All retinas are not created equal: Fovea-to-macula thickness ratio and foveal microvasculature in healthy young children

Abstract

Purpose: Markers for the relationships between structural and microvasculature measures given by optical coherence tomography angiography are necessary to increase the diagnostic and prognostic value of this technique. The aim of this study was to investigate relationships between structural and microvasculature measures around the fovea in healthy eyes of healthy children.

Methods: Observational cross-sectional study involving children aged 8-17 years, born at full-term, with no eye disease. The better of two 3 × 3 mm macular scans obtained with a Cirrus 5000HD-OCT was analysed. Images were corrected for lateral magnification errors. Vessel density and perfusion were measured with ImageJ/Fiji software for the superficial capillary plexus. Structural measures including foveal and macular thicknesses were performed manually.

Results: The sample included 86 participants, 51 (59%) females. Mean age was 12.4 years (SD = 2.5); mean best-corrected acuity was -0.10 logMAR (SD = 0.09); mean refractive error was +0.59 D (SD = 1.3) and mean axial length was 23.1 mm (SD = 0.86). Mean area of the foveal avascular zone (AFAZ) was 0.20 mm² (SD = 0.88); median fovea-to-macula thickness ratio (FMTR) was 0.63 (IQR = 0.08); mean central vessel density was 12.42 mm^-1 (SD = 2.78) and mean central perfusion was 38.66% (SD = 3.83). AFAZ was correlated with central vessel density (p < 0.001), perfusion (p < 0.001), foveal thickness (p < 0.001) and FMTR (p < 0.001). