Outer retinal and choriocapillaris modifications in choroideremia: three differentially impaired retinal regions and the potential diagnostic role of the external limiting membrane

Background The external limiting membrane (ELM) is formed by the apical processes of Muller cells attached to the inner segments of the photoreceptor cells. Both cells are implicated in the pathogenesis of choroideremia (CHM). The purpose of this study was to explore the diagnostic role of ELM in CHM. Methods The study was designed as observational case series. Sixteen CHM eyes were examined by multimodal imaging and were compared to healthy controls. The main outcome was the measurement of ELM thickness and reflectivity over the follow-up, and its relationship with other multimodal imaging quantitative parameters. Results Baseline ELM was characterized by 11 +/- 1 mu m of thickness and 0.68 +/- 0.13 of reflectivity, resulting 8 +/- 1 mu m (p < 0.01) and 0.65 +/- 0.14 (p > 0.05) at the last follow-up. Choriocapillaris (CC) analysis revealed 3 regions. The first was characterized by normal vessel density (VD). The second surrounding the partially preserved islet, showing significantly lower baseline VD and undergoing minor changes over the follow-up. The third was localized in the partially preserved islet, showing significantly lower VD at baseline, and resulted atrophic at the last follow-up. ELM reflectivity and ELM thickness correlated both with outer retinal atrophy progression and the CC status. Conclusions ELM may be considered a useful imaging biomarker in CHM. Its assessment confirmed a primary role of Muller cells impairment in the pathogenesis of CHM.