Clinical uses of the scanning laser ophthalmoscope

Abstract

Summary The scanning laser ophthalmoscope (SLO), designed in the early 1980s, has many advantages over conventional imaging techniques. Continuous, real-time images are displayed on a TV monitor and usually recorded on video-tape. Images can be obtained without pupil dilation and it is often possible to image through cataract and other media opacities. The lasers used for retinal imaging, helium-neon, argon and infra-red diode, are low powered and the light levels used in the SLO are about 1000 times less than in conventional fundus photography. Fluorescein and indocyanine green angiography can be performed and the dynamic sequences recorded on video-tape. Several systems are available, for example Rodenstock, Zeiss and Heidelberg, and each have specific features. Typically the field of view is 20 or 40 degrees. Instruments can work in a non-confocal, confocal or indirect mode. While the non-confocal mode is used for general retinal imaging, in the confocal mode, thin slices of a structure can be imaged, e.g. the optic nerve head, and by means of a computer three-dimensional reconstruction is possible. In the indirect mode, the image is formed from structures that scatter light, e.g. the choroidal vessels. The SLO also has the ability to perform psychophysical measurement. Any graphics which can be displayed on a TV monitor can be encoded into the scanning laser beam. The stimuli can be altered in character, size, contrast and velocity. The tests that allow assessment of visual function include visual acuity, retinal perimetry and measurements of fixation stability. Scrolled text can be used to allow precise evaluation of reading ability at different retinal locations. Using these tests we have investigated residual vision in patients with macular disease. Microperimetry was able to define accurately the size and location of retinal lesions and in some patients residual visual function was detected in areas that appeared to be non-functioning. Some patients had fixation stability which was exceptionally poor whilst in others it was normal. The ability to recognize scrolling text declined with increasing retinal eccentricity and decreasing text contrast. The imaging and psychophysical abilities of SLOs are providing valuable information on patients with retinal diseases.