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Patients with central scotomas must use peripheral retina in place of the damaged fovea. Many patients exhibit a “preferred” retinal locus (PRL) for fixation.1–4 Previous studies have only described monocular fixation behaviour. This case report describes a patient who successfully uses a novel strategy of observing with two non-corresponding PRLs in different eyes for different tasks.
A 78 year man attended the clinic with a 7 year history of exudative age related macular disease (AMD) in the right eye and an 18 month history of exudative AMD in the left eye. The patient reported using his right eye for distance vision and his left eye for reading. He covered the contralateral eye for both tasks. He has no history of amblyopia. As a young man, the patient used his right eye for rifle shooting. He is right handed.
On fundus examination, both eyes show disciform scars at the macula, with a larger lesion in the right eye. Best corrected visual acuity was 0.92 logMAR (6/48) in the right eye and 0.60 logMAR (6/24) in the left. Contrast sensitivity, measured using a Pelli-Robson chart, was 1.05 log units in each eye. With optical correction (using hyperocular lenses) reading speed was 85 words/minute, using the left eye. “Fluent” reading can be defined as being faster than 80 words/minute.5
Fixation behaviour was measured for each eye using a scanning laser ophthalmoscope (SLO). The patient was asked to observe a cross-shaped target of height 2.5° for a period of 10 seconds. Fixation stability was assessed during this task by calculation of the bivariate contour ellipse area (BCEA).6 The BCEA is a measure of the area of an ellipse which encompasses a given proportion of fixation points (in this case, 68%). Smaller BCEA values correspond to more precise fixation. BCEA values were 14 900 minutes of arc2 for the right eye and 5360 minutes of arc2 for the left. BCEA values for normally sighted observers are typically around 450 minutes of arc2.7
The size of the dense scotoma was measured using SLO microperimetry, with stimuli being Goldmann III size targets of 200 cd/m2 presented for 200 ms.8 Digitised SLO images were recorded (figs 1 and 2).
The left eye had better distance visual acuity, a smaller dense scotoma and a less peripheral, more stable PRL than the right eye. It is unsurprising that this patient uses this eye for reading. Unexpectedly, he found his right eye (which has poorer distance visual acuity) to be of more use for navigation and other distance tasks. Such visual behaviour can not be explained on the basis of the retinal lesions. It is assumed that the previous dominance of this patient’s right eye must play a part in determining which eye is preferred for distance vision. Examination of the SLO images shows a large area of healthy retina adjacent to the PRL in the right eye whereas in the left eye the PRL is constrained by the optic disc and the macular lesion.
Although the PRLs are in the same quadrant of retina in each eye, the PRL in the right eye is further away from the previously normal fovea than that in the left. The fact that the patient covers his better seeing left eye while looking to the distance reinforces the fact that these loci are not in corresponding retinal locations. On detailed questioning, the patient cited the greatest difficulty with his vision being for wiring a plug or setting a combination lock on a suitcase: tasks requiring good binocular function.
This patient’s behaviour suggests that for distance vision, a large “window” of functioning retina is of more use than a smaller region of retina with better visual acuity. It also indicates that it is possible to use different, non-corresponding preferred retinal loci in each eye for different tasks. It is not straightforward to predict which retinal location, or even which eye, patients will use from clinical features alone.
This patient has selected different PRLs for near and distance vision to partially ameliorate the symptoms of his macular disease. Although unable to perform detailed binocular tasks, he is able to navigate successfully, to read correspondence, and to remain independent.
The authors thank Jenni Turner for referring the patient to us and the patient for his enthusiastic cooperation in the writing of this case report. MDC is supported by Guide Dogs for the Blind Association ophthalmic research grant 2000–29a. SAK is supported by a Colin Kunkler Memorial Fellowship.
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