Myoclonus may be a direct reflection of seizures but is also a separate nonepileptic manifestation in several neurodegenerative and storage diseases, of which seizures are an important component. For example, a relatively benign idiopathic condition, juvenile myoclonic epilepsy, is accompanied by myoclonic jerks when the patient is tired or has ingested alcohol. A more serious type of myoclonic epilepsy, identified with the names Unverricht and Lundborg, in the beginning is marked by polymyoclonus as an isolated phenomenon, but later is associated with dementia and other signs of progressive neurologic disease. An outstanding feature of the latter is a remarkable sensitivity of the myoclonus to stimuli of all sorts. If a limb is passively or actively displaced, the resulting myoclonic jerk may lead, through a series of progressively larger and more or less synchronous jerks, to a generalized convulsive seizure. In late childhood this type of stimulus-sensitive myoclonus is usually a manifestation of the juvenile form of lipid storage disease, which, in addition to myoclonus, is characterized by seizures, retinal degeneration, dementia, rigidity, pseudobulbar paralysis, and, in the late stages, by spastic quadriplegia.
Myoclonus may be associated with atypical petit mal and akinetic seizures in the Lennox-Gastaut syndrome (absence or petit mal variants); the patient often falls during the brief lapse of postural mechanisms that follows a single myoclonic contraction. Similarly, in the West syndrome of infantile spasms, the arms and trunk are suddenly flexed or extended in a single massive myoclonic jerk (“jackknife” or “salaam” seizures); mental regression occurs in 80 to 90 percent of these cases, even when the seizures are successfully treated. These types of special “myoclonic epilepsies” are discussed further below and in Chap. 15 in relation to epilepsy.
Another form of stimulus-sensitive (reflex) myoclonus, inherited as an autosomal recessive trait, begins in late childhood or adolescence and is associated with neuronal inclusions (Lafora bodies thus Lafora-body disease) in the cerebral and cerebellar cortex and in brainstem nuclei. In yet another familial type (described under the title of Baltic myoclonus by Eldridge and associates), autopsy has disclosed a loss of Purkinje cells but no inclusion bodies. Unlike Lafora-body disease, the Baltic variety of myoclonic epilepsy has a favorable prognosis, particularly if the seizures are treated with valproic acid.
Under the title of cherry-red-spot myoclonus syndrome, Rapin and associates have drawn attention to a familial (autosomal recessive) form of diffuse, incapacitating intention myoclonus associated with visual loss and ataxia. This disorder develops insidiously in adolescence. The earliest sign is a cherry-red spot in the macula that may fade in the chronic stages of the illness. The intellect is relatively unimpaired. A similar clinical syndrome of myoclonic epilepsy is seen in a variant form of neuroaxonal dystrophy and in the late childhood–early adult neuronopathic form of Gaucher disease, in which it is associated with supranuclear gaze palsies and cerebellar ataxia (see Chap. 36).
Diffuse Myoclonus with Acquired Neurologic Disease