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A 67-year-old woman with shaking of the world
Digital Journal of Ophthalmology 2008
Volume 14, Number 12
June 15, 2008
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Stephen Turner | King’s College Hospital, London
Paul Riordan-Eva | King's College Hospital, London
Steve Connor | King's College Hospital, London
Michael Samuel | William Harvey Hospital
Diagnosis and Discussion
In 1970, Hoyt and Keane reported five patients with intermittent monocular oscillopsia and binocular vertical diplopia in whom unilateral repetitive torsional eye movements were seen on slit-lamp examination.(1) They hypothesized that fourth nucleus inhibition was the cause of what they termed superior oblique myokymia (SOM). The earliest similar case was reported as a unilateral rotary nystagmus by Duane in 1906.(2)

SOM can be readily diagnosed on the typical history and clinical findings. Repetitive contraction of the superior oblique muscle gives rise to high-frequency, low amplitude, tremor with torsional and/or vertical monocular eye movements, which may be exacerbated by ipsilateral head tilt or downward and inward movement of the affected eye, such as for reading. The presence of torsional oscillopsia and/or nystagmus seen in this case is fairly specific for superior oblique myokymia. Right SOM is reported to be more common than left.

The diagnosis in this case is essentially a clinical one, with the vast majority of cases either having no identifiable etiology or being due to fourth-nerve compression by an aberrant artery. Occasionally, these findings have been reported with posterior fossa tumors, dural arteriovenous fistulas, adrenoleucodystrophy, as well as after fourth nerve palsies, brainstem strokes and head trauma.

High resolution MRI has confirmed Bringewald’s hypothesis; that is, SOM may result from microvascular compression of the root exit zone of the fourth cranial nerve on the dorsal surface of the midbrain. Similar compression of the fifth, seventh, and ninth cranial nerves is also known to occur in trigeminal neuralgia, hemifacial spasm and glossopharyngeal neuralgia, respectively.(3,4) It is important to distinguish between compression at the root exit zone, which extends up to 1.2mm from the point of exit from the brainstem, and compression of the more distal (cisternal) segment of the fourth nerve. Compression of the distal segment occurs in 60-90% of individuals and is not relevant to the pathogenesis of SOM.(5) Resolution of SOM following surgical microvascular decompression has been reported.(6,7)

Conservative treatment is suitable for some patients. Systemic and topical beta-blockers have been reported to be beneficial, the latter presumably due to a direct stabilization of muscle cell membranes.(8,9) Thus, topical timolol eyedrops are a reasonable first line treatment in patients with troublesome symptoms and no contraindications. Traditionally, carbamazepine has been the preferred oral agent, but other agents besides beta-blockers, such as gabapentin or baclofen also have been reported to be beneficial.(10) More recently, substantial improvement in symptoms with memantine treatment was reported in one patient who had suffered side-effects with gabapentin.(11) If symptoms are precipitated by a particular gaze position, prisms may be tried. In cases resistant to medical treatment, superior oblique tenectomy has been advocated, but usually needs to be combined with inferior oblique muscle weakening to compensate for the induced superior oblique palsy. The role of microvascular decompression has yet to be established. There is a tendency for symptoms to spontaneously improve in some cases, therfore conservative follow-up is recommended before any form of surgical therapy is considered.