[Myoclonus and epilepsy: diagnosis and pathophysiology]

Myoclonus presents as a sudden brief involuntary jerk triggered by the central nervous system. Electromyographic studies enable determining whether the jerk is caused by a muscular contraction, i.e. positive myoclonus, or by an interruption of muscular activity, i.e. negative myoclonus. Many classifications have been proposed, reflecting our lack of understanding about myoclonus. Myoclonus is a symptom and should never be considered as a diagnosis. Clinical history and physical examination are the basis to diagnosis. Clinical neurophysiology testing can reveal a neuroanatomical localization and certain patterns have some etiological specificity. Etiological hypotheses can be put forward on the basis of clinical and neurophysiological data. The cortex is the most commonly identified source of myoclonus, but the subcortical area and spinal area can also be involved. Myoclonus is considered epileptic when it is combined with an epileptiform discharge on the EEG. The International Classification of Epileptic Syndromes should be applied in this situation. Myoclonic epilepsies are a collection of syndromes in which myoclonic seizures are a prominent feature. Myoclonus can occur as one among several seizure components, as the only manifestation of seizure, or as one of multiple seizure types within an epileptic syndrome. Neurophysiological studies are needed to investigate the pathophysiological mechanisms of the myoclonus. Electrophysiological studies report that myoclonic seizures are produced through a cortical generator via a polysynaptic mechanism acting on muscles. Apparently, the epileptiform discharges stimulate the motor cortex resulting in myoclonus jerk. Despite recent progress, advances are still needed to achieve a better understanding of the pathophysiological mechanisms involved in myoclonus. In myoclonic epileptic syndromes, more useful information can probably be obtained from studies grouping several patients with a same epileptic syndrome than from single case reports.