Historical note
The term myoclonus covers a group of neurophysiologically diverse phenomena, of heterogeneous etiology, whose common semiological element is represented by involuntary, jerky movements, most frequently involving antagonist muscles (Marsden et al., 1982). Myoclonus originates from abnormal muscle activation in the form of brief electromyographic (EMG) bursts (positive myoclonus) or, more rarely, from a brief interruption of ongoing EMG activity (negative myoclonus) (Marsden et al., 1982). Clinical phenomenology of myoclonus may also result from a combination of a myoclonic jerk plus postmyoclonic muscle inhibition (Guerrini et al., 1994a).
The nosologic boundaries between myoclonus and epilepsy have been the subject of discussion since the earliest clinical descriptions. Reynolds (1861) observed the frequent association between epilepsy and ‘clonic spasms’, interpreting the latter as interseizure phenomena. Friedreich (1881) coined the term ‘paramyoclonus multiplex’ to describe a sporadic, non-progressive form of myoclonus. Subsequently, a series of conditions revealing a close association between epilepsy and myoclonus were described: familial myoclonic epilepsy (Unverricht, 1891), epilepsia partialis continua (Kojewnikow, 1895), and non-progressive myoclonic epilepsy (Rabot, 1899). The first attempt at classification was undertaken by Lundborg (1903), who subdivided myoclonus into three etiological categories: (i) symptomatic myoclonus, which should include, for example, postencephalitic myoclonus (Dubini, 1846); (ii) essential myoclonus (the appropriate category for Friedreich's paramyoclonus multiplex); and (iii) familial myoclonic epilepsy (subdivided into non-progressive and progressive forms). Later, Muskens (1928) highlighted a close nosologic link between myoclonus and epilepsy, and coined the term ‘fragments of epilepsy’ to designate the myoclonic jerks of patients with epilepsy.
