Clonus is rhythmic, involuntary, and repetitive muscular contraction and relaxation. It may be induced by sudden passive stretching of a muscle or tendon, most usually the Achilles tendon (ankle clonus) or patella (patellar clonus). Ankle clonus is best elicited by holding the relaxed leg underneath the moderately flexed knee, then quickly dorsiflexing the ankle and holding it dorsiflexed. A few beats of clonus is within normal limits but sustained clonus is pathological.
Clonus reflects hyperactivity of muscle stretch reflexes and may result from self reexcitation. It is a feature of upper motor neurone disorders affecting the corticospinal (pyramidal) system. Patients with disease of the corticospinal tracts may describe clonus as a rhythmic jerking of the foot, for example when using the foot pedals of a car. Clonus may also be observed as part of a generalized (primary or secondary) epileptic seizure, either in isolation (clonic seizure) or much more commonly following a tonic phase (tonic-clonic seizure). The clonic movements usually involve all four limbs and decrease in frequency and increase in amplitude over about 30-60 seconds as the attack progresses. Rather different "clonic" movements may occur in nonepileptic seizures. A few clonic jerks may also be observed in syncopal attacks, leading the uninitiated to diagnose "seizure" or "convulsion."