Clonus is a set of involuntary and muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron involving descending motor pathways, and in many cases is accompanied by spasticity (another form of hyperexcitability).
Signs
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).
Cause
Clonus is typically seen in people with
cerebral palsy,
stroke, multiple sclerosis,
spinal cord 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
Serotonin drugs, where clonus strongly predicts imminent serotonin toxicity (serotonin syndrome).
Mechanism
Hyperactive stretch reflexes
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
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
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.
Clonus and spasticity
Clonus tends to co-exist with
spasticity in many cases of
stroke and spinal cord injury likely due to their common physiological origins.
Some consider clonus as simply an extended outcome of spasticity.
Although closely linked, clonus is not seen in all patients with spasticity.
Clonus tends to not be 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.
Clonus results due to an increased motor neuron excitation (decreased action potential threshold) and is common in muscles with long conduction delays, such as the long reflex tracts found in distal muscle groups.
Diagnosis
Clonus at the
ankle is tested by rapidly flexing the foot into dorsiflexion (upward), inducing a stretch to the
gastrocnemius muscle.
Subsequent beating of the foot will result, however only a sustained clonus (5 beats or more) is considered abnormal. Clonus can also be tested in the
knees by rapidly pushing the
patella (knee cap), towards the toes.
Voluntary induction in healthy people
Gregory Bateson described the induction of clonus in healthy people:
In the text, Bateson goes on to describe induction of clonus as a key element of Balinese ritual.
See also
External links
