Purpose: Choroideremia (CHM) is an X-linked inherited retinal degeneration causing loss of photoreceptors, retinal pigment epithelium, and choriocapillaris. Structural abnormalities of the cone photoreceptor mosaic have been reported even within the retained island of functioning retina. Here, we describe the relationship between cone density and visual sensitivity within the retained central retina in CHM.
Methods: The cone mosaics of 31 patients aged 10 to 57 years with CHM were imaged using a multi-modal, custom-built adaptive optics scanning light ophthalmoscope (AOSLO). Retinal sensitivity was measured using fundus tracking perimetry, cross-sectional retinal structure with optical coherence tomography. The relationship between sensitivity at each retinal location tested and structural parameters of local disease severity (cone density and distance to border) was explored using unpaired t-tests and linear regressions. Ellipsoid zone area was also investigated.
Results: Three hundred fourteen individual regions of interest were analyzed. Cone density and retinal sensitivities were significantly decreased compared with healthy controls (P < 0.0002). There was a statistically significant correlation between cone density and visual sensitivity within the fovea and parafovea (P < 0.0005), but not in the perifovea (P > 0.1). There was a significant relationship between the distance to the atrophic border and sensitivity from 400 µm to 1600 µm eccentricity (P < 0.001). Mean sensitivity was significantly related to the ellipsoid extent (P < 0.0001).
Conclusions: The correlation between cone density and retinal sensitivity within 1 mm of the foveal center in CHM suggests central sensitivity loss is driven by cone loss. Outside of the central fixation point, proximity to the atrophic border is correlated with sensitivity, suggesting the presence of a functional transition zone.