Cone and rod function in cone degenerations

Progressive cone dystrophy (CD) is usually marked in the initial stages by reduced visual acuity, color vision deficiency and alterations in the photopic electroretinogram, while morphological alterations can be very mild; in some forms rods are affected in a later stage as well. We examined 40 patients with progressive cone dystrophy to determine the extent of functional losses in the cone system with psychophysical tests. A great variety of visual acuity and fundus alterations was found. Myopia was present in 74% of the patients. An autosomal dominant pattern of inheritance predominated (32%). No prevalence of gender was found. The age of onset ranged between 10 and 30 yr. All patients had progression of their symptoms. The total error score in color arrangement tests, the saturated Farnsworth Panel D-15 and the Farnsworth-Munsell 100-hue test, was pathologic with a predominance of confusions along the tritan and scotopic axis. Especially if visual acuity was below 0.5, color vision defects increased, but color vision defects were also found in patients with normal visual acuity. A general decrease of sensitivity in all three cone mechanisms was observed in measurements of spectral sensitivity. Moreover, cone-cone interaction as tested by transient tritanopia measurements was usually disturbed. In the dark adaptation function the threshold of the cone branch was usually elevated. These tests provide a good means to ascertain the correct diagnosis in early stages of the disease and to monitor progression in patients suffering from cone dystrophy.