Photoreceptor topography in ageing and age-related maculopathy

The relative rate of rod and cone degeneration is a fundamental characteristic of any disorder affecting photoreceptors, including ageing and age-related maculopathy (ARM). The macula consists of a small cone-dominated fovea surrounded by a rod-dominated parafovea. In donor eyes with grossly normal maculas, the number of foveal cones is stable and the number of parafoveal rods decreases by 30% over adulthood. These trends continue in early ARM. In exudative ARM, the photoreceptors that survive over disciform scars are largely cones, and rods decline precipitously in relation to thick subretinal pigment epithelium deposits. The preferential vulnerability of rods over cones has been confirmed by recent functional studies showing that the loss of scotopic sensitivity is greater than the loss of photopic sensitivity throughout adulthood and in patients with early ARM. A hypothesis that these effects are due to to retinoid deficiency at the level of the photoreceptors is proposed. The topography of rod loss in ageing and ARM is consistent with the location of early ARM lesions described in population-based studies and is not consistent with the location of fundus autofluorescence due to lipofuscin.