Macular rod and cone photoreceptor structure in patients with X-linked cone dysfunction

Abstract

Purpose : X-linked cone dysfunction has been shown to cause variable structural and functional disruption of the long (L) and/or middle (M) wavelength-sensitive cone mosaic. Although the condition is considered to be isolated to L and M cones, the effect, or lake thereof, on rod density has not yet been confirmed. Adaptive optics scanning light ophthalmoscopy (AOSLO) enables in vivo visualization of the photoreceptor mosaic. Here we quantify the density of rods – derived from the density recovery profile (DRP) – and cones within the macula in patients who do not have normal L or M opsin, and thereby present with symptoms of blue cone monochromacy.

Methods : Eight patients (aged 16 to 51 years) with quantifiable parafoveal AOSLO images were selected for analysis. All patients had genetically confirmed mutations affecting both L/M opsin genes: four had the Cys203Arg missense mutation, three had abnormal exon-3 haplotypes (two with LVVVA; one with MIAVA + LIAVA), and one had a deletion of the locus control region (LCR). Where possible, rods and cones were counted (using confocal and split-detection AOSLO images respectively) in 1° increments from 3° to 10° along the temporal meridian.

Results : Although the overall mean (SD; n) cone density (cones/mm²) was reduced from normal, ranging from 6,446 (5,817; 4) at 3° to 1,750 (636; 2) at 10°, there were differences between the genotypes, with Cys203Arg trending towards lower cone density (Fig 1). Mean (SD; n) cone-corrected DRP-derived estimates of rod density (rods/mm²) ranged from 54,383 (28,276; 5) at 4° to 81,810 (1,4621; 3) at 9°, although, again, there were genotype differences, with Cys203Arg trending towards higher rod density (Fig 2).

Conclusions : Understanding the cone and rod topography within the macula is crucial for assessing the potential impact and monitoring efficacy of future therapeutic efforts in X-linked cone dysfunction. Our data suggest that, in patients who lack expression of normal L and M opsin genes, rod density and/or distribution across the retina may be affected by genotype.

This is a 2021 ARVO Annual Meeting abstract.

 

Density of cones (direct count) in patients with X-linked cone dysfunction, grouped by genotype.

 

Density of rods (estimates derived from the density recovery profile, corrected for area occupied by cones) in patients with X-linked cone dysfunction, grouped by genotype.