I Cacho¹, C M Dickinson¹, H J Smith¹, R A Harper²
¹UMIST, Optometry and Neuroscience, Manchester, United Kingdom; ²Manchester Royal Eye Hospital, Academic Department of Ophthalmology, Manchester, United Kingdom
Purpose:
To investigate a simple method to prescribe prism spectacles for patients with age-related macular degeneration (AMD). Static (Humphrey) and Kinetic (Bjerrum) perimetry were used to investigate the relationship between the prism power and base direction of a "relocating prism" giving optimum acuity, and the linear distance and direction of the closest non-scotomatous retinal location (NRL) to fovea.
Method:
Seventeen subjects with long-standing AMD were examined. In addition to perimetry, logMAR distance VA, near VA (Bailey-Lovie) and Pelli-Robson contrast sensitivity were measured. Relocating prism was prescribed using a bracketing technique whilst the subject viewed a projected image subtending 30°.
Results:
A base direction in the superior hemi-field was preferred by most patients (56% versus 25% for inferior hemi-field and 19% exactly on the horizontal). The average prism power was 6.4 prism diopters (Range 2 to 12). Correlations were 0.45 between prism power and the linear distance from the fovea to the closest NRL, and 0.54 between prism direction and direction to closest NRL: 10 out of 17 subjects had these two directions within 30°.
Discussion:
The Bjerrum screen appears to be a good semi-quantitative method to determine the most useful NRL for prism relocation and it can be used as a starting point for prescribing. The lack of precise correlation suggests that subjects do not necessarily choose the closest NRL, but may be influenced by the size of this area when the task subtends a large angle. The superior retina appears to be preferred for prism relocation, suggesting an improved function for rehabilitation
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