Myoclonus (Pediatric) Overview
Myoclonus is "sudden, brief, jerky, shock-like, involuntary movements" (Fahn et al. 1986). The movements are quite rapid and may be triggered by attempts at voluntary movement, sensory stimulation, or startle. Myoclonus may cause rhythmic jerks, in which case, it is termed a "myoclonic tremor." Myoclonus is categorized based upon the likely source of movement. Such sources include cortical or subcortical areas or the spinal cord.
Cortical myoclonus is thought to be due to a lack of inhibition in the sensory or motor cortex. Subcortical myoclonus is often due to abnormalities in the brainstem; spinal myoclonus is presumed to be due to abnormalities in spinal inhibitory circuits. Myoclonus may be severely disabling, particularly when it is triggered by movement. In some cases, it may also be very mild. The most common example of a mild myoclonus is sleep myoclonus. In this form of myoclonus, children or adults have occasional brief jerks of an arm or a leg; these "jerks" occur while the individual is falling asleep. Negative myoclonus is a sudden involuntary relaxation of a muscle, rather than a contraction; this is thought to be due to mechanisms similar to those of sleep myoclonus. Cortical reflex myoclonus is triggered by attempts to obtain a knee jerk or other tendon reflex. Epilepsia partialis continua is a type of focal epilepsy that causes myoclonic tremor. Myoclonus is often associated with epilepsy; there is a particular class of degenerative disorders called "progressive myoclonus epilepsies (PME)" in which the association of myoclonus and epilepsy is common.