Scanning laser ophthalmoscope recording of fundus movement in the presence of an ophthalmic prism

Introduction: 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. The purpose of this study is to demonstrate that the above statement is correct. Conversely it has been suggested that the eye changes its centric or eccentric viewing behaviour when a prism is introduced i.e. the eye remains stationary and does not refixate. Method: With the confocal scanning laser ophthalmoscope (CSLO), a laser beam (632.8nm) is focused onto the fundus by the optics of the eye and is reflected back through the scanning system resulting in a high resolution, high magnification, video image in real time. Three normally sighted observers observed a target while the CSLO imaged the right retina. Three situations were compared; a) a 4 prism dioptre (PD) introduced over the left (fixing) eye, causing a fixation shift b) a 4 PD introduced over the right eye causing a fixation shift, but an equal shift of the optics of the CSLO c) 4 PD introduced over the right eye, while the left eye fixated, causing a movement of the retinal image, but no fixation shift. Results were normalised against a model eye (no ocular movement). Results: The movements in these situations were recorded and demonstrate that an eye refixates for a prism and that, if the optical path of the CSLO also passes through the prism, the retinal image of the CSLO will shift by an equal amount. Conclusion: The results support Bailey, Long and Woo's view that there is no reason to believe that the normal eye will change its habitual centric or eccentrc viewing behaviour when a prism is introduced. Future studies will be conducted to determine the behaviour of eyes with central scotoma resulting in eccentric fixation.