Indicators that may be used to predict neurological recovery post-cardiopulmonary arrest include brainstem reflexes, myoclonus status epilepticus, somatosensory evoked potentials (SEP), electroencephalography (EEG), and serum neuron specific enolase (NSE).1 Among these, SEP is a useful test which can be easily performed in the intensive care unit (ICU), with the absence of bilateral N20 responses being a poor prognostic sign known to have a false positive rate close to 0.1

We report a case of myoclonus status epilepticus recurring in a patient with amyotrophic lateral sclerosis while performing SEP study to evaluate the degree of hypoxic injury, and investigate the risk factors and pathophysiology of this condition.

Received: January 18, 2017

Revised: June 1, 2017

Accepted: June 8, 2017

ANNALS OF CLINICAL NEUROPHYSIOLOGY

CASE REPORTAnn Clin Neurophysiol 2017;19(2):136-140https://doi.org/10.14253/acn.2017.19.2.136

Correspondence to

Seo-Young LeeDepartment of Neurology, Kangwon National University Hospital, 156 Baeng-nyeong-ro, Chuncheon 24289, KoreaTel: +82-33-258-2431Fax: +82-33-258-2103E-mail: leeseoyoung@kangwon.ac.kr

http://www.e-acn.org

pISSN 2508-691X eISSN 2508-6960

Copyright © 2017 The Korean Society of Clinical NeurophysiologyThis is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Myoclonic status epilepticus in hypoxic ischemic encephalopathy which recurred after somatosensory evoked potential testingSeongheon Kim, Yeshin Kim, Sunghun Kim, and Seo-Young Lee

Department of Neurology, Kangwon National University Hospital, Chuncheon, Korea

A 77-year-old male with amyotrophic lateral sclerosis had a hypoxic event. After resuscitation, generalized myoclonus appeared and resolved after two days. Five days after the hypoxic event, myoclonic seizures re-emerged right after performing a somatosensory evoked po-tential and persisted for ten days. Electroencephalogram revealed frequent bi-hemispheric synchronous spike and waves in the central areas. We suggest that somatosensory evoked potential testing may trigger myoclonic status epilepticus. Underlying cortical degeneration associated with amyotrophic lateral sclerosis could attribute to this phenomenon.

Key words: Myoclonus; Status epilepticus; Somatosensory evoked potential

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Seongheon Kim, et al. Recurred myoclonic SE in hypoxic encephalopathy after SSEP

Myoclonic status epilepticus in hypoxic ischemic encephalopathy which recurred after somatosensory evoked potential testingSeongheon Kim, Yeshin Kim, Sunghun Kim, and Seo-Young Lee

Department of Neurology, Kangwon National University Hospital, Chuncheon, Korea

A 77-year-old male with amyotrophic lateral sclerosis had a hypoxic event. After resuscitation, generalized myoclonus appeared and resolved after two days. Five days after the hypoxic event, myoclonic seizures re-emerged right after performing a somatosensory evoked po-tential and persisted for ten days. Electroencephalogram revealed frequent bi-hemispheric synchronous spike and waves in the central areas. We suggest that somatosensory evoked potential testing may trigger myoclonic status epilepticus. Underlying cortical degeneration associated with amyotrophic lateral sclerosis could attribute to this phenomenon.

Key words: Myoclonus; Status epilepticus; Somatosensory evoked potential

CASE

A 77-year-old male was resuscitated after 6 minutes of re-spiratory arrest. He started having bilateral upper extremity weakness 4 years ago and was diagnosed with amyotrophic lateral sclerosis, and developed dyspnea a year ago. An hour post-arrest he developed generalized myoclonic jerks which would occur every few seconds, which resolved gradually over the course of 2 days. He was treated with intravenous midazolam infusion (1 mg/kg/hour) and clonazepam via na-sogastric tube (9 mg/24 hour), and midazolam was discontin-ued after the myoclonic jerks had resolved. EEG at that time showed generalized slowing at 1.5–2 Hz between the muscle artifact due to the myoclonus (Fig. 1). Serum NSE levels 30 hours post-event were within normal range at 13.02 ng/mL, and diffusion MRI on day 3 did not demonstrate any acute changes. EEG showed generalized continuous small spikes at frequencies above 1 Hz with maximum amplitudes in bilateral frontocentral areas, with rare gross head movements, making it challenging to determine as to whether this was artifact

