In the context of upper motor neuron lesions, which sign may manifest as rapid, rhythmic contractions?

• A. Decerebrate posturing
• B. CN
• C. DTRs
• D. Clonus

Answer: (D)

Rapid, rhythmic contractions in the setting of upper motor neuron lesions are clonus. This occurs when loss of descending inhibitory control leaves the spinal stretch reflex arc hyperexcitable. A sudden stretch—such as passive ankle dorsiflexion—triggers an ongoing series of involuntary muscle contractions that repeat in a rhythmic, clonic pattern. This sign is most commonly observed at the ankle but can occur elsewhere.

Decerebrate posturing is an abnormal extension posture from brainstem injury, not a rhythmic clonic response. Deep tendon reflexes can be brisk or hyperactive in UMN lesions but do not inherently produce repeated rhythmic contractions like clonus. Cranial nerve findings aren’t about rhythmic contractions either. Clonus specifically reflects this disinhibited, repetitive reflex activity in the spinal cord due to an upper motor neuron lesion.