A 15-year-old male with a past medical history of complex partial seizures and narcolepsy with cataplexy presents with ongoing excessive daytime sleepiness. He is currently not on medical therapy to treat his narcolepsy. He takes multiple naps throughout the day, lasting up to 4 hours daily. He reports, in general, feeling refreshed after a nap and is otherwise performing to an average level in school and does not complain that his daytime sleepiness greatly impairs his daily function. He is interested in obtaining his driver's license. On neurological examination, he shows cranial nerves II-XII are intact with no focal deficits. Epworth Sleepiness Scale score is 8/24. He takes no medications.
A Maintenance of Wakefulness Test (MWT) was ordered to evaluate for safety prior to driving. On the MWT, he was observed to have bilateral, rhythmic, circular roving eye movements. This pattern was observed for about 3 minutes.
QUESTION: Video 1 shows the eye movements in question during the MWT. Figure 1 shows an epoch from the MWT during the movements in question. What is the cause of these eye movements?
A 30-second epoch from the MWT.
C4-M1 = central derivation, Chin = chin derivation, E1-M2 and E2-M1 = electrooculographic (EOG) derivations, EKG = electrocardiogram, F4-M1 = frontal derivation, MWT = Maintenance of Wakefulness Test, O2-M1 = occipital derivation.
ANSWER: Video 1 shows eye movements related to patient watching the ceiling fan.
The eye movement observed in Video 1 is not rotary nystagmus, rather it is from the patient watching the ceiling fan as documented in the technician's notes. He was observed to be alert with wakefulness pattern on EEG.
Nystagmus is a rhythmic, regular oscillation of the eyes. It may consist of alternating phases of a slow drift in one direction with a corrective quick jerk in the opposite direction, or slow, sinusoidal oscillations to and fro which are also classified as pendular. Pendular nystagmus has a sinusoidal oscillation without fast phases. The waveform of pendular nystagmus may occur in any direction. It can be torsional, horizontal, vertical, or a combination of these, resulting in circular, oblique, or elliptical trajectories. It can be different in the two eyes and sometimes even monocular.1,2 Pendular nystagmus can be observed in people falling asleep either naturally or under anesthesia.3 Acquired monocular pendular nystagmus may occur in patients with multiple sclerosis, neurosyphilis, or thalamic and upper midbrain disease.4
Drugs and medications are also known to result in visual disturbances. One of the most frequent adverse effects of antiepileptic drugs is visual dysfunction because the eye is very susceptible to the dose, duration, and mechanisms of action of many antiepileptic drugs.5 Downbeat nystagmus has been reported with phenytoin and carbamazepine toxicity.6 Nystagmus is usually the first sign of drug intoxication.7 Physostigmine has also been shown in increase nystagmus.8
The patient in this case slept on four out of four naps. The mean sleep latency was 20.5 minutes, with a range of 10 minutes to 32.5 minutes. It was felt that these results did not demonstrate adequate alertness for motor vehicle operation and it was recommended that he not drive. It should be noted that the MWT in practice can be helpful for assessing patients where inability to stay awake is a potential safety issue (patients that wish to drive), or to evaluate the efficacy of a alerting medication in patients with narcolepsy or idiopathic hypersomnia.9 The result from the MWT should not be the sole determining factor, but rather added information to assess when using one's clinical judgement. This patient would likely benefit from an alerting medication prior to engaging in activities requiring high vigilance, and a repeat MWT may be helpful when the patient is felt to be adequately treated.
This case highlights the importance of reviewing the technician notes, which can contain vital additional information when evaluating a study with abnormal findings, and proper utilization of the MWT in clinical practice
SLEEP MEDICINE PEARLS
Use resources like video and technician notes when reviewing abnormal findings on EEG or EOG.
Nystagmus can be seen at sleep onset, occurring naturally or under anesthesia.
Antiepileptic drugs such as phenytoin, carbamazepine, and physostigmine can cause nystagmus.
Nystagmus is usually the first sign of phenytoin and carbamazepine toxicity.
An MWT is used to assess the patient's ability to stay awake and can therefore evaluate the efficacy of a patient's treatment regimen; however, it can never “clear” a patient to drive.
All authors have seen and approved the manuscript. The authors report no conflicts of interest. The patient and his guardian have provided permission for use of the video in this publication.
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