Causes Of Myoclonic Jerks

MYOCLONUS

Myoclonus refers to brief lightning-like muscle jerks due to brief electromyographic bursts of 10 to 50 ms, rarely more than 100 ms in duration. The jerks are usually due to positive muscle contractions but can also be due to sudden brief lapses of contraction (i.e., so-called negative myoclonus) such as is seen in asterixis. Asterixis is a tremorlike phenomenon of the extended wrists due to brief lapses of muscle contraction. It is usually encountered in the metabolic encephalopathies that accompany severe hepatic, renal, and pulmonary disorders. Agonists and antagonists usually fire (or are inhibited in negative myoclonus) synchronously. Clinically, there is a wide expression of myoclonus. The jerks may occur singly or repetitively. They may be focal, segmental, or generalized. The amplitude ranges from mild contractions that do not move a joint to gross contractions that move limbs, the head, or the trunk. Myoclonic jerks range in frequency from rare isolated events to many events each minute; they may occur at rest, with action, or with intention movements. Commonly, myoclonic jerks are stimulus sensitive (reflex myoclonus); they can be induced by sudden noise, movement, light, visual threat, or pinprick. Most often, myoclonic jerks occur irregularly and unpredictably. But some occur in bursts of oscillations, and some are very rhythmic, as in palatal myoclonus. They resemble tremor in this last situation. Myoclonus arising from the cerebral cortex (cortical myoclonus) is usually focal and reflex-induced. Epilepsia partialis continua can be considered within the cortical myoclonus family. The cortical origin can be ascertained by enlarged somatosensory evoked potentials or by spikes in the electroencephalogram associated with electromyographic correlated jerks that are revealed by a back-averaging technique. Rasmussen encephalitis is a disorder of childhood and adolescence in which there is a unilateral focal seizure disorder, including epilepsia partialis continua, and a progressive hemiplegia due to focal cortical inflammation and destruction. Myoclonus originating from the brainstem can be either generalized (reticular myoclonus) or segmental (e.g., oculo-palatal-pharyngeal myoclonus). Palatal myoclonus is rhythmical (approximately 2 Hz) and can be primary or secondary. The latter is more common and is the result of a lesion within the Guillain-Mollaret triangle encompassing the dentate, red, and inferior olivary nuclei.

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