associated with fine myoclonus of the head or true potentials originating from the brain itself (Fig. 2). Median nerve SEP testing was performed on day 5 while the patient was still in a comatose state. When both wrists were electrically stimu-lated, normal latencies and waveforms of evoked potentials were observed from scalp leads C3, and C4. After median nerve SEP testing the patient developed recurrent myoclonus in the face and extremities, some of which were spontaneous and others triggered by passive movements or somatosenso-ry stimuli. Due to frequent myoclonic artifact the background was difficult to assess (Fig. 3A), but with intravenous vecuroni-um the myoclonus subsided and spike and slow-wave com-plexes occurring at least 2 Hz with phase reversal at C3, and C4 were observed (Fig. 3B), which could be suppressed with intravenous lorazepam. Levetiracetam (1,000 mg/24 hour) was subsequently administered but multifocal myoclonus continued to recur, which resolved 10 days later. The patient expired 2 months later due to systemic infection without any recovery of consciousness.

Fig. 1. Electroencephalography (EEG) recorded at three hours after hypoxic event. Background EEG shows diffuse arrhythmic delta slowing low to moderate amplitude. Thick arrows indicate large potential changes associated with generalized myoclonus. Artifactual components obscured authen-tic EEG potential. Sharp waves with maximum amplitudes in bilateral frontal area synchronously (thin arrows) were occasionally observed.

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Annals of Clinical Neurophysiology Volume 19, Number 2, July 2017

DISCUSSION

Various types of myoclonus exist in terms of distribution, source, and etiology. Not uncommonly does stimulus-sensi-tive or reflex myoclonus occur, which is provoked by different stimuli. The myoclonus after SEP testing in this case may be classified as multifocal myoclonus in terms of distribution, as cortical or epileptic myoclonus associated with fairly focal ep-ileptiform discharges on EEG, or reflex myoclonus provoked by somatosensory stimuli.2,3

In general postanoxic myoclonus status epilepticus, which occurs in the acute phase several hours post-anoxic injury, spontaneously resolves within several days, and is known to be a poor prognostic factor implying diffuse cerebral injury.1 Hallett et al. suggested that the source of acute postanoxic myoclonus is the medulla oblongata based on the fact that the trapezius was activated prior to the scalp EEG activity and orbicularis oculi, masseter, and arm muscles activity later when performing surface electromyography on various mus-cles innervated by cranial nerves during myoclonus among patients who woke up 2 weeks post-cardiopulmonary arrest.4 In cases of chronic postanoxic myoclonus, otherwise known

as Lance-Adams syndrome, the source is considered to be cortex for the most part, the rationale being that EEG activity is observed to precede myoclonus on EEG back-averaging and at times with giant SEPs.5 However, generalized spikes may be induced on EEG by stimulus in addition myoclonus in a number of cases of stimulus sensitive postanoxic myoclo-nus during the acute stages,6 hence this may be considered to be a type of reflex epilepsy involving the cerebral cortex. The ictal EEG of acute postanoxic myoclonus has been re-ported to manifest in various patterns. Burst suppression or generalized periodic epileptiform discharges are common, but diffuse slowing or low voltage EEG may also be seen, and when burst suppression or generalized periodic epileptiform discharges are detected the myoclonus may or may not be time-locked to the bursts or spikes. This suggests that various mechanisms for postanoxic myoclonus status epilepticus may be involved,7-9 which may also be a result of differences in location and degree of anoxic injury.

This was an unprecedented case of documented spikes with a central focus in acute postanoxic myoclonus. There have been cases of vertex, central, and frontal-dominant generalized spikes,10 and there might be a lack of reports

Fig. 2. Electroencephalography (EEG) recorded at three days after hypoxic event. Generalized small spikes or polyspikes were semiperiodically seen, predominantly in bilateral frontocentral area. During this EEG, visible myoclonus occurred only occasionally in face.

