Electrophysiological investigation of age-related macular degeneration

Age-related macular degeneration (AMD) affects 12.7 million people in Europe and North America (Klein et al., 1995 Klein et al. 1999). As a combination of decreasing birth rate and increasing longevity alter the demographic of the population, the impact of this disease can only increase. This places an immense burden, not only on the individuals afflicted by the condition, but on the financial resources of society as a whole. Unfortunately, treatment for AMD is still very restricted, and even our understanding of the pathogenesis of the disease is far from complete One concern in tackling the growing problem of AMD is that methods used in the assessment of the condition are limited, usually based on fundus appearance and visual acuity. The aim of this study was to develop a battery of electrophysiological tests which would be sensitive to the most subtle changes in retinal function in AMD. Such tests may aid diagnosis, provide a more sensitive measure of disease progression, and allow an early identification of phenotypic subtypes. Protocols were included for the recording of the focal rod ERG, the focal cone ERG, the S-cone ERG and the dynamic focal cone ERG, along with psychophysical tests of colour vision and dark adaptation. These tests were then applied to 31 subjects with ARM (12 with bilateral ARM, 11 with unilateral wet AMD and 8 with unilateral dry AMD), and 28 controls. In the analysis of ERG amplitudes a ratio of focal to full-field amplitude was introduced as a novel means of reducing intersubject variability in response. This was found to increase the accuracy of all tests in distinguishing between subject groups. The greatest separation between ARM and control groups was provided by the dynamic tests of visual function i.e. rod-cone break time of the dark adaptation function, and time constant of recovery of the dynamic focal cone ERG. The time to rod-cone break also showed potential in identifying subjects at increased risk of exudative retinal changes. Subjects were assigned to groups in this study on the basis of fundus appearance. However, individuals within each subject group showed a range of retinal function which belied the homogeneity of retinal signs. This raises the question of whether 'form' or 'function' should form the basis of classification and assessment of individuals with ARM and AMD