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A 54 year old man with intermittent diplopia
Digital Journal of Ophthalmology 2005
Volume 11, Number 8
April 1, 2005
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| Diagnosis and Discussion | Ocular neuromyotonia (ONM) is a rare disorder that most commonly involves the oculomotor nerve followed by the abducens nerve and then the trochlear nerve (3). It was first described by Clark in 1966 (4), but Ricker and Mertens coined the term “ocular neuromyotonia.” (5) It is characterized by(6):
1. intermittent diplopia lasting seconds to minutes
2. The amount and direction of intermittent angles of deviation may vary without any pattern.
3. Provocation of these episodes by changes in the gaze direction or after holding eccentric gaze (7)..
4. Phases characterized by overaction of isolated single muscles.
Previous radiation to the sellar or parsellar regions is the most common cause (8,9). This is the most likely etiology of ONM in our patient, however, recurrence of his nasopharyngeal tumour with metastasis to the cerebrum cannot be ruled out without the proper imaging. ONM have been attributed to other causes like supraclinoid aneurysms, infectious cavernous sinus thrombosis, alcohol abuse, arachnoiditis, basilar artery dolichoectasia, and Graves’ disease (10-13). Other cases of OMN resulting from radiotherapy of nasopharyngeal cancer have been documented (14-15). The latency between radiation and onset of diplopia varies between 2 months and 18 years (6).
Although the exact pathogenesis of OMN is unknown, there is wide acceptance of the segmental demyelination hypothesis (16). The membrane potential traveling within the axons which is normally insulated from the surrounding by the myelin sheath is exposed due to damage from causes mentioned above to result in atypical patterns of neural transmission and muscle activation – ephaptic transmission (lateral or “cross-talk”); reflection (reversal of transmission); and proximal branching of sprouting axons. The growth of axons within a damaged fiber (sprouting) may conduct orthodromic impulses which are reflected and conducted in an anti-dromic fashion. This mechanism can establish a reverberating nerve circuit. The speed, number and specific connections of the sprouting fibers dictate the degree and specifics of the response, giving the dynamic nature of ONM (17). Supporting this hypothesis is electrophysiologic testing revealing neuronal degeneration and impaired nerve conduction after radiation therapy(18,19). In addition, eye movement recordings were consistent with spasms of the involved muscles (20).
Spontaneous firing of neurons supplying a muscle may spread to the other neurons supplying the other extraocular muscles or ephaptic transmissions between the axons may result in the co-contraction of the other extraocular muscles. Abdulla and Eustace described a case where oculomotor nerve neuromyotonia was replaced by optic nerve neuromyotonia (21). In this 48 year old woman, phasic constrictions of the pupil developed after surgery for a giant aneurysm of the internal carotid artery in the cavernous sinus. Two years later this was replaced by intermittent involuntary cyclic spasms elevating the ptotoc lid which were not elicietd with any eye movement or by increased accomodation.
In our patient with the bilateral VI nerve palsy, the weak VI nerves resulted in the small esotropia. However, on sustained lateral gazes to both directions, the overfiring of the lateral rectus resulted in the large exotropia. Radiation may be a relatively effective way to achieve the unique a pattern of nerve injury without cell death that gives the potential to develop ONM (17). A combination of impaired phasic firing in agonist muscles and tonic contraction of the antagonist muscles explains the paroxysms of eye movement limitation in this disorder (22). With horizontal muscle weakness there may be a small amount of intermittent hypotopia, especially when the vertical and oblique muscles cannot compensate in the primary position.
Following the trial of prism glasses, carbamazepine will be suggested. The pharmacological effect of neural membrane stabilization to inhibit inappropriate action potential transmission effective in treatment of ocular neuromyotonia helps to support the pathogenesis. (6) Alternatively, eye muscle exercises may help as Safran and Magistris reported a case where the patient with fourth nerve neuromyotonia discovered that each episode of tonic ocular depression could be terminated by forcefully directing her gaze upwards. (23) Further stretching an abnormally innervated muscle, as is the case with ONM, may eventually cause some crease in the force generated by the stretched muscle. (24) Although not apparent on clinical history and examination, thyroid studies should be ordered to rule out Graves’ disease as this may be an easily reversible cause of GL’s ONM.(13)
In patients presenting with intermittent diplopia, it is important to rule out benign disorders like ocular neuromyotonia that may respond to conservative treatment and not perform strabismic surgery given the variable nature of the deviations. Furthermore, serious pathology leading to the ocular neuromyotonia needs to be ruled out.
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