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Seongheon Kim, et al. Recurred myoclonic SE in hypoxic encephalopathy after SSEP

due to difficulty ascertaining these discharges in the midst of myoclonic artifacts. Acute postanoxic myoclonus may either occur spontaneously or with sensory stimuli, somatosensory stimuli being the most common. This suggests that the so-matosensory cortex might be the most vulnerable to hypox-

ic injury. Even in other causes of myoclonus the EEG activity observed with back-averaging was found to originate from the sensorimotor cortex much like SEPs.3 This patient was suffering from amyotrophic lateral sclerosis, and we assumed that the underlying cortical neuronal degeneration might

A

B

Fig. 3. Electroencephalography (EEG) after somatosensory evoked potential testing. (A) EEG recorded during frequently multifocal myoclonus in face and limbs at eleven days after hypoxic event. Generalized high amplitude spikes or polyspikes were semiperiodically seen, predominantly in bilateral frontocentral area. It is not discernible whether they are only artifacts or epileptic activities mixed with artifactual components. (B) EEG recorded after myoclonic movements were abolished by vecurnoium injection. Semiperiodic spikes were disclosed on C3 and C4 synchronously.

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Annals of Clinical Neurophysiology Volume 19, Number 2, July 2017

have been more prone to irritability after an anoxic insult, es-pecially in the pericentral areas. However, myoclonus or sei-zures are not more common spontaneously in amyotrophic lateral sclerosis.

Perhaps the main topic of discussion is whether or not the postanoxic myoclonus status epilepticus after SEP testing was truly due to the SEP study. To date there have been no studies on the potential association between SEP testing and postanoxic myoclonus status epilepticus. Stimulus-sensitive myoclonus frequently occurs post-anoxia, and considering that hyperexcitability of the sensorimotor cortex is frequently observed, it is possible that the repetitive electrical stimulation of sensory nerves might have induced myoclonic status. One cannot exclude the possibility of this being the nature course of postanoxic myoclonus or the recurrence of myoclonus which improved after decreasing the dose of midazolam, but postanoxic myoclonus has been reported to spontaneously resolve within 5 days in most cases.8 The initial myoclonus seen in this patient resolved within 2 days, which is consistent with its common clinical course, but is unusual for postanoxic myoclonus to recur 5 days post-arrest and persist for 10 days. The majority of cases of chronic postanoxic myoclonus after recovery from anoxia have been reported to demonstrate myoclonus in the acute phase, and in this case the patient might have progressed to chronic postanoxic myoclonus with time. However, neurologically this patient did not recov-er, demonstrating repeated resolution of myoclonus within 10 days, which differs from chronic postanoxic myoclonus in general.

SEP testing may provoke post-anoxic myoclonic status epilepticus. Future studies are required to assess the frequen-cy and risk factors, whether or not SEP is safe to perform in patients who have demonstrated postanoxic myoclonus, or if the underlying condition of amyotrophic lateral sclerosis had attributed to this phenomenon or not.

REFERENCES

1. Wijdicks EF, Hijdra A, Young GB, Bassetti CL, Wiebe S; Quality Stan-

dards Subcommittee of the American Academy of Neurology.

Practice parameter: prediction of outcome in comatose survivors

after cardiopulmonary resuscitation (an evidence-based review):

report of the Quality Standards Subcommittee of the American

Academy of Neurology. Neurology 2006;67:203-210.

2. Shibasaki H. Neurophysiological classification of myoclonus. Neu-

rophysiol Clin 2006;36:267-269.

3. Hallett M. Myoclonus: relation to epilepsy. Epilepsia 1985;26 Suppl

1:S67-S77.

4. Hallett M, Chadwick D, Adam J, Marsden CD. Reticular reflex my-

oclonus: a physiological type of human post-hypoxic myoclonus.

J Neurol Neurosurg Psychiatry 1977;40:253-264.

5. Werhahn KJ, Brown P, Thompson PD, Marsden CD. The clinical

features and prognosis of chronic posthypoxic myoclonus. Mov

Disord 1997;12:216-220.

6. Van Cott AC, Blatt I, Brenner RP. Stimulus-sensitive seizures in pos-

tanoxic coma. Epilepsia 1996;37:868-874.

7. van Zijl JC, Beudel M, vd Hoeven HJ, Lange F, Tijssen MA, Elting JW.

Electroencephalographic findings in posthypoxic myoclonus. J

Intensive Care Med 2016;31:270-275.

8. Hui AC, Cheng C, Lam A, Mok V, Joynt GM. Prognosis following

postanoxic myoclonus status epilepticus. Eur Neurol 2005;54:10-

13.

9. Young GB, Gilbert JJ, Zochodne DW. The significance of myoclonic

status epilepticus in postanoxic coma. Neurology 1990;40:1843-

1848.

10. Madison D, Niedermeyer E. Epileptic seizures resulting

from acute cerebral anoxia. J Neurol Neurosurg Psychiatry

1970;33:381-386.