Clonus

Clonus is a set of repeated muscle movements that happen without a person’s control. These movements happen when muscles tighten and then relax in a steady pattern. Clonus can be a sign that there is a problem with certain parts of the nervous system, especially linked to issues with the upper motor neuron.

Often, clonus happens together with spasticity, where muscles become very stiff. Unlike small muscle twitches called fasciculations, which are linked to problems with the lower motor neuron, clonus causes larger movements. These movements usually start when a reflex is triggered. The beats of clonus usually happen between three to eight times each second and can last only a few seconds or a few minutes.

Signs

Clonus shows that there may be issues with the nerves. It causes quick, repeated muscle movements. It is usually seen in the ankle, where the foot moves up and down. It can also happen in the fingers, toes, or other body parts.

• Ankle (medial gastrocnemius)
• Patella (knee cap)
• Triceps surae
• Biceps brachii

Cause

Clonus can happen in people with health conditions like cerebral palsy, stroke, multiple sclerosis, spinal cord damage, and hepatic encephalopathy. It may also appear during severe pre-eclampsia and eclampsia in pregnant women, or during a seizure in epilepsy. Some medicines can cause clonus, especially those that change serotonin levels, and it can be a sign of serotonin syndrome.

Mechanism

Clonus is a movement where muscles contract and relax quickly over and over. This can happen because of problems with the brain's control of muscles, often after injuries to the central nervous system (CNS) like a stroke or spinal cord injury. These injuries can change how nerve cells normally work, making them send signals more easily.

Another idea is that parts of the nervous system may keep sending signals to the muscles even after the original trigger stops. Clonus often happens with spasticity, where muscles are stiff and hard to control, especially after strokes or spinal cord injuries. However, not everyone with stiff muscles will have clonus. It is most often seen in the ankle because signals travel longer distances in these muscles.

Diagnosis

Doctors look for clonus by moving a person's foot or knee quickly. For the foot, they push it upward to stretch a muscle. This might make the foot move up and down again and again. If this happens five times or more, it is seen as unusual. They can also check the knees by pushing the knee cap toward the toes to see if the same repeated movements happen.

Voluntary induction in healthy people

Gregory Bateson showed that clonus can happen in healthy people. He said our balance depends on actions our spine does without thinking. When we move our legs in a certain way, these actions can make our legs shake rhythmically. This shaking is called clonus.

Bateson noted that in Balinese rituals, creating clonus is an important tradition.