A 43-year-old man with slowly progressive motor weakness, dysphagia, and dysarthria over 30 years was diagnosed with an atypical form of primary lateral sclerosis on the basis of upper motor neuron signs on physical examination, including increased deep tendon reflexes in all limbs, theBabinski reflex on the right side, and tap-elicited sustained clonus of the biceps and triceps brachii, as shown in this video. On surface electromyography, the frequency of the clonus was 8.3 Hz, and the periodic bursts of the biceps and triceps brachii muscles were synchronous.
Tap-elicited clonus is rare in the biceps and triceps brachii, and the patient’s clonus did not need continuous passive stretch of the muscle to be sustained, as is usually the case.
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