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Twitches are brief, sudden, and fine contractions caused by electrical impulses that stimulate muscle fiber activity. Some twitches are under voluntary control, but most are involuntary. Twitches have been called muscle jerks, spasms, and fasciculations, and these terms are often used interchangeably.

Fibrillation is a contraction of a single muscle fiber. It occurs when the muscle fiber loses electrical stimulation from the nervous system. A fibrillation is not visible through the skin. This spontaneous activity often goes unnoticed and it is usually harmless. In rare instances it can be associated with inflammatory diseases of the muscle, such as polymyositis.

Fasciculation, a contraction of a group of muscle fibers that makes up a motor unit, is caused by nerve irritability. Fasciculation is seen in many older adults and most often involves the muscles of the thighs, calves, or hands. It can be chronic, lasting for days, weeks, or even years without any progression or evidence of underlying disease. When fasciculation is associated with muscle weakness or wasting, the likelihood of an underlying problem with the neuromuscular systemsuch as amyotrophic lateral sclerosis increases.

Myoclonus is a rapid, brief contraction of either a proximal or a distal muscle in a nonrhythmic manner that results in movement of a body part. The most common type of myoclonus occurs in people as they are drifting off to sleep. The presence of this type of myoclonus does not imply disease. Myoclonus can be inherited in families or occur spontaneously in otherwise healthy subjects. More generalized myoclonus can occur in patients with diseases such as epilepsy or certain rare neurodegenerative disorders such as Creutzfeldt-Jakob disease. Myoclonic jerking can accompany any severe metabolic disturbance due to respiratory disease, chronic kidney failure, liver failure, or electrolyte disturbances. It can be seen in alcohol and drug withdrawal, or follow severe brain damage due to lack of oxygen to the brain. Myoclonus can be very resistant to treatment, although it may respond to treatment with certain anticonvulsant drugs like valproic acid or to a tranquilizer like clonazepam.

Restless leg syndrome is characterized by numbness and tingling of the calves. This sensation is difficult to describe, but it often causes the irresistible urge to move the legs. The symptom occurs most frequently in the evening, and it can cause significant sleep disturbance when severe. It seems to be aggravated by sleep deprivation or stress. Restless leg syndrome overlaps with a disorder known as periodic leg movements or nocturnal myoclonus. Periodic leg movements can involve the whole leg or smaller portions. The person is usually unaware of these movements, and often sleeps through them. There is an association between both disorders and increasing age, although the exact mechanism is not well understood. Medical treatments that have been tried include medications used for Parkinsons disease, opiates, and tranquilizers.

Philip E. Lee B. Lynn Beattie

See also Deconditioning; Sarcopenia.


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