“The term and concept of myoclonus have been a longstanding source of confusion and debate. Myoclonus should be used in its strictly etymological sense, referring to any brief muscular contraction, as would have been wished by Muskens, the first neurologist to do detailed clinical and experimental studies of it.”⁴²

The development of the concept of myoclonus and its application to epileptic seizures is closely connected with the history of description and conceptualization of progressive myoclonus epilepsy (PME) and other myoclonic epilepsies. Dubini (1846)³⁵ described a cohort of patients with possibly different conditions who had involuntary jerky movements that he named electric chorea. Delasiauve (1854), Reynolds (1861), Féré (1890), Binswanger (1899), and Gowers (1902) described sudden starts or jerks as expressions of motor prodromes, auras, or abortive seizures (petit mal moteur) in their monographs on epilepsy. They did not, however, differentiate them semiologically or nosologically. Credit for the first exact and comparable description of these phenomena belongs to Herpin (1867),⁵⁹ who described them as: “this variety of [seizure] prelude is a jerk [secousse] which runs through the entire body like an electric current.” His twenty-fourth observation was the first description of juvenile myoclonic epilepsy (JME), in a 14-year-old child, the son of a doctor. His jerks were called commotions or impulsions.

“At the beginning of the seizure, the movements or jerks were limited to the upper part of his body; they later became generalized. If the boy were to stand or walk, he might fall, although this is rare. He stands up again immediately [after the seizure]. He throws or drops what he is holding, particularly if he is holding it with his right hand. He says that he cannot see at the moment of the movement, but that he recovers immediately afterwards. His mother called these events (accidents) shakes (tremblement) and the father jerks (secousses).”

In 1881, Friedreich⁴⁰ described a case, the diagnosis of which has remained unclear until today, and named it paramyoclonus multiplex. Many publications followed his observation, but they were comparable only in that they were dealing with involuntary jerks. From the “chaos of motor neuroses and from the cloudy pool of many publications,” Unverricht¹¹⁶ crystallized a unique disease entity from his 1895 study of members of two families and named it familiäre myoclonie. This disease started between the ages of 7 and 15 years, and consisted of a combination of nocturnal convulsive seizures and “lightning-like, arrhythmic, isolated jerks of individual, functionally unrelated muscles of both sides of the body. The jerks occurred less frequently when strained, but more frequently during psychic excitement, and stopped completely during sleep.” Unverricht seemed to have not observed any relationship between the phenomena he named myoclonie and the preparoxysmal or interval jerks of epileptic patients that, at that time, were still called secousses.

In 1899, both Dide²⁷ and Rabot,⁹⁰ at almost the same time, introduced the concept of myoclonie to the vocabulary of epilepsy—Dide from a semiologic viewpoint (“La myoclonie dans l’épilepsie”) and Rabot from a nosologic view (“De la myoclonie épileptique”). Rabot confirmed Herpin’s observation and emphasized that the “jerks” might precede major convulsive seizures for many years, usually occur in the morning after awakening, are prone to recur in series, and occur more frequently days before major convulsive seizures and disappear thereafter for a while, as if switched off.

Some years later, Clark and Prout¹⁹ proposed the concept of myoclonus epilepsy for their cases of Unverricht myoclonie, which they had described as paramyoclonus multiplex associated with epilepsy in 1900. When, in 1903, Lundborg⁷⁶ sought an appropriate term for the disease he observed in 18 members of a Swedish farm family, he noticed the same nomenclature for two different diseases. He then proposed to describe one, which was “identical with the Unverricht’s myoclonie,” as progressive familial myoclonus epilepsy—from which later the term Unverricht-Lundborg myoclonus epilepsy developed—and the other, which was “standing near the essential epilepsy,” as intermittent sporadic myoclonus epilepsy (Rabot’s type). This was described by Lecasble in 1958 as “épilepsie generaliseé avec des myoclonies intermittentes et sporadiques” and by Castells and Mendilaharsu¹⁸ in the same year as “la epilepsia mióelonia bilaterally cosciente” to differentiate it from the epilepsy beginning with partial myoclonus. Janz^66,67 as well as Lennox⁷³ tried to replace the vague but accepted concept of myoclonus with the descriptive terms—at least for the jerks of JME—impulsive-petit mal⁶⁷ (from the Latin impulsio, “jerk”) and jerk epilepsy.⁷³ During the 1970s, a syndromic approach to the problems of myoclonic epilepsies occurred, with a new importance given to electroencephalographic (EEG) findings and to elements of etiology and prognosis. The Marseille school, among others, established several well-accepted syndromic entities using the adjective myoclonic, such as benign and severe myoclonic epilepsy in infancy, and epilepsy with myoclonic absences.⁴⁴

Recently, myoclonus has been the subject of intensive research using advanced technologies. Electrophysiologic studies, especially, have brought new insights into the pathophysiomechanism of myoclonus.¹⁰² The recent advances in molecular genetics have also helped achieve an understanding of the different conditions that cause myoclonus.

