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Lea D Bennett, Martin Klein, Kelly Kiser, Donald Charles Hood, David G Birch; Detectable rod function in patients with retinitis pigmentosa (RP) with or without a measureable rod electroretinogram (ERG) response. Invest. Ophthalmol. Vis. Sci. 2016;57(12):133.
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© ARVO (1962-2015); The Authors (2016-present)
Although it is known that rod photoreceptors are primarily affected in RP, rod visual fields are not well characterized due to the unavailability of commercial devices capable of detecting rod sensitivity in the periphery. The current standard for measuring rod function is the full-field ERG (ffERG), which, despite being the summed electrical response to light from the remaining rods, is not detected in patients with advanced RP. A new scotopic perimeter designed to test rod function throughout the visual field was used in order to determine if patients with RP had measurable rod function in the absence of a rod ffERG response.
Normal controls (n=7; age 36.9 ± 18.8yrs) and patients with RP (n=27; age 53.6 ± 14.4yrs) had one eye dilated and dark-adapted for 45 minutes. Rod ffERG responses were elicited by the ISCEV flash (0.01 cd.s/m2). Patients with RP were separated into 2 groups, those with a rod ffERG response >3µV (n=17) and without a rod ffERG response (<3µV; n=10). A DAC (dark-adapted chromatic perimeter; Medmont International Pty Ltd; Victoria, Australia) was used to test 164 points over 144° with short (505 nm) and long (625 nm) wavelength stimuli. Thresholds to short wavelength stimuli were considered rod-mediated if the sensitivity was within 20 dB of the normal control rod threshold. In some patients rod-mediation was further evaluated by determining the difference in threshold between the 505 nm and 625 nm stimuli.
The average sensitivity across all locations for normal controls was 52.3 ± 4.1 dB, which was higher than for patients with RP (22.1 ± 14.0 dB; p<0.0001). Patients had an average of 53.8 ± 48.2 test points that were rod-mediated. Patients with a rod ffERG response had 80.3 ± 41.7 rod-mediated loci. For those without a rod ffERG response, 4 patients with RP (40%) retained at least 5 rod-mediated loci.
Rod function can be quantified with the DAC perimeter even when the rod ffERG response is non-detectable. This provides a possible outcome measure of rod function for clinical trials involving retinal degenerative diseases.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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