The syndrome of quick, arrhythmic, involuntary single or repetitive twitches of a muscle or group of muscles was described in Chap. 4, where it was pointed out that the condition has many causes. Chapter 36 discusses those caused by hereditary metabolic diseases. Familial forms are known, one of which, associated with cerebellar ataxia, was discussed earlier (dyssynergia cerebellaris myoclonica of Ramsay Hunt). But there is another disease, known as hereditary essential benign myoclonus, that occurs in relatively pure form unaccompanied by ataxia (termed essential, or familial, myoclonus; see Chap. 4). In this condition, it is difficult to evaluate coordination because willed movement is interrupted by myoclonus that may be mistaken for intention tremor. Only by slowing the voluntary movement can the myoclonus be reduced or eliminated. This myoclonic disease is inherited as an autosomal dominant trait. It becomes manifest early in life; once established, it persists with little or no change in severity throughout life, often with rather little disability. It can, by its natural course, be differentiated from some of the hereditary metabolic diseases such as the Unverricht and Lafora types of myoclonic epilepsy, the lipidoses, tuberous sclerosis, and myoclonic disorders that follow certain viral infections and anoxic encephalopathy. Of interest is the response of this form of movement disorder to certain pharmacologic agents, notably clonazepam, valproic acid, and 5-hydroxytryptophan, the amino acid precursor of serotonin, particularly when these agents are used in combination (postanoxic myoclonus responds to the same medications).
Another form of nonprogressive myoclonus, dominantly inherited, is associated with dystonia, which is due to a mutation in a sarcoglycan gene, SGCE.
The main clinical distinctions are from juvenile myoclonic epilepsy (see Chap. 15), drug-induced myoclonus, particularly lithium and opiates; renal failure and other acquired metabolic disorders; asterixis; and from the startle responses and some of the diseases that have this sign as their main characteristic (see Chap. 4). Creutzfeldt-Jakob subacute spongiform encephalopathy may cause difficulty in diagnosis initially but the course of illness clarifies the situation rapidly. Myoclonus is also one component of the complex movement disorder in corticobasal-ganglionic degeneration that was described in an earlier section.
The place in nosology of the hyperekplexias, and in particular their relation to the myoclonias, is uncertain.
This general term designates a group of progressive degenerative disorders of motor neurons in the spinal cord, brainstem, and motor cortex, manifest clinically by muscular weakness, atrophy, and corticospinal tract signs in varying combinations. It is for the most part a disease of middle life and progresses to death in a matter of 2 to 5 years or longer in exceptional cases.