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Laura Wood, Jasleen K Jolly, Robert E MacLaren; Assessment of low luminance visual acuity as a functional measurement in two distinct types of inherited retinal degeneration.. Invest. Ophthalmol. Vis. Sci. 2019;60(9):1817. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Multiple gene therapy clinical trials are ongoing for X-linked retinitis pigmentosa (RPGR) and choroideremia. In choroideremia, vision deteriorates due to a mutation is in the underlying retinal pigment epithelium (RPE), whereas in RPGR vision deteriorates due to a mutation in the photoreceptors. Both conditions present with nyctalopia and progressive visual field loss, but initially good visual acuity (VA); limiting its usefulness in quantifying the effects of gene therapy in early stages. Low luminance VA has been explored as an outcome measure of VA in poor lighting. Its usefulness as a biomarker in the early stages of these inherited retinal degenerations has not yet been formally assessed.
Standard VA was obtained using the early treatment diabetic retinopathy logMAR charts at 4m or 1m as required. Low luminance VA was tested using a 2.0-log unit neutral density filter, with the same chart set up, without formal dark adaptation. The results of 88 male participants aged 14-84 were collated; healthy eyes (n=21), RPGR (n=15) and choroideremia (n=52). Comparative statistics, with right eye only (tested first) were conducted.
Low luminance difference (LLD), was calculated by standard VA minus low luminance VA. The median LLD for RPGR, choroideremia and controls was 24, 18, and 7 respectively. Using the Kruskal-Wallis test all pairwise comparisons (Dwass-Steel-Chritchlow-Fligner), no significant difference between disease groups (p=0.98) and between RPGR versus controls (p=0.13) was found. However, choroideremia was significantly different to controls (p=0.02). Correlation between standard VA and low luminance VA showed a similar correlation between groups (RPGR r=0.61, p=0.06; choroideremia (r=0.74, p<0.01); and controls (r=0.65, p<0.01) until 50 standard VA letters. After this point, the low luminance VA became unmeasurable.
The greater drop in VA under low luminance in choroideremia compared to healthy controls may make a useful biomarker in patients undergoing gene therapy who are otherwise at the near maximum standard VA. The none statically significant RPGR LLD difference could be due to the small sample size. LLD usefulness in both diseases reduces when standard VA is less than 50 letters (20/100). Since this is higher than the predicted VA for rod vision, low luminance VA most likely reflects impaired cone function.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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