Myoclonic jerks can be focal, segmental, or generalized.
When generalized, they can be associated with epilepsy or with a host of static and progressive encephalopathies. To narrow the differential diagnosis at the examination, it is important to look for signs of encephalopathy, dementia, retinal pathology, hearing loss, myopathy, neuropathy, and ataxia. The differential diagnosis of myoclonus consists of determining whether the jerks are caused by an ongoing medical illness, by exposure to a drug or toxin, or by a primary disorder of the central nervous system. An important element of the differential diagnosis is whether the myoclonus is epileptic or nonepileptic. The age at onset is also extremely important because the more common progressive forms of epilepsy that result from degenerative processes tend to occur in the pediatric age group.