The phenomenon of myoclonus was discussed in Chap. 4, where the relationship to seizures was emphasized. Characterized by a brusque, brief, muscular contraction, some myoclonic jerks may be so small as to involve only one muscle or part of a muscle; others are so large as to displace a limb on one or both sides of the body or the entire trunk musculature. Many are brief, lasting 50 to 100 ms; they may occur intermittently and unpredictably or present as a single jerk or a brief salvo.
As mentioned earlier, a series of several small, rhythmic myoclonic jerks may appear with varying frequency as part of atypical absence seizures, and as isolated events in patients with generalized clonic-tonic-clonic or tonic-clonic seizures. As a rule, seizure-associated myoclonus, when occurring in isolation, is relatively common, signifies nothing more than the seizure disorder, and usually responds well to antiepileptic medication. In contrast, there are diseases in which myoclonus is progressive in severity or very frequent. These disorders have their onset in childhood and raise the suspicion of entities such as the myoclonus-opsoclonus-ataxia syndrome, lithium or other drug toxicity or, if lasting a few weeks, subacute sclerosing panencephalitis. Chronic progressive polymyoclonus with dementia characterizes the group of juvenile lipidosis (see Chap. 36), Lafora-type familial myoclonic epilepsy, certain mitochondrial disorders, and other chronic familial degenerative diseases of undefined type (paramyoclonus multiplex of Friedreich) as noted further on in Table 15-3. The large number of adult diseases causative of myoclonus and seizure disorders are discussed in later chapters. Myoclonus as a phenomenon is further described in Chap. 4.
This is the most common form of idiopathic generalized epilepsy in older children and young adults. It begins in adolescence, typically around age 15 years, with a range that essentially spans all of the teenage years. The patient comes to attention because of a generalized tonic-clonic seizure, often upon awakening or because of myoclonic jerks in the morning that involve the entire body; sometimes absence seizures are prominent. The family reports that the patient has occasional myoclonic jerks of the arm and upper trunk that is brought out with fatigue, early stages of sleep, or alcohol ingestion. A few patients in our experience have had only the myoclonic phenomena and rare absence or tonic-clonic seizures that persisted unnoticed for years. The EEG shows characteristic bursts of 4to 6-Hz irregular polyspike activity. A linkage has been established to several loci, mainly of ion channels and of GABA-related receptors.
The disorder does not impair intelligence and tends not to be progressive, for which reason it has been called “benign,” but a proclivity to infrequent seizures usually continues throughout life. A report by Baykan and colleagues has indicated that, over an average of two decades, the majority of patients have long seizure-free periods and a large reduction in myoclonic seizures by the fourth decade but only one-fifth become virtually seizure free. Valproic acid in particular and some other antiepileptic drugs are highly effective in eliminating the seizures and myoclonus but they should be continued indefinitely as discontinuation of medication is associated with a high rate of relapse. Owing to the potential teratogenicity of valproate, women of childbearing age are often given levitiracetam or lamotrigine, acknowledging that they may not be as effective as the first choice of drug. It has been observed that carbamazepine and phenytoin may exaggerate the seizures.