Myoclonus is a brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. Brief twitches are perfectly normal. The myoclonic twitches are usually caused by sudden muscle contractions; they also can result from brief lapses of contraction. Contractions are called positive myoclonus; relaxations are called negative myoclonus. The most common time for people to encounter them is while falling asleep (hypnic jerk), but myoclonic jerks are also a sign of a number of neurological disorders. Hiccups are also a kind of myoclonic jerk specifically affecting the diaphragm. When a spasm is caused by another person it is known as a 'provoked spasm'. Shuddering attacks with babies also fall in this category. Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute.
Contributions by The Anome, 1earthworm, and MrADHD.