Clonus is involuntary muscle contractions caused by sudden strains, often affecting the ankles. It can be a symptom of neurological disorders and can be caused by adverse reactions to certain medications. Mild clonus is not unusual, but sustained contractions for more than five are abnormal. Clonus often occurs on one side of the body and can aid in the diagnosis of certain conditions.
Clonus, also called clonospasm, is the medical term for repetitive, involuntary muscle contractions due to sudden strains. It most commonly affects the ankles. Clonus takes its name from a Greek word meaning “turmoil” and is often a symptom of a neurological disorder.
When caused by neurological problems, clonus is most often associated with lesions on the upper motor neurons that connect the cerebral cortex or brainstem to the spinal cord. Such injuries can be caused by a variety of neurological disorders, including stroke, multiple sclerosis, spinal cord injury, and hepatic encephalopathy caused by liver failure. The condition can also be caused by an adverse reaction to serotonergic drugs in the so-called serotonin syndrome. A wide variety of medications can contribute to serotonin syndrome, including monoamine oxidase inhibitors (MAOIs) and other antidepressants, opioids such as oxycodone, psychedelics such as methylenedioxymethamphetamine (MDMA) and lysergic acid diethylamide (LSD), stimulants including cocaine and even therapeutic herbs including Panax, ginseng and St. John’s Wort.
Mild clonus is not unusual even in healthy people. It is only considered abnormal if it is sustained for more than five contractions. It is usually initiated by reflex actions. It can be tested by flexing the ankle or wrist, or by pressing the kneecap, or patella, toward the toes.
Clonus often occurs on only one side of the body. It is associated with nerve damage in the spinal cord area above where it started. Because it occurs only during muscle stretching and does not produce very large involuntary actions, clonus often goes unnoticed until diagnosed by a neurologist. While clonus can aid in the diagnosis of certain conditions, it by itself doesn’t suggest much, as it can have a huge number of causes. It is important to consider the clonus along with any other symptoms the patient is experiencing to determine the site and cause of any possible neurological damage or disease.
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