Adaptive Optics Imaging of the Cone Mosaic in Oligocone Trichromacy

Abstract

Purpose: : Oligocone trichromacy is a cone dysfunction syndrome characterized by reduced visual acuity, mild photophobia, reduced cone ERG’s with normal rod responses, a normal fundus appearance, normal color vision, and varying degrees of nystagmus. It has been proposed that these patients have a reduced number of functioning cones. Here we describe the appearance of the cone mosaic in three patients with oligocone trichromacy.

Methods: : Retinal images were obtained from two brothers and a third unrelated male diagnosed with oligocone trichromacy. High-resolution images of the retina were obtained using a high-speed adaptive optics camera. Visible structures were analyzed for density using custom MatLab software. Additional images were obtained using SDOCT (Bioptigen, Inc. & Carl Zeiss Meditec, Inc.) and used to visualize retinal lamination and retinal thickness.

Results: : Consistent with previous literature discrepancies, one of our subjects had nystagmus while the other two did not. In the unrelated male with nystagmus, we observed a normal appearing cone mosaic with adaptive optics. However, the two brothers had a sparse mosaic of cones in their fovea, with no structure visible outside the central fovea. Cone density was reduced by ~90% in both brothers. On SD-OCT, the outer photoreceptor complex (ELM, IS/OS, RPE1, RPE2) was evaluated. In the male with nystagmus, all 4 layers were visible, though he had a shallow foveal pit and an unusual optic nerve head. In the 2 brothers, all 4 layers were visible in the central fovea; however more peripherally, the IS/OS decreased in intensity and the RPE1 layer was no longer visible.

Conclusions: : The visual deficit in the two brothers appears to be caused by a reduced number of healthy cones whereas it had no apparent structural basis in the third subject. These data are consistent with the phenotypic heterogeneity described clinically and show that cone dysfunction associated with normal/near normal color vision is a structurally heterogeneous nosological entity at the cellular level. High-resolution imaging may prove an effective means of distinguishing between subtypes and directing genetic screening in certain instances.

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