Myoclonus

Myoclonus is a sudden, brief, shocklike contraction of a muscle or group of muscles. Diagnosis is by history, physical examination, and selective testing. Treatment includes correction of reversible causes and sometimes oral drugs (eg, clonazepam, valproate).

Geriatric Essentials

• In the elderly, myoclonus is usually a symptom of a dementing disorder, a metabolic abnormality, toxic encephalopathy, or degeneration of the basal ganglia.
• Startle myoclonus is a unique form of myoclonus; symptoms occur only when patients are suddenly startled. It can be a sign of Creutzfeldt-Jakob disease.

Common causes include dementia, metabolic abnormalities, encephalopathies
(toxic, physical, hypoxic, or viral), degeneration of the basal ganglia, and drugs (see Table 46-6).

Myoclonus is caused by muscular contractions (positive myoclonus) or inhibition of reciprocal muscles (negative myoclonus) initiated by hyperexcitable neurons in the spinal cord, medial reticular formation, or cerebral cortex.

Symptoms and Signs

Myoclonus can vary in amplitude, frequency, and distribution. Muscle jerks may be induced by a stimulus: sudden noise, movement, light, or visual threat.

Myoclonus may be an early feature of Creutzfeldt-Jakob disease. In this disease, myoclonus typically occurs when patients are startled (called startle myoclonus); it correlates with a periodic synchronous discharge on EEG. Myoclonus can also occur in late stages of Alzheimer's disease. In this disease, myoclonus is briefer and more focal than that in Creutzfeldt-Jakob disease; it can occur at rest, during voluntary activity, or after a stimulus. In patients with dementia, myoclonus more often results from Alzheimer's disease than from Creutzfeldt-Jakob disease because Alzheimer's disease is much more common.

Myoclonus after a hypoxic episode is usually precipitated by voluntary movement. Metabolic abnormalities (see Table 46-6) may be complicated by multifocal, asymmetric, stimulus-induced myoclonus. Facial or proximal limb muscles are predominantly involved. If the metabolic abnormality persists, generalized myoclonic jerks and, ultimately, seizures may occur.

Drugs, such as long-term levodopa treatment in some patients with parkinsonism, can induce myoclonus characterized by single, abrupt, symmetric jerks of the arms and legs, usually when patients are sleeping, drowsy, or at rest. A reduction in dose can alleviate the frequency and severity of the myoclonus. Toxic doses of some drugs (eg, antihistamines, carbamazepine, lithium, monoamine oxidase inhibitors, phenytoin, tricyclic antidepressants, valproate) can also induce myoclonus. High-dose penicillin or cephalosporin infusion can induce nonrhythmic, asymmetric, and stimulus-induced myoclonus.

Nocturnal myoclonus is a misnomer. The term refers to periodic limb movements of sleep; myoclonus is not involved.

Diagnosis

Diagnosis is by history, physical examination, and testing for any clinically suspected causes.

Treatment

Whenever possible, the cause is treated. Several drugs (eg, clonazepam 0.25 mg po once/day, gradually increased as needed up to 1 mg po tid; valproate 125 mg po once/day, gradually increased as needed up to 500 mg po tid) may reduce symptoms.

This topic was last updated June 2006.