Clonus

Clonus (from the Greek for "violent, confused motion") is a series of involuntary muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, and is particularly associated with upper motor neuron lesions such as in stroke, multiple sclerosis, spinal cord damage and hepatic encephalopathy. Unlike the small, spontaneous twitching known as fasciculations (usually caused by lower motor neuron pathology), clonus causes large motions that are usually initiated by a reflex.

Clonus is most common in the ankles, where it is tested by rapidly flexing the foot upward (dorsiflexion). It can also be tested in the knees by rapidly pushing the patella (knee cap), towards the toes. Only sustained clonus (5 beats or more) is considered abnormal.

Clonus appearing after ingesting potent serotonergic drugs strongly predicts imminent serotonin toxicity (serotonin syndrome).