Investigations or perimetry and gaze-stability in healthy and deceased retina

Abstract

Our visual world is composed of images of colours, textures, edges and contrasts. In addition, these images may be moving or flickering. The goal of visual testing is to quantify these functions. Traditionally we have tested visual function as visual acuity (VA) - the capacity to discriminate the fine details of objects and visual field (VF) - the portion of space in which objects are visible at the same moment during steady fixation of gaze in one direction. Perimetry is the systematic measurement of visual field function. The limits of the visual field are mapped to lights of different sizes and brightness. These maps of visual fields are very important when diagnosing diseases of the visual system.

The accuracy of the conventional VF techniques relies on the assumption that the subject's fixation/gaze is kept foveal and stable during the examination and is therefore inadequate for accurate evaluation of the VF and fixation/gaze stability in disorders in the visual system where the patient often has unstable or extrafoveal fixation/gaze. The fixation/gaze is often quantified by using the Bivariate Contour Ellipse Area (BCEA) method for identifying the area used during fixation. Since this kind of measurements can give misinterpretation of the data if the material is not normally distributed, we have developed a method called CG (Center of Gravity), were no consideration is necessary to whether the material is normally distributed or not.

Methods: A Scanning Laser Ophthalmoscope (SLO) was used to investigate the possibility to use the CG-method for studying the fixation and gaze-stability in 31 normals compared to 15 patients who viewed a solar eclipse. Furthermore in the solar eclipse patients the SLO results were compared with the subjective experience, VA and funduscopic appearance during a followup time of twelve months.

Results: The results from the present studies show that the mean distances from the fixation cross to the CG in our normal subjects were 0.27 degs, thus well within the retinal area for highest visual resolution (0.5 degrees). Further our investigations show that, no correlation was found between the location of the presented stimuli and the fixation of each subject. In the solar eclipse patients all scotomas included the central fovea, which corresponds to the fixation mark. These patients also presented foveal lesions such as edemas, lamellar macula holes and distinct retinal pigment epithelia (RPE) disturbance, and such lesions could be detected in all 15 patients with the SLO at all visits, but with traditional funduscopic exams, only in 7 patients during the initial visits and two at visit 2 (three months) and 3 (12 months), respectively.

Conclusion: The results of the present studies thus have implications for the interpretation of the results obtained from a conventional perimetry session where the fixation location never can be exactly measured. The abilities of the SLO make it a helpful diagnostic and data collection instrument that provides information complementary to standard clinical techniques. Together with knowledge about fixation/gaze in healthy subject, this technique may provide a possibility of early detection of macular pathology. This will be important in early intervention, and insights in the development of diseases in the retina and the visual pathway