Familial macular cone dystrophy: diagnostic value of multifocal ERG and two-color threshold perimetry

Background: It is difficult to detect receptor dysfunction in patients with marked bilateral visual loss but only mild morphological alterations of th e fundus. Methods: Two patients, father and son, with visual acuity loss to 20/100 we re examined. Using the multifocal ERG, 61 local cone ERGs from each eye wer e derived from the central Visual field. The dark-adapted two-color thresho ld perimetry using stimuli of 500 nm and 656 nm for rod and cone function w as investigated along the horizontal meridian of the visual field. Results: In the multifocal ERG of both patients a macular response was abse nt. From eccentricity at and anterior to 5 degrees, good multifocal cone ac tivity was recorded. Cone thresholds were markedly diminished in the macula . The rod thresholds were borderline in the father and normal in the son. Conclusions: Multifocal ERG is a novel technique, very well suited to revea l the topography of cone function. Using two-color threshold perimetry affo rds an opportunity to differentiate between rod and cone functional defects . Both together helped to establish the diagnosis of macular cone dystrophy in the present family.