Clonus (from Greek for "violence, confusion") is a series of unconscious, rhythmic, muscular contractions and relaxations. Clonus is a sign of a particular neurological condition, particularly related to upper motor neuron lesions involving decreased motor pathways, and in many cases, accompanied by flexibility (another form of hyperexcitability). Unlike the spontaneous small wrinkles known as fasciculations (usually caused by the pathology of the lower motor neurons), the clonus causes large movements that are usually initiated by reflexes. Studies have shown that clonal beat frequency ranges from three to eight Hz on average, and can last several seconds to several minutes depending on the patient's condition.
Video Clonus
Signs and symptoms
Clonus is most commonly found in the ankle, especially with dorsoflection/plantarflection movements (up and down). Several case studies also reported a clonus of the fingers, toes, and laterals of the ankle (as opposed to typical up and down motions).
- Ankle (medial gastrocnemius)
- Patella (knee cap)
- Triceps surae
- Biceps brachii
Maps Clonus
Cause
Clonus is usually seen in people with cerebral palsy, stroke, multiple sclerosis, spinal cord damage and liver encephalopathy. Clonus also appears after ingestion of potent serotonergic drugs, where consumption strongly predicts the imminent toxicity of serotonin (serotonin syndrome).
Mechanism
Hyperactive stretch reflex
Re-excitation of the hyperactive stretch reflex theory involves recurrent relaxation-recessive cycling of the affected muscle, which creates oscillatory movements in affected limbs. In order for excitation to re-exist, both increased motor neuron stimulation and neural signal delays are required. Increased motor neuron stimulation may be achieved with changes in net inhibition of neurons occurring as a result of CNS injury (stroke/spinal cord injury). This lack of inhibition of the neuron bias to a clean excitatory state, thus increasing the total conduction signal. The signal delay is present due to the increase in nerve conduction time. Long delays are mainly due to long reflex pathways, which often occur in joints and distal muscles. Therefore this may explain why clonus is usually found in distal structures such as ankles. The frequency of the clonal beat has been found to be directly proportional to the length of the reflex path found.
Middle Oscillator
Clonus, in connection with the presence of a central oscillator, serves on the theory that when a central oscillator is turned on by a peripheral event, it will continue to rhythmically excite motor neurons; because it creates clonus.
Although the two proposed mechanisms are very different in [theories] and are still being debated, some research now proposes the potential of both mechanisms together to create the clonus. It is thought that the stretch reflex path can be stimulated first, and through its events, causing a decrease in the current synaptic threshold. The current decrease in synaptic thresholds increases motor neuron stimulation as a nerve impulse more easily, and thus activates this central oscillator. This theory is still under investigation.
Clonus and spasticity
Clonus tends to co-exist with flexibility in many cases of stroke and spinal cord injury possibly due to their general physiological origins. Some consider clonus only as a result of an extension of flexibility. Although closely related, the clonus is not seen in all patients with flexibility. Clonus tends not to be present with flexibility in patients with significantly increased muscle tone, as the muscles continue to be active and therefore are not involved in the on/off cycle of clonal characteristics. Clonus results are caused by increased excitation of motor neurons (reduced action potential threshold) and often in muscles with long conduction delays, such as long reflex tracts found in distal muscle groups. Clonus is usually seen in the ankle but may exist in other distal structures as well.
Diagnosis
Clonus at the ankle is tested rapidly flexing the legs into dorsiflexion (upwards), pushing the stretch to the gastrocnemius muscle. Further beating the foot will result, but only the sustained clonus (5 beats or more) is considered abnormal. Clonus can also be tested on the knee by pushing the patella (knee cap) quickly, toward the toes.
See also
- Myoclonus
- Tonic-clonic seizures
- Deep tendon reflex (stretching)
References
External links
- Gilman on Serotonin Toxicity
- Videos about Clonus at the ankle
- A video about Clonus on the knee (patella)
Source of the article : Wikipedia
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