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Editor,—The electroretinogram (ERG) and visual evoked potential (VEP) to pattern reversal stimulation became degraded in association with visual loss in a young patient with gaze evoked amaurosis who had a mass at the apex of the left orbit. His visual symptoms improved following left orbital decompression and Snellen acuity was maintained at 6/6; however, the pattern VEP was attenuated and degraded compared with before surgery.
A 9 year old boy with neurofibromatosis type II initially presented with a history of intermittent visual disturbance affecting the left eye when in left gaze. The right eye had a retinal hamartoma and a visual acuity of 6/12. He had undergone previous surgery to correct a right divergent squint. A partial third nerve palsy on the left side was present.
On examination, visual acuity of the left eye was 6/6 when viewing in the primary position, and pupillary reactions were normal. However, acuity became perception of light on sustained left lateral gaze, and a mild relative afferent pupillary defect became detectable at this stage in the left eye. Intraocular pressure by applanation tonometry was 10 mm Hg in the right eye and 12 mm Hg in the left. No proptosis was detectable. Ophthalmoscopy revealed a normal right optic nerve and marked swelling of the left optic disc (Fig 1). An enlarged blind spot was evident on visual field testing.
Computed tomography and magnetic resonance imaging demonstrated a small rounded mass just to the left of the brainstem which appeared to involve the third nerve and extended forwards to the cavernous sinus. The mass was thought to represent a third nerve neuroma. In addition, imaging showed the optic chiasm was displaced downwards. There was also small mass extending forwards medial to the optic nerve at the apex of the left orbit and this was thought to be an intraorbital extension of the third nerve neurofibroma (see Fig 2).
ERG and VEP recording following flash and pattern reversal stimulation gave normal responses for each eye when looking straight ahead. However, the pattern ERG (recorded with a corneal electrode) and pattern VEP to 50′ checks occupying a large stimulus field (28 degrees horizontally by 21 degrees vertically) became progressively degraded and eventually non-detectable on sustained left lateral gaze (see Fig3). In contrast, the flash ERG and VEP were attenuated to a relatively minor extent on sustained left gaze (on average, 14% smaller). Retinal and occipital pattern responses normalised rapidly when the eyes returned to straight ahead viewing. He underwent an optic nerve decompression, following which he had some degradation of the pattern VEP but did not experience any further episodes of gaze evoked amaurosis.
Amaurosis fugax is a well recognised symptom associated with carotid occlusive disease in older patients. Our case demonstrates that it may also be a sign associated with an intraorbital mass, and can occur in younger patients.
Gaze evoked amaurosis has been reported to occur occasionally in patients with cavernous haemangiomas, optic nerve sheath meningiomas, and osteoma.1 2 Mechanisms proposed to cause gaze amaurosis include optic nerve or retinal ischaemia, disruption of axonal conduction due to optic nerve compression, and compression leading to ischaemia of the globe.1 3 Our case had an intraorbital extension of the third nerve neurofibroma down the medial branches of the lower division of the third nerve. The left optic nerve appeared to be mildly stretched over the mass, and to become more stretched on extreme left gaze. This process is similar to that described by Manor et al 4 in 1996 involving reading evoked amaurosis. Interestingly, in our patient, pattern ERG and VEP testing indicated that, during the periods of gaze evoked visual loss, macular function was more severely compromised, compared with extramacular function (reflected in flash ERG and VEP results). The rapid reversibility of the pattern ERG changes suggests that ischaemic mechanisms, rather than disruption in axonal conduction by direct compression, may be predominantly responsible for the visual loss. This notion is supported by the observations of Knapp et al5 who used Doppler blood flow studies to demonstrate reduction of blood flow in the central retinal artery during gaze evoked amaurosis. Our patient had no further episodes of amaurosis after the surgery. Visual acuity remained 6/6 postoperatively. It thus appears from our case, and other cases, of gaze evoked amaurosis that the transient visual loss is not predicative of permanent disability, but that its presence can be an important clinical sign and indicate optic function near the threshold of disruption.
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