Electroencephalographic Findings in Posthypoxic Myoclonus

Abstract

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.

The interrelation between clinical presentation and neurophysiology of posthypoxic myoclonus

“…Moreover, in this cohort CM was more likely present in patients with generalized PHM than in focal PHM. This result is in line with earlier studies who demonstrated that generalized PHM can arise from the cortex 5, 6, 33. These findings suggest that the clinical model of typical CM and SM phenomenology that physicians encounter in noncomatose myoclonus patients at the outpatient clinic does not apply for PHM in the first few days after resuscitation 10, 11.…”

“…This finding suggests that patients with a multiple lesioned cortex (and CM) could have a worse outcome compared to patients with a singular, small brainstem lesion (and SM). In addition, studies that correlated the generalized and focal PHM phenotype to outcome found that the severity of myoclonus, and not per se the presence or absence of brainstem damage, was related to poor outcome 1, 5, 6. Yet in contrast, another finding of this study was that in patients presenting PHM within 24 h after CPR, significantly less CM occurred (Table 3).…”

“…EEG background patterns were categorized as (1) iso‐electric, (2) low voltage, (3) burst suppression, (4) generalized status epilepticus, (5) diffuse slowing, and (6) mild encephalopathic or normal 6, 28. EEGs were visually inspected for motor cortex potentials before the onset and in direct relation with PHM (Fig.…”

“…To classify the anatomical origin of PHM, five different neurophysiological methods were used, namely: (1) visual inspection of EEG, (2) EMG burst‐duration, (3) muscle recruitment order, (4) JLBA, and (5) coherence analysis 6, 9, 10, 11, 18, 19, 20, 21, 22. Cortical PHM was presumed if (1) an EEG spike prior PHM was visible on EEG, (2) a mean burst duration <75 msec was present (10 randomly assessed jerks), (3) a cranio‐caudal muscle recruitment order or only single myoclonic jerk(s) were present, (4) JLBA was positive, or (5) reliable corticomuscular coherence was present.…”

“…The incidence of posthypoxic myoclonus (PHM) in resuscitated patients with postanoxic encephalopathy (PAE) is 19%, and only 9–14% of these patients recover with mild to no cognitive deficits 4, 5, 6…”