Sveinsson chorioretinal atrophy:: the mildest changes are located in the photoreceptor outer segment/retinal pigment epithelium junction

Purpose: To locate the mildest and/or earliest changes in the retina and/or choroid in Sveinsson chorioretinal atrophy (SCRA), using more advanced techniques than previous studies. Methods: We used fundus photography, intravenous fluorescein angiography (IVFA) enhanced ocular coherence tomography (OCT) scans, microperimetry and multifocal electroretinography (mfERG) in an attempt to locate the mildest changes in SCRA. Eight patients with SCRA were examined. To improve the resolution of OCT scans, several consecutive recorded B-scans were retrieved for each location of interest. The scans were processed off-line with an averaging algorithm developed to maximally reduce laser speckle (noise). Static microperimetry was performed using the Rodenstock scanning laser ophthalmoscope (SLO). Results: Biomicroscopy and fundus photographs disclosed an apparent thinning of the retinal pigment epithelium (RPE) in the areas minimally affected, where possible changes in the transparent sensory retina were not visible. In minimally affected areas a choriocapillaris fidefect was evident on IVFA, but some choroidal blood vessels remained open. High-resolution OCT scans in normal eyes showed three highly reflective outer layers, probably representing the junction of the inner and outer photoreceptor segments in the case of the innermost layer, the interdigitizing outer photoreceptors and RPE villi in the case of the middle layer, and the outer RPE in the case of the outermost layer. The middle layer was absent in the transition between affected and unaffected areas in all eyes with SCRA. In the more severely affected areas, the innermost layer was discontinuous and associated with increasing thinning of the outermost layer. Microperimetry of the transition areas sometimes showed clearly defined lesions that were non-responsive to stimuli. It also revealed elevated thresholds (10-15 dB) at the margins and normal thresholds in apparently unaffected areas. Conclusions: The mildest changes seen in SCRA on OCT are found at the outer photoreceptor/RPE junction; the changes in the outer RPE, choriocapillaris and inner photoreceptor segments may be secondary. Corresponding functional deficits are confirmed on microperimetry and mfERG.