Blind spot size depends on the optic disc topography: a study using SLO controlled scotometry and the Heidelberg retina tomograph.

AIMS To find out whether the size of the blind spot area, determined by static perimetry, depends on the surface topography of the optic disc and its surrounding area. METHODS Ten eyes were examined; all had a parapapillary atrophy adjacent to the temporal side of the disc. Microperimetry was performed under direct fundus control using a Rodenstock scanning laser ophthalmoscope. The horizontal meridian of the optic discs was examined in 0.5 degree steps using five stimulus sizes (Goldmann I to V), each with 10 different degrees of brightness. Optic disc topography was measured with the Heidelberg retina tomograph (HRT). RESULTS Stimuli with a high luminance level (Goldmann IV, 4 dB), presented on the horizontal meridian, were seen up to 0.75 degree centrally (that is, towards the optic disc centre) from the temporal edge of the parapapillary atrophy but up to 1.85 degrees centrally from the nasal optic disc border (p < 0.01). Horizontal HRT section profiles of the optic disc consistently showed prominent nasal disc borders contrasting with a shallow excavation within the temporal parapapillary atrophy. CONCLUSIONS The size of scotomas depends on the surface topography of the tested area. The prominent nasal part of the optic disc appears less 'blind' than the shallow temporal part, probably because of more intensive light scattering by the prominent nasal part of the disc. These considerations should also apply to other scotomas.