Extent of foveal tritanopia in diabetes mellitus

Abstract

Aim: To use a colour matching technique to test the hypothesis that the foveal tritanopic zone is increased in size in diabetes mellitus.

Method: A Wright tristimulus colorimeter was adapted for small field colour matching and colour matches were performed on bipartite fields in the range 12' to 60' of arc. The reference stimulus was 490 nm desaturated with 650 nm and the matching stimulus consisted of either two wavelengths (530 nm and 650 nm) or three (460 nm, 530 nm, and 650 nm). The size of the zone of foveal tritanopia was measured using two alternative forced choice presentations of dichromatic and trichromatic matches made by the observer for different field sizes. 21 diabetic and 12 controls performed the experiment.

Results: The results for the controls show a normal distribution, with a median foveal tritanopic zone of 18' of arc. The median for the diabetic patients was also 18' of arc, but the distribution showed a significant skew to the right. A non-parametric test shows a significant difference in comparison with the controls (p = 0.01), with several subjects having extensive zones of foveal tritanopia, reaching up to 1 degree.

Conclusions: In the majority of diabetic subjects the extent of foveal tritanopia is normal; however, there is good evidence that in a small number of subjects the size of the zone is significantly increased. This indicates S-cone pathway damage that is sufficiently severe to lead to dichromatic colour vision in the fovea.

Figure 1

Chromaticity diagrams illustrating the principle of colour matching to establish the presence of S-cones. (A) The possible colours obtainable using mixtures of 530 nm and 650 nm light and a separate mixture of 490 nm and 650 nm light. The circle on the 490–650 nm line indicates the position of the test colour. For this combination of primaries, no colour match is possible for a trichromatic subject. (B) This shows that the addition of 460 nm light to 530 nm and 650 nm allows a trichromatic subject to achieve a colour match. (C) Shows that in tritanopia, a colour match is possible as the ratio of 530 nm to 650 nm light can be adjusted so that both colours lie on a tritanopic confusion line.

Figure 2

Results of forced choice presentations of dichromatic and trichromatic colour matches as a function of aperture size for observers NPD and VS. Note that with aperture sizes below 20’ of arc the forced choice response approaches five, which would be expected by chance (that is, the observers have reached a tritanopic zone)

Figure 3

Histograms of the measured size of the zone of foveal tritanopia in the control (solid) and diabetic (open) groups.

Figure 4

Foveal tritanopic zone versus age in control (solid) and diabetic (open) groups.