Clinical and Oculographic Analysis of Inferior Oblique Myokymia

A 63-year-old man experienced transient vertical oscillopsia lasting several seconds for 2 months. Examination disclosed paroxysmal excyclotorsion of the right eye, spontaneously or triggered by adduction. Eye movements using 3D video-oculography showed intermittent, monocular phasic movements which consisted of excyclotorsion of the right eye mixed with a small amount of supraduction and abduction, and a tonic movement with excyclotorsion and slight elevation. Orbital and brain MRI was unremarkable. Administration of oxcarbazepine markedly decreased the severity and frequency of the episodes. The oculographic characteristics in our patient may indicate that inferior oblique myokymia may be attributed to aberrant, spontaneous discharges in the inferior oblique motor unit.

Clinical Observation Clinical and Oculographic Analysis of Inferior Oblique Myokymia Ji-Yeong Jang, MD, Hyeshin Jeon, MD, Seo-Young Choi, MD, Jae-Hwan Choi, MD, Kwang-Dong Choi, MD Abstract: A 63-year-old man experienced transient vertical oscillopsia lasting several seconds for 2 months. Examination disclosed paroxysmal excyclotorsion of the right eye, spontaneously or triggered by adduction. Eye movements using 3D video-oculography showed intermittent, monocular phasic movements which consisted of excyclotorsion of the right eye mixed with a small amount of supraduction and abduction, and a tonic movement with excyclotorsion and slight elevation. Orbital and brain MRI was unremarkable. Administration of oxcarbazepine markedly decreased the severity and frequency of the episodes. The oculographic characteristics in our patient may indicate that inferior oblique myokymia may be attributed to aberrant, spontaneous discharges in the inferior oblique motor unit. Journal of Neuro-Ophthalmology 2017;37:418-420 doi: 10.1097/WNO.0000000000000535 © 2017 by North American Neuro-Ophthalmology Society S uperior oblique myokymia is characterized by monocular, high-frequency, low-amplitude contractions of the superior oblique muscle producing monocular torsional or vertical oscillopsia, which often lasts seconds to hours, and can occur several times a day (1-3). Although the etiology of superior oblique myokymia remains unknown, previous proposed causes include direct involvement of the superior Department of Neurology (J-YJ, S-YC, K-DC) and Ophthalmology (HJ), Pusan National University Hospital, Pusan National University School of Medicine and Biomedical Research Institute, Busan, Korea; and Department of Neurology (J-HC), Pusan National University School of Medicine, Research Institute for Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital, Yangsan, Korea. The authors report no conflicts of interest. Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the full text and PDF versions of this article on the journal's Web site (www. jneuro-ophthalmology.com). Address correspondence to Kwang-Dong Choi, MD, Department of Neurology, College of Medicine, Pusan National University Hospital, 1-10 Ami-dong, Seo-gu, Busan 602-739, Korea; E-mail: [e-mail] 418 oblique muscle, vascular compression of the fourth nerve, or a brainstem etiology. No treatment for superior oblique myokymia is consistently effective, but individual patients may respond to carbamazepine, baclofen, gabapentin, or topically administered beta blocker eye drops. Inferior oblique myokymia is another unique eye movement disorder. To date, 2 patients have been reported with descriptions based on only clinical observation (4,5). We report an additional case and describe the oculographic characteristics of inferior oblique myokymia. CASE REPORT A 63-year-old man with hypertension and dyslipidemia reported a 2-month history of transient vertical oscillopsia lasting several seconds. The frequency of attacks ranged from 1 to 2 times per month to several times per day. Examination disclosed paroxysmal excyclotorsion of the right eye, occurring spontaneously or triggered by adduction (See Supplemental Digital Content, Video, http:// links.lww.com/WNO/A233). Eye movements using 3D video-oculography (SLMED, Seoul, Korea) disclosed intermittent, monocular phasic movements occurring several times over 5 minutes (Fig. 1). Simultaneous binocular recording confirmed that the movements were strictly monocular. The phasic movements primarily consisted of excyclotorsion of the right eye mixed with a small amount of supraduction and abduction. Amplitude and frequency of the torsional component ranged from 1.4 to 2.1°/s and from 