The axial length/disc area ratio in anisometropic hyperopic amblyopia : a hypothesis for decreased unilateral vision associated with hyperopic anisometropia

Abstract Objective Hyperopic anisometropia is a risk factor for amblyopia. Disc areas and axial lengths are reduced in amblyopic eyes. This study's aim was to investigate the differences in the relative size of the optic disc in 5 groups of eyes: (1) normal right eyes; (2) smaller eyes of nonamblyopic, anisometropic, bilateral hyperopes; (3) larger eyes of nonamblyopic, anisometropic, bilateral hyperopes; (4) fellow (nonamblyopic) eyes of eyes of amblyopic, anisometropic, bilateral hyperopes; and (5) amblyopic eyes of bilateral hyperopic anisometropes. To determine the relative size of the discs, the relationships of axial length to disc area in the above groups were evaluated. Design Case–control study. Participants Nine hundred twenty-seven records from the author's private practice, which included axial length measurements and optic disc photographs or digital images, were analyzed to form the 5 groups above. Methods Disc areas were measured using objective techniques developed by Bengtsson and Krakau. Axial lengths were determined by ultrasound biometry or with a Zeiss IOL Master. Main outcome measures Optic disc areas, corrected for magnification, axial lengths, and a derived ratio, axial length/disc area (AXL/DA). Results The AXL/DA for the amblyopic eyes (group 5) was significantly greater than that of all of the other groups, indicating that amblyopic eyes have discs that are both absolutely smaller and, when adjusted for eyes' size, relatively smaller than those of nonamblyopic eyes. Conclusion The relatively small size of the discs of amblyopic eyes points to an organic as opposed to a functional amblyopia in these patients.