A myoclonic jerk is the brief, involuntary twitching of a muscle or group of muscles. It may be caused either by a sudden muscle contraction, or a sudden lapse of contraction. This happens when a person is on the verge of falling asleep, and suddenly have a sensation or feeling that they are free falling through the air. Contractions are called positive myoclonus; relaxations are called negative myoclonus. When falling asleep, it is common for people to experience a type of myoclonic jerk known as a hypnic jerk. Hiccups are also a kind of myoclonic jerk specifically affecting the diaphragm.
Myoclonus is a symptom of a disease in which a muscle or group of muscles suddenly starts twitching or jerking. Some common examples of myoclonus are:
- a sudden violent jerking or twitching motion when one is startled; or,
- the twitching of an arm or leg as you begin to fall asleep.
Myoclonic jerks may be perfectly normal, or they may be a sign of a number of neurological disorders, such as multiple sclerosis, Parkinson's disease, Alzheimer's, subacute sclerosing panencephalitis, Creutzfeldt-Jakob disease, MERRF syndrome and some forms of epilepsy. They may originate from lesions of the cortex, subcortex, or spinal cord. Myoclonic seizures occur in epilepsy and cause abnormal movements on both sides of the body at the same time.
Severe cases of myoclonus can distort movement and limit a person's ability to perform normal functions such as speech, sleep, and walking.
Myoclonus may develop in response to infection, head or spinal cord injury, stroke, brain tumours, renal or hepatic failure, lipid storage disease, or poisoning. Benign myoclonic movements are commonly seen during induction of general anesthesia with intravenous medications such as etomidate and propofol. Hypoxia may result in posthypoxic myoclonus. The specific mechanisms underlying myoclonus are not yet fully understood.
Treatment focus on medications that may help reduce symptoms. The drug of first choice is clonazepam. Dosages or clonazepam are increased gradually until the patient improves or side-effects become harmful. Drowsiness and loss of coordination are common side-effects. Barbiturates, phenytoin, primidone, and sodium valproate are also used to treat myoclonus.
Alcohol taken before sleep seems to help this condition, but the long-term use of alcohol is not recommended and is not a cure.