Les myoclonies sont des mouvements involontaires brefs et soudains, à l'origine de secousses musculaires, pouvant être d'origine pathologique ou physiologique. En raison de la grande variété de pathologies en cause, et de l'extrême distribution des lésions pouvant provoquer des myoclonies, elles représentent un enjeu diagnostique difficile. L'objectif de ce travail est de proposer un fil conducteur permettant d'orienter la démarche diagnostique à partir des données cliniques et électrophysiologiques recueillies devant des myoclonies de l'adulte. Successivement, les éléments cliniques permettant de distinguer une myoclonie d'autres mouvements anormaux, les caractéristiques des myoclonies pouvant contribuer à orienter le diagnostic topographique et étiologique, l'apport du contexte clinique et celui des explorations electrophysiologiques sont détaillés. Il est également important de rappeler que la détermination du site d'origine d'une myoclonie ne préjuge en rien du siège du processus pathologique.

Myoclonus, defined as shock-like involuntary movement, may be physiological or caused by a very wide variety of hereditary and acquired conditions. Because myoclonus can originate from different disorders and lesions affecting quite varied levels of the central and peripheral nervous systems, it represents from many points of view a diagnostic challenge. Moreover, new entities have been recently individualized, such as cortical tremor, which deserve renewed attention. The aim of this review is to propose a rationale for a diagnostic approach based on clinical and electrophysiological grounds. In this setting, we successively address 1) the clinical features allowing a positive diagnosis of myoclonus; 2) the clinical clues to the etiology; 3) the relevance of the clinical context to the diagnosis; and 4) the contribution of neurophysiology. Differentiating myoclonus from tics, spasm, chorea and dystonia can be difficult, and a careful reappraisal of clinical features allowing precise identification is presented. Moreover, the topographical distribution of myoclonus, the temporal pattern of muscle recruitment, the condition of occurrence and the rhythm of the event, may provide clinical clues relevant to the diagnosis. Myoclonus without associated epilepsy, myoclonus with epilepsy, myoclonus with encephalopathy, parkinsonism and/or dementia represent overlapping clinical categories, although they remain useful for the diagnostic approach. Using electrophysiology (including back-averaging EEG, MEG, SEP, C-reflex studies) to determine the origin of myoclonus may not allow us to focus on the underlying condition. Indeed, in many instances, the myoclonus is cortical in origin, but the pathology is found elsewhere.