Clonus

Clinic

• Clonus is a set of involuntary and rhythmic muscular contractions and relaxations.
• It isassociated with upper motor neuron lesions involving descending motor pathways
• It is accompanied by spasticity
• Unlike fasciculations, it causes large motions that are usually initiated by a reflex.
• It is a sign of UMN entity

Cause

• Cerebral palsy
• Stroke
• Multiple sclerosis
• Spinal cord damage
• Hepatic encephalopathy
• Epilepsy as part of a generalized tonic–clonic seizure
• Pre-eclampsia and eclampsia

Mechanism[edit]

Hyperactive stretch reflexes[edit]

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. Signaling delay is present due to an increased nerve conduction time. Long delays are primarily due to long reflex pathways, which are common in distal joints and muscles. This may therefore explain why clonus is typically found in distal structures like the ankle. Frequency of clonus beats have been found to be directly proportional to the length of the reflex pathway it is found in.

Central oscillator[edit]

Clonus, with respect to the presence of a central oscillator, functions on the theory that when the central oscillator is turned on by a peripheral event, it will continue to rhythmically excite motor neurons, therefore creating clonus.

Although the two proposed mechanisms are very different in [theory] and are still debated, some studies now propose the potential of both mechanisms co-existing to create clonus. It is thought that the stretch reflex pathway may be stimulated first, and through its events, cause a decreased synaptic current threshold. This decreased synaptic current threshold would enhance motor neuron excitability as nerve impulses would be more readily conducted, and thus turn on this central oscillator. This theory is still being investigated.

Related entities

• Fasciculations are spontaneous twitches usually caused by LMNS. But clonus causes large motions that are usually initiated by a reflex.
• Myoclonous: It differs from clonus, because of its irregularity.
• Spasticity

Clonus vs Spasticity

• Clonus tends to co-exist with spasticity in many cases of stroke and spinal cord injury likely due to their common physiological origins
• Although it is not seen in all patients with spasticity.
• Clonus is not present with spasticity in patients with significantly increased muscle tone, as the muscles are constantly active and therefore not engaging in the characteristic on/off cycle of clonus.