Detect by comparing the response in the limbs to painful stimuli. If pain produces an asymmetric response, then limb weakness is present. (If the patient 'localises' with one arm, hold this down and retest to ensure that a similar response cannot be elicited from the other limb).
Failure to 'grimace' on one side in response to bilateral supraorbital pain indicates a facial weakness.
Both patients are in coma; both have an asymmetric response to pain indicating a right arm weakness and focal brain damage.
Pain stimulus applied to the toe nails or Achilles tendon similarly tests power in the lower limbs. Variation in tone, reflexes or plantar responses between each side also indicates a focal deficit. In practice, if the examiner fails to detect a difference in response to painful stimuli, these additional features seldom provide convincing evidence.
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