Clonus

Clonus is a neurological sign characterized by rhythmic, involuntary muscle contractions that occur after a sudden stretch. It most often involves the ankle plantar flexors and is seen in conditions affecting the upper motor neurons or spinal cord. Clonus can be inducible, produced by a rapid passive stretch, or spontaneous, occurring without provocation. It is commonly associated with spasticity and hyperreflexia.

Clonus arises from loss of inhibitory control over the stretch reflex in upper motor neuron disease, leading to hyperexcitability of alpha motor neurons and muscle spindle afferents. It is typically velocity dependent; its frequency is usually around 5 to 7 Hz and the beats can last for seconds to minutes while the limb remains stretched.

On examination, clonus is elicited by maintaining a sustained passive stretch of a relaxed limb and briefly dorsiflexing the ankle with the knee extended, producing a sequence of rhythmic beats. It may occur in other limbs and is often accompanied by hyperreflexia and increased resistance to movement.

Clonus is most commonly associated with stroke, spinal cord injury, multiple sclerosis, cerebral palsy, and other upper motor neuron disorders. It must be distinguished from myoclonus, tremor, fasciculations, or convulsions, which have distinct patterns and etiologies.

Management focuses on treating the underlying condition and reducing spasticity. Nonpharmacologic measures include stretching and physical therapy. Pharmacologic options include baclofen, tizanidine, benzodiazepines, dantrolene, and gabapentinoids; focal clonus may respond to botulinum toxin injections. Prognosis varies with the underlying disease and the degree of motor recovery.