Clonus

Clonus is a set of involuntary and rhythmic muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many cases is accompanied by spasticity (another form of hyperexcitability). Unlike small spontaneous twitches known as fasciculations (usually caused by lower motor neuron pathology), clonus causes large motions that are usually initiated by a reflex. Studies have shown clonus beat frequency to range from three to eight Hz on average, and may last a few seconds to several minutes depending on the patient’s condition. Clonus is most commonly found at the ankle, specifically with a dorsiflexion/plantarflexion movement (up and down). Some case studies have also reported clonus in the finger, toe, and laterally in the ankle (as opposed to the typical up and down motion). Ankle (medial gastrocnemius) Patella (knee cap) Triceps surae Biceps brachii Clonus is typically seen in people with cerebral palsy, stroke, multiple sclerosis, spinal cord damage and hepatic encephalopathy. It can occur in epilepsy as part of a generalized tonic–clonic seizure, and in pregnant women presenting with severe pre-eclampsia and eclampsia. It can also be an adverse drug reaction, such as after ingestion of potent serotonergic drugs, where clonus strongly predicts imminent serotonin toxicity (serotonin syndrome). The self re-excitation of hyperactive stretch reflexes theory involves a repetitive contract-relax cycle in the affected muscle, which creates oscillatory movements in the affected limb. In order for self re-excitation to exist, both an increase in motor neuron excitability and nerve signal delay are required. Increased motor neuron excitability is likely accomplished by alterations to the net inhibition of neurons occurring as a result of injury to the central nervous system (CNS) (stroke/ spinal cord injury). This lack of inhibition biases neurons to a net excitatory state, therefore increasing total signal conduction.