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Visual field defects respecting the vertical midline are a common occurrence associated with focal neurological lesions. However, unilateral nasal hemianopsias are rare defects, documented to be associated with pituitary adenomas, temporal optic nerve lesions, and suprasellar aneurysms.1 Cataracts are known to depress the overall sensitivity of the visual field,2,3 but localised visual field defects due to cataract are extremely rare and, to our knowledge, only three other cases have been reported in the literature.4–6 We report a case of a right unilateral nasal hemianopsia resulting from central posterior subcapsular lens opacity.
A 51 year old woman treated for normal tension glaucoma in her right eye for 2 years attended for a review of her glaucoma following a change of medication with the addition of bimonidine eye drops to dorzolamide eye drops. At this 3 monthly review the patient gave a 1 month history of a sudden onset of misty vision affecting her right nasal visual field noticed while driving her car. There were no other associated neurological symptoms. Just before this she had been diagnosed with “borderline” systemic hypertension. There were no other risk factors for a vascular event, although there is a positive family history—her father had had a cerebrovascular accident.
On examination, her visual acuity had deteriorated from 6/6 to 6/24 in the right eye, remaining unchanged at 6/6 in the left since the previous visit. It had also been documented that letters on the nasal side of the Snellen chart were not seen with the right eye. Confrontation visual field demonstrated a nasal hemianopsia of the right eye. Her pupils were equal with normal reactions to light and accommodation. Dilated slit lamp biomicroscopy revealed marked central posterior subcapsular lens opacity with very mild subcapsular changes in the other eye, previously documented as normal. Retinal examination was normal and the optic discs pathologically cupped with inferior rim thinning; changes consistent with glaucoma, although there were no documented changes from the previous visit 6 months earlier.
Further neurological and cardiovascular examination, other than a blood pressure of 170/70 mm Hg, were also unremarkable.
The right nasal hemianopsia respecting the vertical midline was confirmed on a clinically reliable Humphrey 24-2 plot with a change in mean deviation from −5.94 to −17.43 from the previously documented visual field (see Fig 1). The visual field of the left eye was normal. Routine blood tests and chest x ray were normal. A computed tomograph (CT) scan of the brain, orbits and visual pathways was also unremarkable. In the absence of a focal neurological lesion this woman subsequently underwent an uncomplicated right phacoemulsification and intraocular lens replacement. A repeat red spot visual field revealed complete reversal of the previously documented right nasal hemianopsia, and a restoration of the visual acuity to 6/6.
Media opacities are known to cause visual field defects,7 the degree of which varies from generalised depression of the visual field8 to apparent scotomatous areas. Localised paraxial lens opacities causing defects mimicking neurological abnormalities are extremely rare. These opacities necessitate a posterior position in the lens to produce a relative scotoma. An opacity in the media anteriorly placed produce generalised reduction in the visual field. In the previous three reported cases all the cataracts were posterior subcapsular in nature.
Our case is unusual in that the cataract was placed centrally, not temporally as may be expected with a nasal defect. Further, the history suggested a sudden onset which necessitated neurological examination and investigation.
We feel it would have been inappropriate to proceed to surgical intervention without previous investigation, which should always include a CT scan (more preferably, if readily available, an magnetic resonance imaging (MRI) scan), of the optic nerves and visual pathways. This unusual visual field defect may have been present for some time before the appearance of the cataract but, if subtle, may not have been evident. However, the presence of the cataract may have decreased the sensitivity of the eye and made the visual defect more prominent. Our concern with this woman was that removing the cataract may have disguised a more sinister underlying pathology—that is, a neurological defect, so we decided on a red spot visual field to confirm that such a defect was indeed not present.
Whereas in this case the right nasal hemianopsia was due to a posterior subcapsular cataract, we believe that a neurological cause for the field defect should always be sought, particularly with a history of sudden onset. Cataract extraction should be considered only establishing the absence of the same. Further, consider performing a postoperative red spot visual field test to confirm the absence of focal neurology in such eyes.