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David G. Birch, Yuquan Wen, Kelly I. Locke, Donald C. Hood; Rod and Cone Contributions to fdOCT Measures of Photoreceptor Structure in Retinal Degenerative Diseases. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6606.
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© ARVO (1962-2015); The Authors (2016-present)
The degree to which fdOCT reflects rod versus cone integrity is poorly understood. Here we evaluate the effects of selective rod or cone loss on fdOCT measures of photoreceptor structure in patients with retinal degenerative diseases.
5 patients with cone or cone/rod dystrophy (CD/CRD) and 8 patients with retinitis pigmentosa (RP) were recruited from the Southwest Eye Registry based on diagnosis and ERG findings: patients comprised three groups 1) selective loss of cone function; 2) greater loss of cone than rod function; 3) greater loss of rod than cone function. FdOCT horizontal line scans (Spectralis, Heidelberg) were segmented to obtain the thicknesses of the outer segments plus RPE (OS+) and total receptor thickness. Normalized thicknesses were obtained after dividing by mean normal values (n=30). Horizontal sensitivity profiles were obtained from the horizontal midline with short- and long-wave stimuli under dark- and light-adapted conditions using a modified MP-1 (Nidek Tech) retinal perimeter.
Normal subjects showed sensitivity to short-wave stimuli that was on average 18 dB higher than sensitivity to long-wave stimuli in the dark (rod-mediated) and nearly equivalent short- and long-wave sensitivity in the light (cone-mediated). Patients with normal rod sensitivity, but decreased cone sensitivity, showed normal thickness of the OS+ layer outside the fovea. The total receptor layer was thinned in the fovea, consistent with loss in cone nuclei and Henle’s fiber layer. Patients with greater loss of cone than rod sensitivity showed OS+ thickness that was linearly related to rod sensitivity. Patients with sensitivity in the dark that was mediated by cones showed OS+ thickness that was linearly related to cone sensitivity.
Both rods and cones can support an intact IS/OS junction and normal photoreceptor thickness measures. The measured thickness of the OS+ segment is proportional to either rod-mediated or cone-mediated sensitivity, depending upon which is the least abnormal. 1. Hood et al., 2009, IOVS; 2. Crossland et al., 2010, ARVO.
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