10 to 30 Hz, respectively. The waveform of the torsional phasic movements consisted of linear offsetting movements with the velocity ranging from 21 to 41°/s. In addition to the phasic offsetting movements, the right eye, on occasion, tonically extorted up to 4.6° and slightly elevated. The tonic deviation persisted for 233 milliseconds. Orbital and brain MRIs with constructive interference in steady state images were normal. The patient was prescribed Jang et al: J Neuro-Ophthalmol 2017; 37: 418-420 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Observation FIG. 1. Oculographic analysis of inferior oblique myokymia. There are a number of phasic movements (arrows) and tonic (arrowhead) excyclotorsion of the right eye with a small amount of supraduction and abduction, followed by returns with incyclotorsion, infraduction, and adduction. Upward deflection in each tracing of the right eye indicates clockwise torsional (RT), upward (RV), and horizontal (RH) movements. oxcarbazepine 300 mg/d, which markedly decreased the severity and frequency of the episodes. DISCUSSION Although superior oblique myokymia has been well described (1-3), little is known about inferior oblique myokymia. We are aware of only 2 previously reported cases of inferior oblique myokymia (Table 1). Chinskey and Cornblath (4) described a 59-year-old man who experienced inferior oblique myokymia of the right eye with a frequency of 5 times in 7 days and duration of 1 minute, usually triggered by supraduction. The episodes did not improve with timolol eye drops or oxcarbazepine. The other report documented a 41-year-old woman with alternating right superior and inferior oblique myokymia and torsional movements of greater amplitude occurred after some, but not all, blinks (5). Our patient experienced inferior oblique myokymia with a variable frequency ranging from 1 to 2 times per month to several times per day, and duration of several seconds. Brain imaging in all 3 patients was unremarkable. Our patient was the only case in which medical therapy diminished the eye movement disorder. Oculographic measurements in our patient demonstrated a number of phasic and a large tonic right eye movements similar to the recordings of superior oblique myokymia (6,7). Phasic offsetting movements consisted of prominent excyclotorsion mixed with small amplitude of supraduction and abduction of the right eye, consistent with the primary, secondary, and tertiary actions of the inferior oblique muscle. The velocities of the torsional offsetting movements were similar to those of torsional saccades recorded during torsional vestibular nystagmus or torsional optokinetic nystagmus (8). Inferior oblique myokymia may be attributed to aberrant, spontaneous discharges in the inferior oblique motor unit. Phasic movements may result from aberrant motor neuron firing, and the return toward baseline indicates that the firing rate is not sustained and generally occurs as intermittent bursts. By contrast, tonic deviation of the eye might result from a more sustained, regular increase in firing rate. TABLE 1. Clinical characteristics of patients with inferior oblique myokymia Patient (Reference) Age/Sex Frequency 1 (4) 59/M 5 times/wk 2 (5) 41/F ND Current report 59/M 1 times/mo w several times/d Duration Triggering Factors Other Features 1 min Supraduction None ND Blink? Several seconds Adduction Brain Imaging Treatment Normal No improvement with timolol eye drops or oxcarbazepine Normal ND Alternating superior and inferior oblique myokymia None Normal Marked improvement by oxcarbazepine 300 mg/d ND, not described. Jang et al: J Neuro-Ophthalmol 2017; 37: 418-420 419 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Observation Although the mechanism of inferior oblique myokymia is unknown, neurovascular compression, the presumed pathogenic mechanism for superior oblique myokymia, seems unlikely. The inferior oblique muscle is innervated by the inferior branch of the third nerve, with the division occurring in the region of the superior orbital fissure. Possibly, inferior oblique myokymia results from the myokymic movements of the inferior oblique muscle due to excessive neuronal firing of the muscle itself. STATEMENT OF AUTHORSHIP Category 1: a. Conception and design: K.-D. Choi, J.-Y. Jang, and H. Jeon; b. Acquisition of data: K.-D. Choi and S.-Y. Choi; c. Analysis and interpretation of data: K.-D. Choi, J.-Y. Jang, and J.-H. Choi. Category 2: a. Drafting the manuscript: K.-D. Choi and J.-Y. Jang; b. 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