Structural and Functional Characteristics of Color Vision Changes in Choroideremia.

Jasleen K Jolly,Robert E Maclaren,Holly Bridge,Marco P Bellini,Odysseas Georgiadis,Matthew P Simunovic,Adam M Dubis,Anthony G Robson,Edward Bloch,Lyndon Da Cruz,Amandeep S Josan

doi:10.3389/fnins.2021.729807

Abstract

Color vision is considered a marker of cone function and its assessment in patients with retinal pathology is complementary to the assessments of spatial vision [best-corrected visual acuity (BCVA)] and contrast detection (perimetry). Rod-cone and chorioretinal dystrophies—such as choroideremia—typically cause alterations to color vision, making its assessment a potential outcome measure in clinical trials. However, clinical evaluation of color vision may be compromised by pathological changes to spatial vision and the visual field. The low vision Cambridge Color Test (lvCCT) was developed specifically to address these latter issues. We used the trivector version of the lvCCT to quantify color discrimination in a cohort of 53 patients with choroideremia. This test enables rapid and precise characterization of color discrimination along protan, deutan, and tritan axes more reliably than the historically preferred test for clinical trials, namely the Farnsworth Munsell 100 Hue test. The lvCCT demonstrates that color vision defects—particularly along the tritan axis—are seen early in choroideremia, and that this occurs independent of changes in visual acuity, pattern electroretinography and ellipsoid zone area on optical coherence tomography (OCT). We argue that the selective loss of tritan color discrimination can be explained by our current understanding of the machinery of color vision and the pathophysiology of choroideremia.

Highlights

The primary retinal neurons for conscious vision are duplex, consisting of two broad types: the rods and the cones
The cones are present throughout the retina, their high density in the fovea is responsible for high spatial resolution in normal subjects, which is measured clinically by best-corrected visual acuity (BCVA), which in turn provides a functional metric for foveal cones
Patients were divided into two groups for analysis: preserved BCVA (≥80 ETDRS letters) based on population studies defining the upper limit of normal (Brown and Yap, 1995) and reduced BCVA (

Summary

Introduction

The primary retinal neurons for conscious vision are duplex, consisting of two broad types: the rods and the cones (intrinsically photosensitive retinal ganglion cells—ipRGCs—which primarily drive circadian function and the pupillary light reflex are not considered further in this paper). The physiological basis of trichromatic color vision is the cone photoreceptor, which has three sub-types: the long- (L-), medium- (M-), and short- (S-)wavelength sensitive cones. These have spectral sensitivities with peaks at around 558 (L-cones), 531 (M-cones), and 419 nm (S-cones) (Dartnall et al, 1983). The second, more recent sub-system, compares quantal catches from the M- vs L-cones (red-green color discrimination). Of note, the latter is proposed to have evolved from a mechanism specialized for extracting spatial detail from the visual scene (Mollon, 1999). Reductions in visual acuity caused by retinal pathology are often accompanied by acquired red-green color deficiency, whilst acquired tritan deficiency may occur in the presence of normal visual acuity (Simunovic, 2016)

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