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Unilateral ocular neuromyotonia (ONM) is a rare entity, and bilateral ONM even more so, with and only 1 case having been reported previously in the literature. We present a rare case of bilateral ocular neuromyotonia whose course of treatment was complicated by side effects of carbamazepine.
Case Report
A 35-year-old female was referred by her oncologist with a 12-month history of blurring that was most noticeable on right lateral gaze. During the episodes of blurring, she felt a tugging sensation that could last for up to 1 minute. These episodes had become more frequent over the past 6 months, increasing to 6 to 7 times per day. Notable medical history included nasopharyngeal cancer (NPC), which was treated with 4 cycles of cisplatin and 5-fluorouracil (5FU) and concurrent daily radiotherapy for 6.5 weeks, 8 years prior. One month after completing her treatment, she experienced paraesthesia in her left lower face, neck, and left arm. Investigation of this led to a diagnosis of hypopituitarism and hypothyroidism, well controlled with growth hormone injections and thyroxine. Four years later, she experienced dysphagia, which was attributed to late deterioration from radiotherapy-induced damage. Her dysphagia caused significant weight loss, and she was referred for a feeding tube.
On examination, best-corrected visual acuity was –0.15 logMAR bilaterally. At rest, there was a stable near exophoria of 4 prism diopters. In primary position, the patient was orthophoric. After sustained gaze to the right, the divergent deviation increased to 25 prism diopters. After sustained gaze to the left, the divergent deviation increased to a lesser extent by 16 prism diopters. Following prolonged gaze testing, she was noted to have reduced adduction of the right eye with left abducting nystagmus. She also was noted to have reduced adduction of the left eye. Horizontal eye movements are demonstrated in Fig. 1. After sustained upgaze, she consistently showed upper lid lag in depressed positions (bilateral pseudo von Graefe sign) and a depression deficit in the left eye (Fig. 2). There was no anisocoria, and both pupils were reactive to light and accommodation.
Fig. 1Horizontal eye movements. Direction of gaze indicated on image by arrow or stated as being in primary position (1°). (A) Right gaze: no abnormality seen; (B) primary position: no abnormality seen; (C) left gaze: no abnormality seen; (D) right gaze after sustained left gaze: left exotropia; (E) primary position after sustained left gaze: left exotropia; (F) sustained left gaze: no abnormality seen; (G) sustained right gaze: no abnormality seen; (H) primary position after sustained right gaze: right exotropia; and (I) left gaze after sustained right gaze: right exotropia.
Fig. 2Vertical eye movements. Direction of gaze indicated on image by arrow or stated as being in primary position (1°). (A) Primary position: no abnormality seen; (B) left lid lag on down gaze; (C) downgaze after sustained upgaze: left levator and SR myotonia; and (D) primary position.
Anterior segment intraocular pressures and fundus examination were unremarkable. Investigations included serology, electromyography (EMG), and magnetic resonance imaging (MRI). Serology, including acetylcholine receptor antibodies, was unremarkable. The divergent position of the eyes was noted during the short tau inversion recovery sequence of the MRI. Repetitive stimulation EMG and single-fibre EMG were normal. Findings were consistent with bilateral ocular neuromyotonia, and treatment with carbamazepine 200 mg twice a day was initiated.
She responded well to treatment, and her symptoms resolved after 1 week. However, she developed a widespread itchy skin rash. Her treatment was reduced to 100 mg twice daily, her rash resolved, and her symptoms remained controlled.
Discussion
ONM is a rare clinical entity, characterized by episodic involuntary contraction of ≥1 extraocular muscles.
The pathophysiology behind the aberrant firing of the affected neurons is still debated but is thought to be linked to unstable neuronal membranes, especially in view of its successful treatment with membrane-stabilizing medications.
Of particular note is the suggestion by Kau et al that cisplatin and 5FU, which were used in the present case, are known to cause cranial nerve toxicity. It was hypothesized this may increase sensitivity to radiation and increase the chance of developing ONM.
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