Color Vision Loss Progression in IRD Patients: Retinal Structural Changes Correlations, and a Novel Color Discrimination Test for Extreme Low Vision Patients

Inherited retinal dystrophies (IRDs) encompass a broad and genetically diverse class of monogenic disorders marked by progressive degeneration of retinal photoreceptors-namely, rods and cones-which ultimately culminates in irreversible visual dysfunction. As the degenerative process advances, patients frequently develop a combination of central and peripheral visual loss alongside progressive impairments in color discrimination. Among retinal structural biomarkers, the ellipsoid zone (EZ)-a hyperreflective band corresponding to the mitochondria-dense segment of the photoreceptor inner segments, historically denoted as the IS/OS junction in spectral-domain optical coherence tomography (SD-OCT)-has emerged as a key indicator of residual photoreceptor viability and visual function. Prior studies, particularly those focusing on retinitis pigmentosa (RP), have demonstrated a compelling association between EZ integrity and chromatic discrimination ability, reinforcing the critical relationship between anatomical preservation and functional outcomes in retinal dystrophies.

This prospective investigation is designed to quantitatively characterize color vision performance in individuals with IRDs and to evaluate its association with retinal microstructural features as revealed by multimodal imaging modalities, including SD-OCT-derived EZ measurements, total retinal thickness, and best-corrected visual acuity (BCVA). By integrating these anatomical measures with genotypic data, the study will further examine the influence of specific mutation types, allelic patterns, and patient age on chromatic function, thereby offering a comprehensive analysis of genotype-phenotype relationships in IRD-associated dyschromatopsia.

A persistent challenge in the evaluation of color vision among IRD patients-particularly those with advanced vision loss-is the limited applicability of conventional color vision assessments. Standard psychophysical tests, such as the Ishihara pseudoisochromatic plates and the Farnsworth-Munsell 100 Hue Test, are fundamentally dependent on preserved spatial resolution and pattern recognition, thereby rendering them ineffective in individuals with substantial visual acuity deficits. Although the Low Vision Cambridge Color Test (lvCCT) offers a more accessible alternative for patients with moderate acuity impairment, those with BCVA at or below the level of Counting Fingers (logMAR ≥ 1.6) are frequently unable to complete the test, resulting in inconclusive or misleading data regarding true chromatic capability. In such cases, performance failure often reflects the constraints of visual resolution rather than an authentic deficit in color discrimination, complicating both diagnosis and longitudinal monitoring.

To address these limitations, the present study incorporates the Moji Low-Vision Color Discrimination Test (Moji LVCDT)-a novel chromatic assessment tool specifically developed for individuals with profound visual impairment. This test employs high-contrast, large-format color stimuli, enabling the assessment of chromatic discrimination thresholds independent of fine spatial vision. Preliminary validation data indicate that the Moji LVCDT can successfully detect residual color perception in patients who are unable to engage with standard color vision testing paradigms, thereby providing clinically meaningful results even in the most advanced stages of IRD.

As part of the study protocol, the Moji LVCDT will be administered in parallel with established color vision tests, including the Ishihara plates and the low-vision-adapted Cambridge Color Test, to evaluate its diagnostic sensitivity, specificity, and correlation with both functional vision and structural retinal metrics. Through comparative analysis, the study aims to validate the Moji LVCDT as a reliable and scalable tool for quantifying color vision in populations historically excluded from color function assessments due to visual acuity constraints.