Correlation of Optical Coherence Tomography and Autofluorescence in the Outer Retina and Choroid of Patients With Choroideremia

Abstract

Purpose: To evaluate the relationships between RPE, photoreceptor, and choroidal degeneration in choroideremia.

Methods: Enhanced-depth imaging optical coherence tomography (EDI-OCT), scanning laser ophthalmoscopy (SLO), and autofluorescence (AF) were performed on 39 patients (78 eyes) with choroideremia. The edges of surviving outer retina on OCT and residual AF were aligned. The distribution of outer retinal tubulations was mapped over a range of ages (16-71 years), and comparison made between pre- and postsubretinal gene therapy. Subfoveal choroidal thickness (SFCT) was compared between 23 choroideremia patients (42 eyes) and 20 age- and refraction-matched male controls (40 eyes).

Results: The edges of RPE AF aligned with a reduction in outer nuclear layer thickness (Spearman's rho = 0.9992). Correlation was also found between the quality of AF and integrity of ellipsoid zone within islands of surviving retina. Tubulations existed in 71 of 78 (91%) eyes with choroideremia and remained stable following gene therapy. Subfoveal choroidal thickness was reduced at baseline in choroideremia (179.7 ± 17.2 μm) compared with controls (302.0 ± 4.8 μm; P < 0.0001), but did not undergo significant thinning until end-stage retinal degeneration (43.1 ± 6.5 μm).

Conclusions: The data suggest that RPE loss is the primary cause of photoreceptor degeneration in choroideremia. The choroid is thinner than controls from early stages, in keeping with a mild developmental defect. Photoreceptors appear to lose outer segments following loss of underlying RPE and form tubulations at the edges of degeneration. The preservation of tubulations over time and after subretinal injection would be consistent with these structures maintaining attachment to the inner retina and hence being potentially light responsive (ClinicalTrials.gov, NCT01461213).

Figures

Figure 1

Correlation between edges of AF islands and OCT structural changes in choroideremia. Panels (A–C) show representative images from three choroideremia patients. By plotting pairs of intersecting circles of proportional diameters from fixed reference points (e.g., vessel branch points) on the AF and SLO images, precisely corresponding point locations could be identified between the AF and OCT B-scans, and vice versa. The intersection between two red circles on the AF image marking the left-hand border of RPE degeneration (Ai), with high magnification view shown (Aii), corresponds to the position on the OCT indicated by a vertical red line (Aiv), while the intersection between green circles marking the right-hand border of RPE degeneration (Ai, Aiii) corresponds to a second position on the OCT indicated by a vertical green line (Av). The same applies to panels (B, C). The edges of surviving RPE on the OCT coincide with the start of an acute decline in ONL height, which diminishes to zero in a triangular transition zone, denoted by horizontal yellow bars. Immediately beyond the loss of ONL, outer retinal tubulations were frequently observed (yellow arrows). Scale bars: 200 μm. A summary of the characteristic OCT to AF image alignment features is provided in (D). Correlation between the transfoveal island width (y) measured on AF images and those predicted based on OCT outer retinal width (x) was performed in 49 choroideremia eyes (data summarized in the Table). OPL, outer plexiform layer; BM, Bruch's membrane.

Figure 2

Correlation between the pattern of AF and integrity of the EZ. In three young choroideremia patients aged 16 (A), 22 (B), and 23 years (C) who have large residual islands remaining on AF imaging, central ovoid zones with relatively preserved AF were observed, which aligned with regions of preserved EZ integrity on the OCT (green arrow bars). The size and eccentricities of these ovoid zones in relation to the anatomical foveal dip (white arrows) are consistent with representing areas of greatest cone density., The rest of the islands showed mottled AF, which corresponded to disrupted EZ (yellow arrow bars). Areas of absent AF corresponded to complete EZ loss (red arrow bars). Note that in (A) (OD & OS) and (C) (OS), at least one edge of the AF island fell outside the OCT image boundaries, therefore the foveola has been used as one of the reference points during image alignment.

Figure 3

Distribution of outer retinal cysts in six patients with choroideremia aged 16 to 71 years ([A–F] in ascending age order). The distribution of tubulations could be inferred from the positions of all outer retinal cysts (ORCs), which represent cross-sections of tubulations, seen on all B-scans of the OCT. For each eye, all ORCs were mapped to corresponding locations on the AF image (shown as white dots). The Table displays the mean distance of ORCs from the nearest edge of AF island (±SD) and genetic mutations in the CHM gene for each patient.

Figure 4

Outer retinal cysts on OCT represent cross-sections through outer retinal tubulations following RPE loss at the edges of degeneration. Plotting the locations of ORCs (white dots) seen on every B-scan of the OCT (horizontal green lines) over the corresponding high magnification infrared SLO image revealed that the ORCs lie over thin ‘pseudodendrites' containing retinal pigment along the edges of the surviving retina (outlined in dotted blue). These pseudodendrites do not match the background autofluorescence of the choroidal vasculature (top panel) but represent distinct tubular outer retinal structures.

Figure 5

The effects of subretinal gene therapy on outer retinal tubulations. Gene therapy using recombinant AAV2-REP1 vector for choroideremia were performed on six eyes (A–F) in which the residual AF islands were surgically detached by subretinal injections. The locations of outer retinal cysts (white dots) seen on all B-scans of the OCTs, which represent cross-sections of tubulations, have been mapped onto the corresponding AF images at baseline (left column), 1 month post (center column) and 6 months post (right column) gene therapy.

Figure 6

Scatter plot of SFCT as measured on enhanced-depth OCT in choroideremia patients with preserved, disrupted or lost ellipsoid zone (as illustrated by representative OCT images above) versus age and refractive error-matched healthy controls. The mean of each group (in black) is shown with 1 SD on either side (in gray). Intergroup statistical comparisons were performed using Kruskal-Wallis test with P values shown. See raw data in Supplementary Table S1.