Changes in fixation in the presence of prism monitored with a confocal scanning laser ophthalmoscope

Background: The main optical effect of an ophthalmic prism is to deviate uniformly the entire field of view seen through the prism, resulting in an equal eye movement, if fixation on the same object is to be maintained. Conversely, it has been suggested that, in the case of low vision due to central scotoma, the eye changes its eccentric viewing behaviour when a prism is introduced, that is, the eye remains stationary with a change of retinal image location. Method: A new method is described in which retinal image position was recorded with a confocal scanning laser ophthalmoscope in four experimental conditions. Results: Results showed that the normally-sighted eye rotates rapidly to compensate for the introduction of a prism. Conclusion: Future studies are required involving subjects with central scotoma but it is likely that many subjects with low vision will also refixate behind a prism and that the prescribing and wearing of bilateral prism in cases of central vision loss will have limited effectiveness in the demonstration or training of eccentric viewing.