Contributed by Leon C. Bass, M.D., David M. Hiestand, M.D., PhD., Kevin Thomas, RPSGT, University of Louisville, Department of Medicine, Division of Pulmonary, Critical Care, and Sleep Disorders Medicine., Health Science Center, Ambulatory Care Building, 550 S. Jackson Street, Louisville, KY 40202
Case Presentation:
A 13 year old male with a history of severe obstructive sleep apnea, meningomyelocele, Arnold-Chiari type II, hydrocephalus, and seizures underwent a positive airway pressure titration sleep study at our university sleep lab.
Notice the two minute sleep fragment below during NREM sleep (fig. 1)
and the REM sleep (fig. 2) which illustrates multiple arousals scored by our sleep technologist.
Question 1.
What is the most unique feature about this two minute sleep fragment?
A) Periodic myoclonus.
B) Normal sleep.
C) Regularity of arousals.
D) Arrhythmia on ECG.
Question 2.
What is the most likely explanation for these regular features?
A) Seizure activity.
B) Respiratory events followed by arousals.
C) External vibratory artifact.
D) Vagal nerve stimulator (VNS).
Answer 1: C
Notice the regular shift in the EEG (fig. 1 & 2) with a concomitant increase in the chin amplitude and change in the ECG. These changes occur at 25 second intervals.
Answer 2: D
The key here is the regularity of the changes noted on the polysomnogram. These findings are artifacts from a VNS, which were misinterpreted as arousals by the sleep technologist.
Discussion:
Vagal nerve stimulators are FDA approved implantable devices in the adjunctive treatment of seizure disorders (). These devices can be quite effective in mitigating further seizures (1,2). One study reported at least a 50% reduction in seizures among 56 children from 8 different medical centers, 3).
Once implanted, the VNS emits an electrical pulse at preset intervals via the left vagus nerve. The indirect stimulation of deeper central nervous system structures and release of norepinephrine has been shown to result in decreased cortical hyperexcitability (4). Although this has been proven to be effective in refractory seizures, it is not without its side effects. One notable side effect is sleep disordered breathing that is thought to occur from pharyngeal muscle constriction due to the electrical stimulation (3).
Although our case did not demonstrate respiratory events from the VNS, it was the scoring of the actual electrical impulses as spontaneous arousals which was the interesting finding. One could argue that a highly voltaged VNS could result in physical discomfort and manifest as periodic arousals on the polysomnogram (PSG). However, review of the sleep technologist’s notes from this study did not reflect any patient discomfort or complaints during the night, making this less likely. Referring back to the two minute sleep fragment of NREM sleep one can appreciate that there is a shift in the EEG lasting for at least 3 seconds and preceded by at least 10 seconds of stable sleep (5). The increase in frequency of the EEG along with the simultaneous increase in amplitude of the chin and ECG leads is even better illustrated during the REM sleep fragment. The regular periodicity of the electrical impulse and patient history of an implanted VNS is what confirmed that these PSG findings were indeed artifact.
References:
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Lado FA, Moshe SL. How Do Seizures Stop? Epilepsia. 2008. October;49(10): 1651-1664.
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Wheless J, Helmers S, Frost M, Gates J, Levisohn P, Tardo C, Conry J Venkataraman V, Vaisleib I. VNS in Children: Efficacy. Epilepsia. 1999;40(suppl):120-121.
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Marzec M, Edwards J, Sagher O, Fromes G, Malow BA. Effects of Vagus Nerve Stimulation on Sleep-Related Breathing in Epilepsy Patients. Epilepsia. 2003. July;44(7): 930-935.
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Fisher RS, Handforth A. Reassessment: Vagus Nerve Stimulation for Epilepsy. Neurology. 1999;53:666-669.
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Iber C, Ancoli-Israel S, Chesson A, and Quan SF. The American Academy of Sleep Medicine Manual for the Scoring of Sleep and Associated Events:Rules, Terminology and Technical Specifications. 1st Edition.: Westchester, IL: American Academy of Sleep Medicine, 2007.
