Repeatability of ellipsoid zone width measurements in retinitis pigmentosa using longitudinal reflectivity profiles

Abstract

Purpose : Ellipsoid zone (EZ) width has been established as a structural marker for boundaries of the visual field in retinitis pigmentosa (RP).¹ Central to the use of EZ width as an objective measure of disease progression is establishing its repeatability. Here we examined the intra-observer repeatability of measurements of EZ width in patients with RP using longitudinal reflectivity profiles (LRPs).²

Methods : We retrospectively examined Bioptigen OCT scans from 15 subjects (1 simplex RP, 5 adRP, 3 arRP, 2 XLRP, 2 USH1, 2 USH2). Mean ± standard deviation (SD) age was 42.7 ± 22.4 years (range = 7-8 years; 8 male, 7 female). The eye with better image quality was analyzed. Nominal scan lengths were 6, 7, or 10 mm, and the lateral scale of each scan was calculated based on estimated lateral magnification from axial length measurements. LRPs were generated from OCT line scans, and peaks corresponding to the external limiting membrane, EZ, interdigitation zone, and retinal pigment epithelium were manually identified in ImageJ (NIH, Bethesda, MD) by a single observer. The nasal and temporal locations at which the EZ peak disappeared were considered the boundaries of the EZ band. This process was repeated with the same observer masked to previous measurements. Estimates of EZ width were obtained for both horizontal (HW) and vertical (VW) scans.

Results : Mean ± SD EZ width was not significantly different between HW (3179 ± 1399 µm) and VW (2459 ± 1279 µm) (p=0.1064), though HW was greater than VW for all subjects, ranging from 139-1453 µm difference. We observed excellent repeatability with ICCs of 0.994 (95% CI = 0.988-1.00) for HW and 0.998 (95% CI = 0.996-1.00) for VW. The mean test-retest difference was 76.4 ± 94.5 µm. Measurement error was 166.7 µm (~5.9%), calculated from the within-subject SD.

Conclusions : Our observation of greater HW than VW is consistent with previous observations in RP.^3-5 EZ width can be measured using LRPs with excellent repeatability. The test-retest differences observed here are comparable to other methods.^4,5 One advantage of using LRPs is that the method does not rely on layer segmentation and can facilitate measurement of band thickness and intensity.
¹Hood PMID:21559123
²Huang PMID:9804149
³Hariri PMID:27031504
⁴Cai PMID:25342618
⁵Ramachandran PMID:24349883

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.