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 the fundus.

Methods: Two patients, father and son, with visual acuity loss to 20/100 were examined. Using the multifocal ERG, 61 local cone ERGs from each eye were derived from the central visual field. The dark-adapted two-color threshold perimetry using stimuli of 500 nm and 656 nm for rod and cone function was investigated along the horizontal meridian of the visual field.

Results: In the multifocal ERG of both patients a macular response was absent. From eccentricity at and anterior to 5 degrees, good multifocal cone activity 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 reveal the topography of cone function. Using two-color threshold perimetry affords 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.