Macular Cone Abnormalities in Retinitis Pigmentosa with Preserved Central Vision Using Adaptive Optics Scanning Laser Ophthalmoscopy

Abstract

PurposeTo assess macular photoreceptor abnormalities in eyes with retinitis pigmentosa (RP) with preserved central vision using adaptive optics scanning laser ophthalmoscopy (AO-SLO).MethodsFourteen eyes of 14 patients with RP (best-corrected visual acuity 20/20 or better) and 12 eyes of 12 volunteers underwent a full ophthalmologic examination, fundus autofluorescence, spectral-domain optical coherence tomography (SD-OCT), and imaging with a prototype AO-SLO system. Cone density and spatial organization of the cone mosaic were assessed using AO-SLO images.ResultsIn 3 eyes with RP and preserved central vision, cones formed a mostly regular mosaic pattern with small patchy dark areas, and in 10 eyes, the cone mosaic patterns were less regular, and large dark regions with missing cones were apparent. Only one eye with RP demonstrated a normal, regular cone mosaic pattern. In eyes with RP, cone density was significantly lower at 0.5 mm and 1.0 mm from the center of the fovea compared to normal eyes (P<0.001 and 0.021, respectively). At 0.5 mm and 1.0 mm from the center of the fovea, a decreased number of cones had 6 neighbors in eyes with RP (P = 0.002 for both). Greater decrease in cone density was related to disruption of the photoreceptor inner segment (IS) ellipsoid band on SD-OCT images (P = 0.044); however, dark regions were seen on AO-SLO even in areas of continuous IS ellipsoid on SD-OCT. Decreased cone density correlated thinner outer nuclear layer (P = 0.029) and thinner inner segment and outer segment thickness (P = 0.011) on SD-OCT.ConclusionsCone density is decreased and the regularity of the cone mosaic spatial arrangement is disrupted in eyes with RP, even when visual acuity and foveal sensitivity are good. AO-SLO imaging is a sensitive quantitative tool for detecting photoreceptor abnormalities in eyes with RP.

The Stiles–Crawford Effect: spot-size ratio departure in retinitis pigmentosa

“…But as we move towards the periphery, the discrepancy in departure increases, both for n-fold and 1/n-fold departures symmetrically towards nasal and temporal regions with pupil entry point r. That means when we move from the fovea outward to the parafovea and beyond, the difference in departure is directly proportional to the magnitude of the compensation the retina exerts. From Figures 5-7 we see that with more and more oblique incidence, the gap between the departures, for normal and RP eyes, gets widened, and for the perfect axial entry, the departures are identical pointing to preserved central vision in RP eyes in spite of cone density loss [15].…”

“…One possible reason is the decreased cone density in RP with proportionate erosion in directional sensitivity, attributes of the cones. And the other reason may be the disruption in the regularity of the cone mosaic spatial arrangement in spite of favourable visual acuity and foveal sensitivity [15]. The retina can ill afford to go for a big compensation as it lacks the number in the form of cones and shape in the form of normal morphology of the cones to tackle the problem at hand.…”

“…Thus, the retina does not have to go for a bigger compensatory response in RP eye compared to its normal counterpart. This is true as the experimental evidence shows that in order for the response to be bigger, the cone population and cone shape should be more and perfect, respectively [15]. As these are affected in RP eyes with preserved central vision, there is neither the requisite number of cones available either at sites away from the fovea centralis or the available cones have the perfect size to register a strong response with increasing oblique incidence.…”

“…Bailey et al showed that the photoreceptors have normal orientation but inferred that the optical characteristics of photoreceptors have changed. And recently, the use of adaptive optics scanning laser ophthalmoscopy has conclusively shown that in spite of good visual acuity and foveal sensitivity, the normal cone morphology is impaired as manifested in decreased cone density and disrupted cone mosaic spatial arrangement in eyes with RP [15].…”

Spontaneous remission of acute zonal occult outer retinopathy: follow-up using adaptive optics scanning laser ophthalmoscopy

“…Images focused on the photoreceptor layer were recorded in the area, and a montage of AO-SLO images was created. The present study examined cone density, ratio of hexagonal Voronoi domain, average nearest-neighbor distance (NND)/expected NND in each cone mosaic to estimate the spatial organization of the cone mosaics as reported [7]. Voronoi domains were constructed for each cell by defining points in the regions that were closer to that cell than to any other cell in the mosaic.…”