Myoclonus is a brief, involuntary of a
muscle or a group of muscles. It describes a
medical sign and, generally, is not a
diagnosis of a disease. These
myoclonic twitches,
jerks, or
seizures are usually caused by sudden muscle contractions (
positive myoclonus) or brief lapses of contraction (
negative myoclonus). The most common circumstance under which they occur is while falling asleep (
hypnic jerk). Myoclonic jerks occur in healthy persons and are experienced occasionally by everyone. However, when they appear with more persistence and become more widespread they can be a sign of various
neurological disorders.
Hiccups are 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 in babies fall in this category.