Symptomatic myoclonus.
A huge number of neurological disorders are associated with myoclonus. This paper describes these disorders whose diagnosis partly relies on the presence of myoclonus. The diagnostic approach is related to certain clinical features of myoclonus, which, after their integration in the clinical context, help orientate towards diagnosis. Myoclonus is frequent during dementia. Although its presence is well-known to take part in the diagnosis of Creutzfeldt-Jakob disease (CJD), myoclonus can also be present to a significant degree in Alzheimer's disease and Lewy body dementia (LBD), which raises a diagnostic issue. Both its clinical and electrophysiological features may help differential diagnosis, given that myoclonus with fast-evolving dementia and focal neurological signs should favor the diagnosis of CJD. Myoclonus in a context of progressive ataxia suggests one clinical form of the Ramsay-Hunt syndrome (progressive myoclonic ataxia, PMA), whose most frequent causes are: coeliac disease, mitochondriopathies, some spino-cerebellar degenerations, and some late metabolic disorders. In addition to ataxia and myoclonus, the presence of opsoclonus directs diagnosis toward the opsoclonus-myoclonus syndrome (OMS), whose origin, in adult, is idiopathic or paraneoplastic. Palatal tremor (myoclonus) with ataxia may represent either a sporadic pattern, which often reflects the evolution of degenerative or lesional disorders, or a familial pattern in some degenerative affections or metabolic diseases. Of more recent knowledge is the association of progressive ataxia, myoclonus, and renal failure, which corresponds to a recessive autosomic disease. In a context of encephalopathy, myoclonus is frequent in metabolic or hydro-electrolytic disorders, and in brain anoxia. One should distinguish these various forms of myoclonus which may occur in the acute post-anoxic phase, from those occurring as sequels at a later stage, i.e. the Lance and Adams syndrome whose clinical aspects are also multiple. Myoclonus is less frequent during toxic or drug exposures. Irrespective of its acute or insidious onset, Hashimoto's encephalopathy is accompanied by myoclonus and tremor. Myoclonus may also be present during encephalic and/or spinal infectious disorders. Myoclonus with focal neurological signs may be observed in thalamic lesions, responsible for unilateral asterixis or unilateral myoclonus superimposed on dystonic posture. Segmental spinal myoclonus or propriospinal myoclonus may be associated with several spinal-cord disorders. Myoclonus associated with peripheral nerve lesions is exceptional or even questionable for some of these.