Clonus, also called clonospasm, is the medical term for repetitive, involuntary muscular contractions due to sudden stretching. It most commonly affects the ankles. Clonus takes its name from a Greek word meaning "tumult" and is often a symptom of neurological disorder.
When caused by neurological problems, clonus is most often associated with lesions on the upper motor neurons connecting the cerebral cortex or brain stem to the spinal cord. Such lesions can be caused by a variety of neurological disorders, including stroke, multiple sclerosis, spinal cord injuries, and hepatic encephalopathy caused by liver failure. The condition can also be brought on by an adverse reaction to serotonergic drugs in so-called serotonin syndrome. A large variety of drugs can contribute to serotonin syndrome, including monoamine oxidase inhibitors (MAOIs) and other antidepressants, opiates such as oxycodone, psychedelics like 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 considered abnormal only if it is sustained for over five contractions. It is usually initiated by reflex actions. It can be tested by flexing the ankle or the wrist, or by pressing the patella, or kneecap, towards the toes.
Clonus often occurs on only one side of the body. It is associated with nerve damage to the area of the spinal cord above the place where it is initiated. Because it only occurs during muscular 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 does not suggest much on its own, as it can have a huge number of causes. It is important to consider clonus along with any other symptoms the patient is experiencing to determine the site and cause of any possible neurological damage or disease.