Cone Density And Color Discrimination In Oligocone Trichromacy

Abstract

Purpose: : Oligocone trichromacy (OT) is typically associated with reduced visual acuity, photophobia, reduced cone ERG’s with normal rod responses, but normal or near-normal color vision. To investigate structural and phenotypic heterogeneity, we performed adaptive optics retinal imaging and color discrimination measurements in two unrelated females previously diagnosed with OT.

Methods: : Subject Co1001 (50 yrs) has previously been found to carry a single heterozygous PDE6C missense mutation. The underlying molecular basis in subject Co1002 (26 yrs) is unknown. High-resolution images of the cone mosaic were obtained with the Kongsberg Adaptive Optics Ophthalmoscope and cross-sectional retinal images were obtained with an SD-OCT (Spectralis, Heidelberg Engineering, Germany). Individual axial lengths were measured with IOLMaster (Carl Zeiss, Germany). Color discrimination was measured with a battery of color tests including the Cambridge Colour Test (CCT) before and after dark-adaptation.

Results: : Both were hypermetropic with best-monocular-corrected Snellen acuity of 1.0 (Co1001) and 0.8 (Co1002). The outer photoreceptor complex was discernible in the central fovea in both, but appeared mottled. The IS/OS reflex was weakened beyond the parafovea, but more so in Co1002. The cone mosaic in the fovea appeared irregular in Co1001 with some regions of evenly packed cones and some with very few visible cones, i.e. cone density was normal around 0.4°T, but reduced by 50% around 0.4°N. In Co1002 the mosaic was discontinuous and cone density was sparse being reduced by more than 50% throughout the fovea. Cone structures were only sparsely visible beyond 1.5° eccentricities in both. Co1001 had slightly elevated color discrimination thresholds, whereas Co1002 had a tritan color-vision deficiency. Both behaved as if they had no S-cone function when tested with the CCT after dark-adaptation, which is different to normal trichromats (p < 0.05).

Conclusions: : The findings indicate that cone density is disrupted in some regions in Co1001, whereas both cone density and the distribution of cones is disrupted in Co1002. This is consistent with cone degeneration in Co1001 and disruption in normal cone packing in Co1002. S-cone function is altered in both, but whether this has a molecular basis is unknown at present.

Keywords: imaging/image analysis: clinical • photoreceptors • retina: distal (photoreceptors, horizontal cells, bipolar cells)