Subtle changes in diabetic retinas localised in 3D using OCT

Abstract

Purpose: To detect and localise subtle changes in retinas of diabetic patients who clinically have no diabetic retinopathy (DR) or non-proliferative DR (NPDR) as compared to age- and sex- matched controls. Spectral Domain Optical Coherence Tomography (SD-OCT) and software to examine all retinal layers, including deeper layers, were used to quantify foveal avascular zone size and inner and outer retinal layer thicknesses, as well as to detect axial location of prominent lesions.

Methods: Diabetic subjects, 19 total with 16 having no DR and three having non-proliferative retinopathy, were matched with 19 controls with respect to age and sex. Macular-centred SD-OCT grids of 20 × 15° were taken with the Spectralis. En face or transverse images were generated from the SD-OCT data by automatically segmenting all retinal layers. The transverse images were investigated for foveal avascular zone (FAZ) size, retinal vessel calibre, and structural changes. The size of the FAZ was compared for diabetics vs controls using vendor software and manual marking in Photoshop. Inner retinal layer (IRL_FAZ ) and outer nuclear layer (ONL_FAZ ) thicknesses at the margins of the FAZ were measured using vendor software.

Results: The FAZ area was larger for diabetics (mean ± S.D. = 0.388 ± 0.074 mm² ) than controls (0.243 ± 0.113 mm² ), t₁₈ = 5.27, p < 0.0001, using vendor software. The mean IRL_FAZ was thicker for the diabetics (86.8 ± 14.5 μm) than controls (65.2 ± 16.3 μm), t₁₈ = 4.59, p = 0.00023, despite lack of exudation by clinical exam. There was no significant association between FAZ area and mean IRL_FAZ for the diabetics, r = 0.099, p = 0.69. Vessels not clinically detected were visible in the NFL transverse image of most diabetics, especially for a mild NPDR patient. A prominent lesion found in the en face infra-red image of a mild NPDR subject was localised in the photoreceptor layer by SD-OCT, as well as additional outer retinal changes in other subjects.

Conclusions: Our results demonstrate changes in inner and outer diabetic retinas not readily detectable by clinical exam. IRL_FAZ had not thinned at the margins of the large FAZs, indicating neural mass did not yet decrease despite potential ischemia.

Keywords: diabetic macular oedema; diabetic retinopathy; foveal avascular zone; imaging; optical coherence tomography; transverse.

Figure 1.

SDOCT images of a diabetic patient and age-matched control showing the computed transverse/en face images, specification of slabs, and cross-sectional images. (a) segmentation of the NFL (red boundary demarcation) and (b) 20 µm thick NFL slab of a type 1 diabetic with mild NPDR. (c) segmentation of the NFL (red boundary demarcation) and (d) 20 µm thick NFL slab of an age-matched control. (e) NFL transverse image displayed en face of the diabetic subject and (f) NFL transverse image displayed en face of an age-matched control subject.

Figure 2.

Noise reduction in the transverse images. (a) NFL transverse image corresponding to a 20×15-degree field centered on the macula of a diabetic subject with mild NPDR before the intensity mask was applied. (b) the NFL transverse image after the intensity mask was applied. The arrows in (a) show noise that is absent in (b).

Figure 3.

Marking the FAZ area in the transverse images. 20×15-deg field transverse images of (a) NFL, (b) GCL, (c) IPL, (d) INL, (e) OPL, (f) ONL, (g) PRL, and (h) RPE centered on the macula of a control subject. Arrows show the FAZ in each retinal layer. The FAZ (a) is most visible in the NFL transverse image.

Figure 4.

(a) FAZ marked in red in the NFL transverse image of a control subject. (b) Magnified image of the FAZ region of the same subject, shown in reverse contrast. Scale bar = 200 µm.

Figure 5.

FAZ and retinal thickness measurements of a diabetic subject with mild NPDR. (a) NFL transverse image showing FAZ demarcation (green demarcation). The vertical FAZ diameter (FAZ_V) corresponds to the vertical yellow line, and the horizontal FAZ diameter (FAZ_H) corresponds to the horizontal yellow line as shown by the blue arrow. (b) Corresponding segmented B-scan showing the inner retinal layer (IRL_FAZ) thickness at the margins of the FAZ. Measurements were taken from the inner limiting membrane (top red line) to the OPL/ONL boundary (middle pink line), as shown by the dotted blue arrows. The outer nuclear layer (ONL_FAZ) thickness at the FAZ margins were taken from the OPL/ONL boundary to the external limiting membrane, as shown by the solid red arrows.

Figure 6.

Mean plot with standard deviation error bars showing the mean FAZ area of the diabetics versus age-matched controls from the vendor software (a) and manual method (b). The mean FAZ area of the diabetics was significantly greater than that of the controls.

Figure 7.

A Bland-Altman plot of the difference (diff) between the FAZ area (mm²) measured by the custom vendor software and manual marking as a function of the mean of the FAZ area from the two techniques. The 95% limits of agreement were found by mean of the differences (Avg_diff) ± 1.96*standard deviation of the differences (s).

Figure 8.

FAZ area plotted as a function of age and IRL_FAZ plotted as a function of FAZ. (a) Plot showing overlap between the FAZ area of the diabetics and controls and lack of significant association between FAZ area and age for diabetics and controls. (b) Plot showing overlap between the IRL_FAZ of the diabetics and controls and lack of significant association between IRL_FAZ and FAZ for diabetics and controls.

Figure 9.

FAZ areas shown in reverse contrast. (a) FAZ regions of diabetics. (b) FAZ regions of age-matched controls to (a). (c) FAZ regions of diabetics. (d) FAZ regions of age-matched controls to (c). (e) FAZ regions of diabetics. (f) FAZ regions of age-matched controls to (e). The FAZ regions vary in shape and size among individuals for both diabetic patients and controls. Scale bar = 200 µm.

Figure 10.

20×15-deg en face, NFL transverse, and b scan images centered on the macula of a 62 yr old type 1 diabetic with mild NPDR and b scans of other type 1 diabetics with non-proliferative retinopathy. (a) Blue circle shows areas of reflectivity changes in the IR image. There is a striking reflectivity change indicated by the blue arrow, and focal regions of lower reflectivity. These reflectivity changes obscure the visualization of underlying retinal structures. (b) Arrows show vessels that appeared to be non-perfused in the corresponding NFL transverse image, but were detected in one or more of the other layers or the en face image. Small, hyperreflective dots are consistent with early vascular changes. (c) B scan with blue arrow showing the axial location of the lesion in (a) to be a damage to the PRL and RPE. (d-e) B scans of 51 and 53 yr old type 1 diabetics with mild and moderate non-proliferative retinopathy, respectively, showing outer retinal changes (blue arrows) similar to that seen in panel c.

Figure 11.

Comparison of a 20×15-deg en face image and successively deeper transverse images centered on the macula, showing that the origin of the greatest reflectivity change (red box) is in the deeper layers for a 62 yr old type 1 diabetic with mild NPDR. (a) IR image, (b) NFL, (c) GCL, (d) IPL, (e) INL, (f) OPL, (g) ONL, (h) PRL, and (i) RPE transverse images of a type 1 diabetic with mild NPDR, from the same data set and retinal region. The lesion in (a) is absent in the inner retinal layers in panels b-e, with no box shown, but present in the outer retinal layers (red box) f-i. The boxes in panels f-i correspond to that in panel a.