ABCA4 and atrophic macular degeneration: Because of peripapillary sparing, the nasal edge of the RPE atrophy in Case 4a is concave towards the disc, while the nasal edge of the RPE atrophy in Case 4b is convex towards the disc; (5) the presence of a small island of central foveal sparing surrounded by an annular area of atrophy (as in Case 4a) is more suggestive of ABCA4-RD, although this can occasionally be seen in extrafoveal geographic atrophy from AMD; (6) the presence of a shallow irregular PED adjacent to the area of atrophy in Case 4b is associated with AMD rather than ABCA4-RD.