Outer retinal thickness and correlation with visual acuity in patients with diabetic macular edema

Abstract

PURPOSE: To investigate the correlation of outer retinal thickness (distance between the retinal pigment epithelium and the external limiting membrane) and visual acuity in patients with diabetic macular edema. SETTING: Department of Ophthalmology, University of Hong Kong METHODS: Eyes with conditions other than DME, and conditions that would render media unclear for OCT scans were excluded. Each subject was assessed by a Snellen visual acuity chart, and OCT scan using the Heidelberg Spectralis (Heidelberg, Germany). The scanned images were then analyzed with the software provided by Heidelberg (Heidelberg Explorer). The hyper-reflective lines in the outer retina were identified, and the distance between the external limiting membrane (ELM) and the retinal pigment epithelium (RPE) was measured using the built-in caliber. Those with disruption of the ELM at the fovea were excluded from further analysis. A P value <0.05 was considered statistically significant. RESULTS: A total of 108 eyes of 108 subjects were recruited, but 30 (27.8%) were excluded due to disruption of the ELM, resulting in 78 eyes for analysis. The mean age was 58.1±11.5 years. 51 of them were male (47.2%). Mean refractive error was -1.3±2.5 dioptres. The mean SnellenVA was 0.51±0.18. Mean total retinal thickness (TRT) at fovea was 398.0±74.3mm. Mean outer retinal thickness (ORT) at fovea was 115.7±35.6mm. When both the TRT and ORT were compared against VA, both had P<0.001. When TRT was correlated with VA, r was 0.34. When ORT was correlated with VA, r was 0.65. CONCLUSIONS: This was the first study to use the distance between the ELM and the RPE to correlate with VA in subjects with DME. There was good correlation between VA and outerretinal thickness (ELM to RPE). Our results were comparable to published series using total retinal thickness. It serves as a novel marker when assessing patients with DME.