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I. Cacho, C.M. Dickinson, R.A. Harper, B.C. Reeves, H.J. Smith; The Use of Visual Function Measures for Predicting a Relocating Prism for Patients With Amd . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3090.
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Purpose: The power and base direction of the prism required to relocate the retinal image on to functioning retina for subjects with age-related macular degeneration (AMD) can be determined subjectively. This study investigated how well the required prism can be predicted from measures of visual function. Methods: We recruited 192 subjects (120 females and 72 males) with a mean age of 80 years (range 56-97) to a randomised controlled trial of prism relocation spectacles. Visual functions measured included distance logMAR acuity (ETDRS charts), word reading acuity (Bailey-Lovie M scale), contrast sensitivity (Pelli-Robson chart) and scotoma area (Bjerrum perimetry). Prism power and base direction were determined using a subjective bracketing technique while the subject viewed a projected image subtending 30°. Results: The average prism power was 7.3Δ (range 2 to 15). Prism base direction in the superior hemi-field was chosen by most patients (63% versus 22% for inferior hemi-field and 15% exactly on the horizontal), with 90°commonly being the preferred position. The mean logMAR distance visual acuity was 0.92 log MAR (range 0.42 to 1.60). Mean values for other visual functions were 3.56 M for word reading (range 0.60 to 10.00), 0.96 log CS for contrast sensitivity (range 0.05 to 1.65) and 160 grid units2 for the area of the scotoma (range 0 to 1529). Regression analyses indicate strong associations between all visual functions and prism power; prism power was estimated to increase by 0.37Δ per logMAR line (p<0.0001), by 0.24Δ per M scale unit (p<0.0001), by 1.8Δ per log CS unit (p<0.003) and by 0.32Δ per 100 grid units2 (p<0.0001). Conclusions: Subjectively prescribed prism power shows strong associations with logMAR visual acuity, word reading acuity, contrast sensitivity and area of scotoma. However, the magnitude of variance in prism power strength explained by visual function variables is relatively small, implying that for a large number of patients, the predicted power would not closely match the power obtained by the subjective method. The superior retina appears to be preferred for prism relocation, suggesting improved function for rehabilitation.
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