Clonus

Clonus is a set of involuntary and rhythmic muscular contractions and relaxations. It is a sign of certain neurological conditions, especially those linked to problems with the upper motor neuron lesions. Often, clonus happens together with spasticity, which is when muscles are very stiff. Unlike small twitches called fasciculations that are usually caused by issues with the lower motor neuron, clonus causes larger movements. These movements usually start when a reflex is triggered. Studies show that the beats of clonus happen between three to eight times each second on average and can last from just a few seconds to several minutes, depending on the person's health condition.

Signs

Clonus is a sign of some nerve problems and involves repeated, quick muscle movements. It is most often seen in the ankle, where the foot moves up and down. Sometimes, it can also happen in the fingers, toes, or other parts of the body.

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

Cause

Clonus can happen in people with certain 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 as part of a seizure in epilepsy. Sometimes, taking certain medicines can cause clonus, especially drugs that affect serotonin levels, and it can be a sign of a serious reaction called serotonin syndrome.

Mechanism

The movement called clonus happens when muscles contract and relax quickly and repeatedly. This can happen because of issues in 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 more likely to send signals.

Another idea is that special parts of the nervous system may keep sending signals to the muscles even after the original trigger stops. Both of these ideas might work together in some cases. Clonus often happens along with spasticity, where muscles are stiff and hard to control, especially after strokes or spinal cord injuries. However, not everyone with stiff muscles will experience clonus. It is most commonly seen in the ankle because of the way signals travel over longer distances in these muscles.

Diagnosis

Doctors check for clonus by moving a person's foot or knee quickly. For the foot, they push it upward to stretch a muscle, which may cause the foot to move up and down repeatedly. 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 similar repeated movements occur.

Voluntary induction in healthy people

Gregory Bateson described how clonus can be created in healthy people. He explained that balance depends on automatic actions in our spine. When we move our legs in a special way, these actions can start to shake rhythmically. This shaking is called clonus.

Bateson also mentioned that in Balinese rituals, creating clonus is an important part of their traditions.