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Although fasciculations, cramps, and restless legs are usually harmless, they may at times suggest serious medical illness. Here is what you need to know about recognizing and managing a selection of such movement disorders.
HUMANS ARE COMMONLY bothered by an assortment of involuntary focal motor events. These include fasciculations and cramps, which probably have very similar physiologies; muscle jerks including myoclonic jerks and tics, which can arise from any level of the central nervous system-even from peripheral nerves-and can be caused by a wide variety of diseases; and restless legs syndrome (RLS), which is usually associated with a common disorder of sleep and is often familial.
FASCICULATIONS
Fasciculations are visible contractions of muscle fibres innervated by the same motor unit that occur spontaneously in resting muscle. They do not generate sufficient force to move a joint and are usually felt. Fasciculations are most often triggered by depolarization of the nerve terminal.1
Fasciculations can be provoked by anticholinesterase drugs that generate action potential in motor nerve terminals that travel antidromicallyopposite to the normal direction of conduction-up a branch of the motor neuron, spreading to activate the motor unit. The fasciculations seen in different pathologic conditions may have a similar origin, although it is uncertain what triggers them. Thus, in amyotrophic lateral sclerosis (ALS), small doses of curare will eliminate fasciculations and anticholinesterase medications will markedly increase them. Spinal anesthesia and proximal nerve blockade do not reduce the frequency or abundance of contractions.
In normal individuals, fasciculations occur, usually after exercise, and especially in the calves,2 where they are seen and felt. Because they raise the specter of ALS they are worrisome for people who have little medical knowledge. Most fasciculations recur irregularly, at very low frequencies. When a single motor neuron fires repetitively at faster frequencies causing more prolonged contraction of the motor unit, this is called myokymia.
Visible contractions of multiple individual motor units can be seen in normal individuals during gentle activation if there is very little subcutaneous fat and if the motor units are large. Such benign "contraction fasciculations" are most often seen in the proximal muscles of elderly patients; if the patient relaxes they disappear. Tongue fasciculations are hard to distinguish from involuntary postural movements if the tongue is protruded; such...