Early Cone Photoreceptor Outer Segment Length Shortening in <b><i>RPGR</i></b> X-Linked Retinitis Pigmentosa

<b><i>Introduction:</i></b> Introduction of retinal gene therapy requires established outcome measures along with thorough understanding of the pathophysiology. Evidence of early, thinned outer segments in <i>RPGR</i> X-linked retinitis pigmentosa could help understand how the level of cone photoreceptor involvement translates to visual potential. <b><i>Objective:</i></b> Analysis of foveal photoreceptor outer segment length in a young cohort of <i>RPGR</i> patients to help clarify the reason for absent maximal visual acuity seen. <b><i>Methods:</i></b> Case-control study of <i>RPGR</i> patients. Quantitative measurement of photoreceptor outer segment by OCT. <b><i>Results:</i></b> Eighteen male <i>RPGR</i> patients and 30 normal subjects were included. Outer segment thickness differed significantly between the <i>RPGR</i> and normal eyes (<i>p</i> &#x3c; 0.0005). Mean outer segment values were 35.6 ± 2.3 µm and 35.4 ± 2.6 µm for <i>RPGR</i> right and left eyes, respectively. In normal eyes, the mean outer segment thickness was 61.4 ± 0.7 µm for right eyes and 62.4 ± 0.7 µm for left eyes. <b><i>Conclusions:</i></b> Patients with <i>RPGR</i> X-linked retinitis pigmentosa show thinning of the foveal photoreceptor outer segment thickness early in the disease course, which could be an explanation for the lower maximum visual acuity seen. These findings must be taken into consideration when assessing efficacy outcome measures in retinal gene therapy trials.