Ratio of optic cup to optic disc: In relation to axial length of eyeball and refraction

Abstract

The ratio of horizontal diameter of optic cup to that of optic disc has already been established as genetically determined. In a series of 75 normal subjects with normal ocular tensions and axial lengths of eyeball between 21 13 and 27-84 mm., the area of the optic cup relative to the total area of the optic disc (cup-to-disc area ratio) tended to be high when the eyeball was long and the refraction myopic, but low when the eyeball was short and the refraction hypermetropic; these observations were statistically significant at the 5 per cent. level. The likeliest explanation is that the absolute area of the optic disc probably varies with the size of eyeball, whereas the volume of nerve fibres congregated at the optic nerve head is fairly standard. To determine the importance of inheritance of axial length in the heredity of cup-to-disc diameter or area ratio will require a separate family study.