What causes severe myoclonic jerks?

Numerous causes. Brief list: normal jerks on falling asleep, but problem if it awakens (plms). Associated with epilepsy (juvenile myoclonic), spinal cord inflammation (aid's), following hypoxia (lance-adams), organ failure (kidney, liver), metabolic and medication reactions. Meds that seem to work best here include depakote, keppra, (levetiracetam) and maybe lamictal. Other drugs used in epilepsy, may make worse.