Functional consequences of disrupted cone photoreceptor reflectivity visualized using multimodal adaptive optics retinal imaging in oligocone trichromacy

Abstract

Purpose : Dark cones are photoreceptors that retain inner segments despite disrupted waveguiding properties, which may be attributed to partial disruption of the cone outer segments. Although dark cones have been observed in numerous conditions, their clinical significance remains uncertain. In this study, we longitudinally followed and evaluated the presence and impact of dark cones discovered in a patient with oligocone trichromacy (OT) using multimodal adaptive optics (AO) imaging alongside clinical assessments.

Methods : A patient with OT was imaged annually for 3 years with a custom-built multimodal AO imaging system, which incorporates confocal reflectance, nonconfocal split detection, and spectral-domain optical coherence tomography (SD-OCT) modes. The proportion of dark cones was determined using spatially co-registered confocal reflectance and split detection images to assess cone reflectivity and inner segment status, respectively. Cone inner segment spacing and density were quantified. In addition, this patient underwent functional clinical assessments, including visual acuity (VA), electroretinogram (ERG), and color vision tests (Hardy-Rand-Rittler, anomaloscope, and Cambridge Color Test).

Results : Overall, dark cones were observed throughout the retina, comprising more than 50% of the total imaged cones. These dark cones were relatively stable, persisting across visits. At non-foveal regions, the proportion of dark cones was 10-20% higher than in the fovea, with corresponding apparent disruptions in the outer retinal layers on AO SD-OCT. On average, quantitative assessment of cone inner segments in the fovea showed that cone spacing was approximately two-fold higher when compared to published normative data (73% decrease in cone density). The results of functional clinical assessments, in which bilateral best corrected VA was 20/32, cone ERG responses were reduced, and color vision was normal at both central and peripheral locations, suggested that cone function is only partially disrupted.

Conclusions : There is a widespread population of persistent dark cones identified in this patient with OT. Despite the disrupted cone structure, color vision is maintained across eccentricities. This characterization of the structure and function of dark cones represents an important step toward understanding their clinical significance.

This is a 2020 ARVO Annual Meeting abstract.