ON-pathway disturbance in two siblings.

Purpose: To present two clinical cases diagnosed with predominant cone dystrophy and demonstrating early disturbance in the on-centre bipolar cells (ON-pathway).
Methods: Electrophysiological findings are presented in two siblings with predominant cone dystrophy. The subjects showed no remarkable ophthalmoscopic or fluorescein angiographic retinal changes, but demonstrated progressive visual disturbance during their 20s.
Results: The electroretinograms (ERGs) showed reduced dark-adapted responses but the positive component of the photopic ERG was absent. Response to 30 Hz flicker was severely reduced. Electroretinograms elicited by long-duration stimuli showed a loss of the b-wave, and the off-response was slightly reduced. In both patients, multifocal ERGs (m-ERGs) were more reduced within the central 10 degrees, where the ON-pathway is normally a major contributor.
Conclusion: We conclude that these patients may be affected by an abnormality of the synapses of the cone receptors and that their decrease in vision might, at least initially, be due to selective ON-pathway dysfunction.