The effect of prism on preferred retinal locus

Whether prism, especially base-up prism, affects the area of the retina used for fixation in a patient with central scotoma has been a controversial subject for 35 years. Our pilot study employed microperimetry to evaluate the effect of base-up prism on the fixation locus, or preferred retinal locus (PRL), in subjects with central scotoma.

We used a microperimeter to assess the PRL in 13 visually impaired subjects with central scotoma under four conditions: no lens, a lens with no prism (control lens), 6^Δ base-up, and 10^Δ base-up. The PRL was measured in degrees in horizontal and vertical co-ordinates from the centre of the optic disc using graphical analysis.

The PRL with the control lens was not significantly different from the PRL with no lens. The preferred retinal loci with the two powers of prism were compared to the control lens and showed a superior shift in 22 of 26 cases (84.6 per cent). The amount of movement was significantly different from zero (p = 0.001 for 6^Δ and p = 0.004 for 10^Δ ). The vertical movement with the 10^Δ prism (1.73 ± 1.73 degrees) was not significantly greater (p = 0.562) than with the 6^Δ prism (1.37 ± 1.08 degrees). The shift was significantly less than the prism powers used (p < 0.001), and the amount of vertical relocation was not significantly different from the amount of horizontal movement.

In our study, base-up prism appears to shift the PRL in the direction of the prism base most of the time, but our findings do not support the use of prism as a way of predictably relocating the PRL. More study is indicated to evaluate whether such a small shift is clinically or functionally significant.