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A. M. Dubis, D. M. Tait, J. Rha, K. Stepien, J. Carroll; Same Retina, Different Picture: Imaging the Photoreceptor Layer With OCT and Adaptive Optics. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1065.
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The purpose of our study was to examine the photoreceptor layer using different imaging modalities in order to evaluate their relative sensitivity for detecting photoreceptor disruption. Adaptive optics has been shown to reveal pre-clinical changes in the cone photoreceptor mosaic in individuals with red-green color vision defects. Optical Coherence Tomography (OCT) has seen exponential growth in clinical applications, and is now accepted as the gold standard for detecting pathology. Here we examined the thickness of the outer nuclear layer (ONL) in normals and people known to have structural disruption in their photoreceptor mosaic.
Sixty individuals with normal vision were imaged with the Bioptigen SD-OCT. Six individuals with novel X-linked opsin genes, previously shown to have disrupted cone mosaics using adaptive optics retinal imaging, were imaged using either Bioptigen SD-OCT, Zeiss Stratus OCT, or Zeiss Cirrus HD-OCT. Retinal layers were manually segmented, and total retinal thickness and ONL thickness was calculated. All image processing was done using ImageJ and custom MatLab software.
Previous works have shown variation in ONL thickness in normal individuals. Individuals with photoreceptor degenerations typically exhibit reductions in ONL thickness. We also observed significant variability in ONL thickness among normals. In the individuals with disrupted cone mosaics, we observed surprising variation in the thickness of the ONL. While these individuals exhibited a diverse array of cone mosaic patterns and packing (Reductions in cone density varied from 20-40%), this numerical reduction in cone number did not directly correlate with the ONL thickness.
Based on the packing densities of the photoreceptors, it would be predicted that the ONL thickness should be dependent on the number of photoreceptors present. One possible explanation is that in some cases of disruptions, the affected cone cells have not completely degenerated, thus cell bodies remain in the ONL layer. This would be consistent with the genetic heterogeneity observed in our subjects: some had genes encoding inactive pigments, while others had mutations that eliminated pigment expression altogether. The disconnect between AO and OCT results suggests that "normal" OCT images do not necessarily imply that there is no cellular damage in the retina.
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