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Editor,—Gaze evoked amaurosis is an uncommon symptom usually associated with intraconal masses such as cavernous haemangioma1 or optic nerve sheath meningioma.2 3 Patients describe transient loss of vision in eccentric positions of gaze with full recovery of vision on returning to the primary position. The cases previously published have all been unilateral. We describe a case of bilateral gaze evoked amaurosis in a patient with dysthyroid orbitopathy.
A 62 year old smoker gave a 12 month history of transient bilateral loss of vision on upgaze associated with supraorbital discomfort. His hobby was flying radio controlled model aeroplanes but after crashing two of these during amaurotic episodes he presented to an ophthalmologist. Two years previously he had required admission for cardiac failure associated with thyrotoxicosis but after treatment with iodine-131 and carbimazole his thyroid status had been stabilised.
Ophthalmic examination revealed symmetrical axial proptosis (26 mm), restriction of upgaze, and lid signs consistent with dysthyroid orbitopathy. In primary position the corrected distance acuities were 6/6 in both eyes with 17/17 Ishihara colour plates seen and full visual fields. On upgaze the distance acuities were reduced to less than 6/60 with none of the Ishihara colour plates seen. The intraocular pressures (IOP) increased from 20 mm Hg (both eyes) in primary position to 30 mm Hg (right eye) and 31 mm Hg (left eye) in elevation. Pupil examination using infrared pupillography showed 0.20 mm dilatation of both pupils and 15% reduction in light reflex amplitude on upgaze compared with the primary position. The disc and fundal appearances were normal and did not change with direction of gaze. On general examination the patient had pretibial myxoedema but appeared clinically euthyroid.
The pattern VEP in both eyes was normal in the primary position but reduced in amplitude on upgaze (Fig 1A). Fluorescein angiography showed normal perfusion of the optic disc and retina both in primary position and in upgaze. Orbital computed tomography revealed moderate enlargement and oedema in all extraocular muscles; no tumour or other mass lesion was present (Fig 2). The patient was treated with oral steroids (prednisolone 40 mg/day). One month later there was symptomatic improvement, 2–3 mm less proptosis, and the distance acuities had improved to 6/36 (right eye) and 6/60 (left eye) on upgaze. There was no change in the IOP rise associated with elevation of the eyes. The pupils dilated less on upgaze (0.05 mm) and there was less attenuation of the light reflex amplitude (5%) compared with before treatment. The pattern VEP showed improvement on upgaze (Fig 1B) compared with before treatment. The steroid dose was tapered off over the next 6 months with no relapse of his symptoms. The patient has now returned to his hobby of flying model aeroplanes.
The visual loss in this patient was transient, reversible, and related to the position of the globe in the orbit. The only orbital pathology found on imaging was enlargement of the extraocular muscles related to his dysthyroid orbitopathy. The mechanism of this visual failure remains intriguing. Its rapid onset and reversibility suggest vascular compromise but fluorescein angiography showed normal disc and retinal perfusion on upgaze. Vascular compromise of the retrobulbar optic nerve cannot be ruled out. Of interest, in the rat model retrobulbar ischaemia produces a depolarising conduction block which takes minutes rather than seconds to develop and which would generate photopsia before the visual failure.4 In previous reports, raised IOP has been invoked as the cause of the visual loss.5However, in our patient the rise in IOP was only moderate and persisted after steroid treatment whereas the vision improved. A third possibility is that compression of the optic nerve by enlarged extraocular muscles or stretching of the nerve as a result of dural tethering produced a mechanical conduction block in upgaze. This has been described in peripheral nerves6 but it classically takes several days for full recovery of function; whether optic nerve axons behave similarly is not known. Gaze evoked amaurosis has not been previously described in thyroid eye disease but was noticed by this patient because it interfered with his hobby of flying radio controlled model aeroplanes.