The correct approach when scoring NIHSS item 5b (motor arm, right) in a patient with known right hemiplegia from an old stroke who now has a new left hemispheric stroke is to score what the right arm actually does now, not what the old deficit predicts. You examine the right arm, observe and score its current movement, and you do not assume the pre-existing right-sided weakness explains the finding. The new left hemispheric stroke does not directly weaken the right arm, so any right arm motor score reflects the arm's current status, including the residual old deficit plus any new change.
This question sits at the intersection of stroke severity scoring, chronic deficit attribution, and the practical reality that NIHSS items are designed to be scored on observed performance rather than inferred anatomy. Getting item 5b right in this scenario matters because the total NIHSS drives treatment decisions, research eligibility, and communication between clinicians. The trap is subtle: a clinician who remembers the patient has "right hemiplegia" may reflexively attribute a poor right arm score to the old stroke and mentally discount it, or may fail to document why the score looks the way it does. Both errors degrade the reliability of the score.
The scoring mechanism for item 5b
The NIHSS motor arm item asks the examiner to place the patient's arm in a specific position, hold it at 90 degrees if sitting or 45 degrees if supine, and observe for drift over 10 seconds. The score runs from 0 to 4. A score of 0 means no drift. A score of 1 means drift but the arm does not hit the bed or other support. A score of 2 means some effort against gravity, with the arm drifting down to the bed or support but retaining some antigravity movement. A score of 3 means no effort against gravity, with the limb falling. A score of 4 means no movement at all.
The critical design feature is that the item is scored on the observed behavior of that limb at that moment. The examiner does not adjust the score for prior deficits, for expected findings, or for the patient's history. The scoring instructions direct the examiner to score the first attempt and to score each arm separately. There is no instruction anywhere in the NIHSS to subtract a pre-existing deficit from the observed score. This is deliberate. The NIHSS is a measure of current neurological status, not a measure of change from baseline.
A plain-language version: you test the arm, you watch what it does, and you score that. The old stroke is context, not a correction factor.
The reason the instrument is built this way is psychometric. The NIHSS was designed for reliability across examiners and for sensitivity to change over time. If examiners were allowed to adjust scores based on presumed chronic deficits, inter-rater reliability would collapse and scores would no longer be comparable across patients, sites, or time points. The item measures the limb, not the brain.
Why the new left stroke does not change the right arm's direct motor score
A left hemispheric stroke affects the right side of the body through the crossed corticospinal tract. The right arm's motor cortex and corticospinal tract lie in the left hemisphere. So a new left hemispheric stroke can produce new right-sided weakness. This is the counterintuitive detail that many clinicians miss in this specific scenario.
The patient already has right hemiplegia from a stroke three years ago. That old deficit came from a prior lesion, likely in the left hemisphere or elsewhere in the right arm's motor pathway. Now a new left hemispheric stroke occurs. The new lesion can add to the old deficit on the same side. The right arm is therefore at risk of worsening, not because of the old stroke, but because the new stroke is on the side that controls the right arm.
This reframes the question. The correct approach is not to discount the right arm score because of the old hemiplegia. The correct approach is to score the right arm carefully because the new left hemispheric stroke may be worsening it. The old deficit is a floor, and the new stroke can push the score higher. A right arm that was scoring 3 before the new stroke might now score 4. That change is clinically meaningful and should be captured.
The practical implication is that the examiner should score the right arm as observed and, separately, document the baseline right arm function if it is known. The score itself is the observed score. The baseline is context for interpretation, not a modifier of the score.
Quantified context for severity and change
The NIHSS total ranges from 0 to 42. Motor arm contributes up to 8 points across both arms, 4 per arm. Item 5b alone contributes 0 to 4. A change of 1 point on item 5b is a meaningful change in that limb's function, and a total NIHSS change of 4 or more points is commonly used as a threshold for clinically meaningful worsening in stroke trials and quality metrics.
In thrombolysis and thrombectomy decision-making, the total NIHSS is a central input. Many treatment protocols use a minimum NIHSS threshold, often 4 to 6, for intravenous thrombolysis consideration, and thrombectomy trials have used thresholds such as 6 or higher for anterior circulation large vessel occlusion. A single item scored incorrectly can shift the total by 1 to 4 points, which can matter at those boundaries.
The old stroke being three years out matters for a different reason: chronic deficits are stable by that point. Most spontaneous recovery after stroke occurs in the first 3 to 6 months, with the steepest gains in the first weeks. A deficit still present at three years is a stable baseline. That stability is what makes it useful as a reference point. If the right arm was scoring 2 at the three-year mark and now scores 3 or 4, the change is unlikely to be spontaneous recovery in reverse and more likely reflects the new left hemispheric stroke or another process.
None of these numbers should be applied as rigid rules without the treating team's protocol. They illustrate why item-level accuracy matters, not what any individual patient's treatment should be.