Myoclonus consists of involuntary, quick movements distinctive for their abrupt, lightning-like character, occurring in a mild or vehement manner, spontaneously or in stimulus-sensitive conditions, affecting the entire body or in regional or localized distal or proximal parts of the body. It may be diffuse, predominating symmetrically in the axial muscles and the upper limb (massive myoclonus), or segmental, affecting randomly the different parts of the body, although the most involved muscles are those of the extremities, face, and trunk (erratic myoclonus); if the legs are involved, the person may be thrown to the ground. Myoclonus can be asymmetrical or even focal, limited to a restricted part of one limb or the face (epilepsia partialis continua).³⁶ It may occur at random over time, or at more or less fixed interval, and in sporadic or rhythmic fashion. Most myoclonia are caused by abrupt muscle contraction (positive myoclonus); rarely, a brief interruption of ongoing activity may be observed (negative myoclonus, previously named asterixis).¹ No distinguishable, even momentary, loss of consciousness occurs with single jerks, but if jerks occur in rapid succession or in the form of myoclonic status epilepticus, there may be a blurring of awareness.

Phenomenologically, epileptic myoclonus cannot be distinguished from the brisk, nonepileptic movements associated with physiologic or pathologic conditions. The classification of seizures as an epileptic manifestation requires knowledge of the clinical context and/or electrophysiologic confirmation, although physiologic correlates are not always demonstrable. Hallett⁵⁴ proposed to define epileptic myoclonus as an elementary electroclinical manifestation of epilepsy involving descending neurons, whose spatial or temporal amplification can trigger overt epileptic activity.

In this chapter, we use the term myoclonic seizure as synonymous with epileptic myoclonus. The term myoclonus was coined by Friedreich.⁵⁵ Myos means muscles, clonus means tumult or a quick movement, and (myo)clonia is the plural. Because a myoclonic movement is a jerk, the term myoclonic jerk is redundant.⁴²

The term epileptic myoclonus implies a central nervous system origin, presumably accompanied by neuronal depolarization shifts with subsequent hyperpolarizing potentials. To clarify the underlying pathophysiologic mechanisms of myoclonus, electrophysiologic studies are useful (i.e., electromyogram [EMG], somatosensory evoked potential [SEP], C-reflex, and jerk-locked back averaging [JLA]). Hallett et al.^54,57 divided epileptic myoclonus into three categories: cortical reflex myoclonus, primary generalized epileptic myoclonus, and reticular reflex myoclonus. Guerrini et al.⁴⁸ also divided this disorder into cortical, thalamocortical, and reticular form.

From an EMG point of view, myoclonus appears as short bursts of synchronized activity, which may involve agonist and antagonist muscles at the same time. The duration of EMG burst during epileptic myoclonus ranges between 10 and 100 msec, usually less than 50 msec (Fig. 1), and that of the EMG silent period of epileptic negative myoclonus from 50 to 400 msec (Fig. 2).⁴⁴ By recording the EMG, the nature of myoclonus can be described (i.e., positive or negative myoclonus, duration of EMG burst, rhythmicity and periodicity, synchrony of agonist and antagonist muscles, distribution of myoclonus, spreading pattern of myoclonus, and so on).

Patients may refer to their myoclonus as a jerk, shock, jump, start, involuntary movement, or even stiffness and numbness. Patients may describe being thrown down, as though pushed by someone. Usually, patients do not notice any warnings, but sometimes smaller seizures serve as precursors to more severe ones. Myoclonus may be predominant in axial muscle groups or in the distal ones. Injury caused by falls or by striking a nearby object with a hand or limb may occur during an attack. Myoclonus occurs spontaneously, sometimes predominantly on awakening, and may be precipitated by various internal or external, simple or complex stimuli. Myoclonus may occur in isolation or in patients experiencing generalized tonic–clonic seizures (GTCs) or other forms of epileptic attacks.

In the following section, we describe the clinical and electrographic features of myoclonus in generalized forms. Because the clinical characteristics and EEG expressions of myoclonus are distinct in accordance with the epileptic syndromes, the descriptions accompany the syndromic classification. Detailed descriptions of the individual syndromes can be found elsewhere.⁹²

Harper⁵⁸ called attention to the fact that myoclonic seizures occur in childhood, just as in adolescence or adulthood; he described true myoclonic epilepsy in childhood in the age group of 3 to 7 years. Aicardi^3,4 further argued that approximately two thirds of the myoclonic epilepsies occur during the first 5 years of life, and that the true myoclonic epilepsies of childhood—which should be delineated from epilepsies with other types of brief axial motor seizures, such as epileptic spasms and tonic seizures of the LGS—might be subdivided into several forms. International classification of epilepsies and epileptic syndromes²¹ covers several distinctive syndromes occurring in infancy and early childhood in which myoclonus is a main seizure type.