Selective cone dystrophy with protan genotype.

Purpose: To determine the functional defects in two male patients with progressive cone dystrophy and hybrid L-M cone pigment genes.
Methods: Clinical evaluation, standard electroretinography, and electrooculography were performed in two affected patients and two family members. Measurements of spectral sensitivity and transient tritanopia were made in both patients.
Results: In the patients, visual acuity varied between 20/50 and 20/100. The electroretinogram showed reduced flicker responses. When light adapted, a-wave amplitudes were borderline, but b-wave amplitudes were reduced severely. Electroretinography with chromatic stimuli showed a difference between well-preserved responses to green and markedly reduced responses to red stimuli. Spectral sensitivity measurement revealed a lack of L (long-wavelength sensitive; red) cone function and normal function of the S (short-wavelength sensitive; blue) and M (middle-wavelength sensitive; green) cones. Transient tritanopia was abnormal, indicating a severe disturbance of cone-cone interaction.
Conclusions: Progressive cone dystrophy with predominant dysfunction of L cones exists in both patients. The cone dystrophy may be caused by a rearrangement of the X-chromosome pigment gene array that is associated with the deletion of L-cone sequences and the formation of hybrid L-M cone pigment genes. It cannot be excluded, however, that both patients have protanopia and that cone dystrophy developed because of other causes.