[Differential diagnosis of cone dystrophies]

Electrophysiological and psychophysical findings recorded in 70 patients with three hereditary diseases of the cone system,--blue cone monochromatism, cone-rod dystrophy and selective cone dystrophy--were compared. Blue cone monochromatism is distinguished from the other two diseases by a reduction of visual acuity since childhood, without progression and with a sex-linked mode of inheritance. In addition, nystagmus is generally observed only in the time shortly after birth and the green and red cones are found to be missing on spectral sensitivity measurements. Cone-rod dystrophy can be distinguished from the blue cone monochromatism by a reduction in visual acuity later in life with progression of the symptoms. Spectral sensitivity measurements reveal reduced function of all three cones in cone-rod dystrophy and a single cone mechanism in selective cone dystrophy. Moreover, in cone-rod dystrophy the ERG reveals a reduction in the amplitudes of the photopic system and often mild involvement of the scotopic part. Measurement of the spectral sensitivity and the ERG can thus help in the diagnosis of these three hereditary diseases.