MYOCLONIC EPILEPSIES OF INFANCY AND CHILDHOOD Myoclonus and myoclonic seizures

Myoclonus refers to sudden, brief, shock-like involuntary movements, caused by muscular contractions (positive myoclonus) or inhibitions (negative myoclonus) arising from the central nervous system. This definition excludes the muscular twitches or fasciculation due to lesions of the lower motor neurons. 1 Myoclonus has been observed in patients with various neurological conditions including epilepsy, and the clinical manifestations, etiology or other associated symptoms are not always homogeneous. Various attempts to classify myoclonus have been made on the basis of clinical appearance, predominant distribution of the myoclonus, temporal pattern, etiology and, most recently, the anatomical origin of the jerks based on the underlying neurophysiological mechanism. 2 Although there have been a few proposals of anatomical classification, they have largely subdivided the origin into the cortex, subcortical structures including reticular formation, and spinal cord. Although myoclonus is a major and distinct seizure type in epileptology, the neuro-physiological background of myoclonic seizures has long remained uninvestigated. Recent progress in the neurophysiological understanding of myoclonus has thus prompted the ILAE to reconsider the classification of myoclonic seizures from an epileptological standpoint, and to propose the classification of myoclonus into cortical myoclonus, thalamo-cortical myoclonus, reticular reflex myoclonus and negative myoclonus. 3 Cortical myoclonus has a proven cortical origin for which a computer-assisted averaging technique can only identify the preceding small cortical spike discharges, thus, the underlying mechanism is similar to focal seizures. 2,3 Some epilepsia partialis continua can be classified into cortical myoclonus. Thalamo-cortical myoclonus corresponds to MS, the most common form of myoclonus in epilepsy. 3 The ictal EEGs of such patients consist of generalized bilaterally synchronous spike and wave complexes, which are presumably generated by thalamo-cortical hyper-excitability. Reticular reflex myoclonus has a brain stem origin, in which the myoclonus impulse spreads along the body away from the source in a time-related sequential fashion. 2,3 There seems to be no counterpart in epileptic seizure classification, although generalized tonic seizures and epileptic spasms may be candidates. Negative myoclonus is a special type of myoclonus producing negative motor phenomena, including epileptic negative myoclonus, and may be a brief form of atonic seizures, as a counterpart in epileptic seizure classification. It is difficult to make a definite diagnosis of cortical and reticular reflex myoclonus because it requires a sophisticated computer-assisted technique. Among the four types of myoclonus, thalamo-cortical myoclonus and negative myoclonus have long received attention in epileptology, because they are the main seizure types in various important …