The history of myoclonus has been described by Marsden et al.²⁵ and Hallett.¹⁷ Friedreich first defined myoclonus as a discrete entity in a case report published in 1881 of a patient with essential myoclonus. He wanted to separate the involuntary movement that he saw from epileptic clonus, a single jerk in patients with epilepsy, and chorea. For the next 10 to 20 years, many other types of involuntary movements, such as tic and myokymia, were also called myoclonus, but in 1903 Lundborg²³ proposed a classification of myoclonus that cleared up much of the confusion. Lundborg classified myoclonus into three groups: Symptomatic myoclonus, essential myoclonus, and familial myoclonic epilepsy.

It is important to recognize that certain types of myoclonus are essentially identical to epilepsy. Single jerks in patients with epilepsy have been recognized since ancient times. In the phrase of Muskens,³⁰ myoclonus can be a “fragment of epilepsy.” One of the tasks for the clinician is to determine what is epilepsy and what is not.

Myoclonus is characterized by quick muscle jerks, either irregular or rhythmic.¹⁹ Myoclonic movements are always simple in nature, and this is often a critical feature separating myoclonus from other types of involuntary movements. Myoclonus can be focal, involving only a few adjacent muscles; generalized, involving many or most of the muscles in the body; or multifocal, involving many muscles but in different jerks. Myoclonus can be spontaneous, can be activated or accentuated by voluntary movement (action myoclonus), and can be activated or accentuated by sensory stimulation (reflex myoclonus).

In differentiating myoclonus from other movement disorders, in addition to the simplicity, the principal features that favor myoclonus are the quickness of the movement and the absence of ability for voluntary suppression. Some simple tics look identical to myoclonus and cannot be visually distinguished. A point in favor of tic is the ability to suppress the movements voluntarily, with a frequent concomitant rise in psychic tension dispelled when the movements resume. Some movements of chorea are quick, but slower movements and sustained postures are also present. The major differential diagnosis for rhythmic myoclonus is tremor and the distinction here is often just convention.

Some disorders of the peripheral nervous system can be confused with myoclonus. Electrodiagnosis can help make the diagnosis since these disorders all show characteristic physiologic findings. Fasciculation is the spontaneous firing of a single motor unit. Myokymia typically looks like an irregular oscillation of a muscle, but it can have the appearance of small jerks. Hemifacial spasm is characterized by jerks of the facial muscles and occasional tonic spasms.

A brief, paroxysmal pause of tonic muscular activity may also give rise to a jerk in the affected body part that is often visually indistinguishable from a movement produced by a burst of electromyographic (EMG) activity. This is called negative myoclonus, and the most commonly encountered form is asterixis. Negative myoclonus, like positive myoclonus, can also arise as an epileptic event.^36,40

There are many types of myoclonus, and there are no common etiologic, physiologic, or therapeutic features. For this reason, recognizing that an involuntary movement is myoclonic in nature is only the beginning of the investigation. In those patients with epileptic seizures, it would be reasonable to suspect that the myoclonus would be related.

There have been many schemes proposed for classifying the large number of myoclonic disorders, and there are at least two useful approaches, etiologic and physiologic, that have usually been discussed separately.¹⁹ From the point of view of differential diagnosis, the physiologic classification is most relevant, and it can help guide symptomatic treatment. The first consideration of therapy, however, should take the etiologic classification into account since the cause should be dealt with first, if possible. Here, the classifications will be combined using the physiologic classification as the primary index and the etiologic classification as a secondary index.

The starting point for a physiologic classification is to decide whether the myoclonus is a fragment of epilepsy. On this basis, the myoclonus is said to be epileptic or nonepileptic.^16,43 There are a number of subtypes in each category.

A significant feature in favor of epileptic myoclonus is if the patient has epileptic seizures. This would be more definitive, of course, if the myoclonus was clearly a fragment of the seizure including a part of the aura. The association of the myoclonus with paroxysmal activity in the electroencephalogram (EEG) is also indicative, and more will be said about the electrophysiologic evaluation in the next section.

Three types of epileptic myoclonus are now recognized: Cortical myoclonus, reticular myoclonus, and primary generalized epileptic myoclonus.^19,37,43 Cortical myoclonus is a fragment of focal or partial epilepsy, and can be subclassified as spontaneous cortical myoclonus, cortical reflex myoclonus, and epilepsia partialis continua. Each myoclonic jerk involves only a few adjacent muscles, but larger jerks with more muscles involved can be seen. The disorder is commonly multifocal and accentuated by action and sensory stimulation.
Reticular reflex myoclonus is a fragment of a type of generalized epilepsy. These jerks are usually generalized with predominance, which is proximal more than distal and flexor more than extensor. Voluntary action and sensory stimulation increase the jerking. Primary generalized epileptic myoclonus is a fragment of primary generalized epilepsy. The most common clinical manifestation is small, focal jerks, often involving only the fingers; thus, the myoclonus is sometimes called minipolymyoclonus. The term minipolymyoclonus was originally coined to refer to small jerks seen in patients with motor neuron disease. Minipolymyoclonus of central origin and minipolymyoclonus of peripheral origin have a similar clinical appearance and are probably most easily separated by the company they keep: Epilepsy and muscle denervation, respectively. A second clinical presentation of primary generalized epileptic myoclonus is generalized, synchronized whole body jerks, not unlike those seen with reticular reflex myoclonus. Electrodiagnosis can help define the type of epileptic myoclonus (see below).

Negative myoclonus can be isolated, but it usually occurs together with positive myoclonus.^15,36,40,43 Clinically, the appearance is called asterixis, and there can be large movements, small multifocal movements, and such frequent movements that the appearance is of an irregular tremor or tremulousness. Negative myoclonus can have a similar physiology to cortical myoclonus with electrophysiologic correlates and production by sensory stimulation. There may be associated seizures.

Myoclonus resulting from a defined pathologic process, the etiologic category of “symptomatic myoclonus,” is usually epileptic in type. The principal conditions follow, along with a brief consideration of their treatment.