The knowledge of the exact location of the fovea is important, especially in low-vision patients with changes of the area of fixation. Because most of those patients have morphologic alterations of the macular area, it may be helpful to know the location of the fovea in relation to the optic disc. Our findings indicate that there are interindividual differences in the distance between the center of the optic disc and the fovea and those differences are larger as assumed by others.
10 11 We chose the center of the optic disc instead of the temporal and inferior margins as the reference, because the size of the disc itself varies between different persons.
12 Otherwise, in myopic eyes with tilted nerve heads, additional corrections would be necessary. Nevertheless, the intraindividual differences between right and left eyes are small, especially for the horizontal distance. The effect of rotation may cause greater variation in the vertical distance. Recently, Keilhauer et al.
13 have reported a significant increasing distance between optic disc and fovea with age in 28 subjects caused by an increasing vertical distance. Our results in a higher number of eyes did not show any change correlated with age
(Fig. 2) and especially no increasing tilt of the eye with age. When there is normal visual acuity and stable central fixation, it is even unlikely that the position of the eye would change during lifespan.
From the data for horizontal and vertical distance between fovea and optic disc, it is possible to calculate the angle between the horizontal through the center of the disc and the line connecting the center of the disc to the fovea. Our mean angle α of −5.6 ± 3.3° shows a smaller tilt than reported in the small study of Keilhauer et al.
13
The determination of the location of the fovea may be possible by use of the perimacular vessels and the center of the avascular zone, even in patients who have macular disease. However, in advanced stages, when central fixation is lost due to pathologic alteration, the avascular zone as well may no longer exist. Therefore, the theoretical option of a more precise definition of the former location of the fovea using the perimacular vessels often is no longer possible. The indirect definition of the location of the fovea according to the distance to the optic nerve head as given in this study may be helpful in such patients, although accuracy is limited.