Electroencephalographic Findings in Posthypoxic Myoclonus

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The physical examination findings of early posthypoxic myoclonus (PHM) are associated with poor prognosis. Recent findings indicate that patients with multifocal PHM, assumed to have a cortical origin, have a comparable outcome to resuscitated patients without PHM. Generalized PHM, assumed to have a subcortical myoclonus origin, is still associated with a bad clinical outcome. It is not known whether the electroencephalographic (EEG) findings differ between the multifocal and generalized myoclonus groups nor is the clinical significance clearly defined. Forty-three patients with PHM were retrospectively derived from an EEG database. Patients were categorized as having multifocal (i), generalized (ii), or undetermined (iii) PHM. Outcome was expressed in cerebral performance category scores. The EEG background was categorized into isoelectric (I), low voltage (II), burst suppression (III), status epilepticus (SE; IV), diffuse slowing (V), and mild encephalopathic or normal (VI). 17 patients had generalized PHM and 23 had multifocal PHM (3 undetermined). The EEG showed more SE in generalized compared to multifocal PHM (64% vs 13%, P < .001). Diffuse slowing was more often present in multifocal PHM (52% vs 17%, P < .05). Early-onset myoclonus occurred significantly more often in generalized PHM, and early generalized PHM was invariantly associated with poor outcome. In conclusion, patients with generalized PHM showed more SE. These EEG findings might be either subcortical corollaries or primarily cortical phenomena. Our retrospective results conflict with currently used clinical criteria for myoclonus classification, and we suggest that more refined difference may be needed for accurate assessment of PHM. To better understand PHM, prospective research with standardized clinical assessment and quantitative EEG analysis is needed.

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