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Bilateral Abducens Neuromyotonia

Luis H Ospina, Nipat AuiAree, Duncan P Anderson


Department of Ophthalmology, University of British Columbia, St Paul's Hospital, Vancouver

Correspondence: Dr Duncan P Anderson, Department of Ophthalmology, University of British Columbia, St. Paul’s Hospital
1081 Burrard St., Vancouver, B.C. V6Z 1Y6, Canada
E-mail: danderson@providencehealth.bc.ca


Accepted for publication:  August 1, 2004

 

 

A 48-year-old female presented occasional transient horizontal diplopia with up to 10 episodes per day. The range of eye movements was full and the eye alignment appeared iniatially normal, with a small esophoria in all positions of gaze. However, after prolonged eccentric gaze to the right or left, she developed a large angle exotropia measuring up to 40 prism diopters. After 40 to 70 seconds, the alignment gradually returned to normal.

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Introduction

Ocular neuromyotonia(ONM) is a rare eye movement disorder characterized by brief intermittent and paroxysmal ocular deviations with diplopia.[1] It is most commonly seen after cranial radiation therapy, especially to the sellar-parasellar region[1,2,3,4] however compressive lesions from mucormycosis, aneurysms, basilar artery dolichoectasia, arachnoiditis and Graves’ orbitopathy have also been described.[1,3,4,5,6,7,8] Idiopathic cases with no identified lesion have been reported as well.[1,3,9,10,11] In this video we present the unusual eye movements of a patient with bilateral sixth nerve neuromyotonia after irradiation for nasopharyngeal carcinoma.

 

Case Presentation

A 48-year-old female presented with a 2 year history of occasional transient horizontal diplopia that became progressively more frequent with up to 10 episodes per day, lasting around 40 seconds, sometimes occurring spontaneously but more often after sustained gaze to the right or left. Her double vision could occur any time during the day. She noticed also that sudden changes of light intensity would elicit her symptoms. Between attacks she was asymptomatic.

She had been treated with radiation and chemotherapy for nasopharyngeal carcinoma 11 years before the onset of diplopia.

Neuro-ophthalmological examination revealed normal visual acuity, pupils and fundi. The range of eye movements was full and the eye alignment appeared iniatially normal, with a small esophoria in all positions of gaze. However, after prolonged eccentric gaze to the right or left, she developed a large angle exotropia measuring up to 40 prism diopters. After 40 to 70 seconds, the alignment gradually returned to normal, as shown in the video.

Neuroimaging was normal and treatment with carbamazepine 200 mg twice a day was initiated.


Comments

Recognizing the characteristics of neuromyotonia, a rare disorder, allows differentiation from other entities that cause episodic diplopia. Conditions like ocular myasthenia, demyelinating diseases, decompensating phorias, convergence spasm, early stages of Graves’ disease, cyclic third palsies and superior oblique myokymia are in the differential diagnosis.12 Two disorders with episodic involuntary muscle contraction deserve special mention: patients with superior oblique myokymia experience exacerbations and remissions and have microtremors. ONM can also involve the superior oblique, but does not cause oscillopsia and microtremor is not seen.3,4 Episodes of spasm occur in patients with cyclic oculomotor palsy, that is usually a congenital condition and tends to be diagnosed earlier in life. The muscle paresis persists between episodes. Conversely, individuals with ocular motor neuromyotonia are usually asymptomatic and have normal eye movements between attacks.4 Most cases are diagnosed in adults.

As in our patient, a history of brain irradiation is common. The latency between irradiation and the onset of symptoms varies between 2 months and 18 years.13 When suspecting the diagnosis, it is necessary however, to rule out other etiologies because of the association with varied compressive lesions as described previously.

The paroxysms seen in ONM are associated with tonic involuntary contraction of extraocular muscles innervated by the third, fourth, or sixth cranial nerves, and can often be triggered by maintained eccentric gaze as illustrated. This feature is relevant, since a brief exam may not reveal the typical findings. Prolonged periods of eccentric gaze may be necessary to elicit the tonic episodes, which last from several seconds to a few minutes and can recur many times per day.4 The majority of cases are unilateral, but like in our patient, bilateral involvement is possible.

ONM may be the product of ephaptic transmission.4 Demyelination of axons resulting from radiation, and compressive or microvascular damage, would facilitate spread of electrical excitation between contiguous axons, thus explaining the hyperactivity and prolonged responses, and the repetitive attacks. A similar mechanism is thought to play a role in conditions like hemifacial spasm14, peripheral neuropathy15 and trigeminal neuralgia. Alternative mechanisms have also been proposed, including central synaptic reorganization after retrograde degeneration of axons and nuclear hyperexcitability.16,17

Treatment with carbamazepine has often been successful in controlling ONM, possibly because of its membrane-stabilizing effect on the hyperexcitable axons.

Our video illustrates the typical eye movements of neuromyotonia, an uncommon treatable entity that must be considered when evaluating a patient with variable diplopia and strabismus, and that can easily be missed if its distinctive features are not recognized.


References

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14. Nielsen VK. Pathophysiology of hemifacial spasm: I. Ephaptic transmission and ectopic excitation. Neurology 1984;34:418-26.

15. Lance JW, Burke D, Pollard J. Hyperexcitability of motor and sensory neurons in neuromyotonia. Ann Neurol 1979;5:523-32.

16. Lepore FE, Glaser JS. Misdirection revisited: A critical appraisal of acquired oculomotor synkinesis. Arch Ophthalmol 1980;98(12):2206-2209.

17. Elston JS. Ocular Neuromyotonia. Arch Ophthalmol987;105:24