Common misconceptions and a professional caveat
The most common misconception is that the old right hemiplegia should be "subtracted" from the right arm score. No such adjustment exists in the NIHSS. The score is the observed score.
A second misconception is that a new left hemispheric stroke cannot affect the right arm. It can, because the left hemisphere controls the right arm. This is the anatomic point that makes the scenario tricky.
A third misconception is that the right arm score is uninformative because it was already abnormal. It is informative precisely because it has a known baseline. A stable chronic deficit that suddenly worsens is a red flag for new injury on the same pathway.
A fourth misconception is that the examiner should score the "better" arm and infer the other. Each arm is scored separately and independently.
A professional caveat: the NIHSS is weighted toward anterior circulation and motor and language domains. It underrepresents posterior circulation deficits, gait, and some cognitive findings. A normal or near-normal total does not exclude a clinically important stroke, and item-level findings should always be interpreted alongside the full examination and imaging.
Practical actions and red flags
Score item 5b on the observed right arm performance at the time of examination, using the standard 0 to 4 scale and the standard positioning and 10-second observation. Document the known pre-existing right hemiplegia explicitly in the note, and state whether the current right arm score represents a change from the documented baseline if a baseline is available. Do not adjust, discount, or "correct" the right arm score for the chronic deficit. Record the observed score and the context separately. Re-examine the right arm after any acute change, and compare item-level scores over time rather than relying on the total alone. Communicate the baseline and the current item scores during handoffs, because the next examiner may not know the patient's history.
Red flags that warrant urgent escalation:
A new or worsening right arm deficit in a patient with a known old right hemiplegia, because the new left hemispheric stroke may be extending or evolving. Any acute change in the total NIHSS of 4 or more points, or any new item 5b worsening, in the setting of suspected new stroke. New language, gaze, or visual field findings alongside the motor change, which suggest a larger left hemispheric syndrome. A right arm score that does not match the expected chronic baseline, prompting reassessment of both the old and new lesion burden.
Comparison of scoring approaches
| Approach | What it scores | Strengths | Weaknesses | Appropriate use |
|---|---|---|---|---|
| Observed current performance (correct) | Right arm as it moves now | Reliable, reproducible, standard | Requires examiner discipline | All NIHSS scoring |
| Baseline-adjusted or "corrected" score | Observed minus presumed old deficit | Intuitive to some clinicians | Not part of the NIHSS, unreliable | Not recommended |
| Score the better arm and infer | Unaffected limb used as proxy | Fast | Ignores the affected limb | Never appropriate for item 5b |
| Score from imaging or lesion location | Predicted deficit from scan | Anatomically appealing | Imaging–clinical mismatch is common | Not a substitute for examination |
| Score from history alone | Prior records | Available before exam | Does not capture current status | Context only, never the score |
The table makes the core point explicit: only the observed current performance is the scored value.
Timeline of deficit evolution and scoring implications
| Time point | Expected right arm status | Scoring implication |
|---|---|---|
| Acute new left stroke, hours 0–24 | Possible new or worsened right arm weakness | Score observed; compare with baseline if known |
| Days 1–7 | Edema and penumbra evolution possible | Re-score daily; watch for item 5b change |
| Weeks 2–6 | Early recovery begins in some patients | Score observed; do not assume recovery |
| Months 3–6 | Most spontaneous recovery plateau | Stable baseline established |
| Year 1 | Chronic deficit stable in most | Useful reference point |
| Year 3 (this patient) | Chronic right hemiplegia stable | Baseline for comparison |
| New stroke onset | New left lesion may worsen right arm | Score observed; do not discount |
The timeline shows why the three-year-old deficit is a stable reference and why the new stroke is the variable that can move the score.
Limitations and what the score cannot tell you
The NIHSS item 5b tells you what the right arm does during a brief, standardized examination. It does not tell you why the arm performs as it does, whether the old or new lesion is responsible for a given point of weakness, or what the patient's function is in daily life. A score of 3 on item 5b in this patient could reflect the old deficit alone, the new stroke alone, or a combination. The score does not decompose the contributions.
The instrument also does not capture proprioception, coordination, or fine motor control beyond the gross antigravity observation. A patient can score 0 on item 5b and still have meaningful upper limb impairment. And a patient with a chronic deficit can score stably for years and then change for reasons other than stroke, including medication effects, pain, fatigue, or a new musculoskeletal problem.
The most important limitation for this scenario is attribution. The examiner can score the arm reliably but cannot, from the score alone, assign the deficit to the old or new lesion. That attribution requires imaging, clinical correlation, and serial examination. The score is a measurement, not an explanation.
This article is general educational information about NIHSS scoring principles, not a diagnosis, treatment plan, or substitute for clinical judgment. It does not address any individual patient's care and should not be used to make treatment decisions. For clinical decisions about a specific patient, consult the treating stroke team and local protocols, and for immediate neurological deterioration, contact local emergency services.