 is also used, particularly when discussing milder strokes in the context of treatment decisions, with some literature describing this range as capturing patients with minor deficits 1
The most recent guidelines from the American Heart Association specifically identify NIHSS 3-5 as a critical decision-making range for thrombolysis consideration, distinguishing these patients from those with NIHSS ≤3 who may be candidates for dual antiplatelet therapy instead 2
Clinical Context Matters
For thrombolysis decisions, NIHSS ≤3 represents "minor stroke" where dual antiplatelet therapy (aspirin + clopidogrel for 21 days) may be preferred over thrombolysis for non-disabling deficits, as recommended by the American Stroke Association 2
NIHSS 3-4 with disabling deficits (such as isolated hemianopia affecting driving or independence) warrants thrombolysis consideration, with the American College of Emergency Physicians noting 3-fold increased odds of excellent outcome with treatment 2
The definition of "minor" versus "mild" stroke lacks complete consensus, with some studies using NIHSS ≤6 to capture this population, though functional outcomes vary significantly even within this range 3
Important Caveats
Approximately 31-40% of patients with mild stroke (NIHSS ≤5) have poor outcomes (mRS ≥2) at 3 months when untreated, highlighting that low NIHSS scores do not guarantee good outcomes 3
Posterior circulation strokes require lower NIHSS thresholds for outcome prediction, with optimal cutoff of NIHSS 4 for posterior circulation versus NIHSS 8 for anterior circulation, meaning a PC stroke with NIHSS 2-4 carries similar poor outcome risk as an AC stroke with higher scores 4
Motor impairment items on the NIHSS are particularly predictive of poor outcomes, even when the total NIHSS score is low, so the presence of motor deficits should heighten concern regardless of total score 3, 5
Practical Algorithm for Classification
NIHSS 0-2: Very mild stroke, often non-disabling; consider dual antiplatelet therapy over thrombolysis unless clearly disabling deficits present 2, 6
NIHSS 3-4: Mild stroke requiring careful assessment of whether deficits are disabling; if disabling, proceed with thrombolysis; if non-disabling, consider dual antiplatelet therapy 2
NIHSS 5: Upper boundary of mild stroke in most definitions; generally warrants thrombolysis consideration if within time window and no contraindications 1
For posterior circulation strokes, lower these thresholds by approximately 2 points to account for the NIHSS underestimating severity in this territory 4