Macular dystrophy with protan genotype and phenotype studied with cone type specific ERGs

Purpose. To determine the L- and M-cone driven ERG responses in a male pati ent with macular dystrophy and a protan phenotype. Methods. We measured large field ERG thresholds to stimuli which modulated exclusively the L- or the M-cones or the two in various combinations (both in-phase and in counter-phase). In none of the stimuli, the S-cones were mo dulated. Additionally, standard and multifocal ERGs were measured. Analysis of the L- and M-cone pigment genes was performed by means of PCR, RFLP ana lysis and DNA sequencing techniques. Results. Macular dystrophy was revealed by the markedly abnormal multifocal ERGs in presence of near normal standard ERGs. The large field ERG respons es were exclusively driven by the M-cones with enlarged thresholds when com pared with otherwise normal protanopes. In addition, the M-cone driven ERG response phases were abnormal. Pigment gene analysis confirmed a protan gen otype with the presence of a single 5'red/3'green hybrid pigment gene. Conclusions. Our novel stimulus technique allows a reliable analysis of the separate cone pathways even in cases with macular dysfunction. The increas ed thresholds and the abnormal phase behavior of the M-cone driven ERGs ref lect altered mechanisms of the retinal physiology in this patient. The data strongly suggest that the macular dystrophy and the protanopia have indepe ndent origins.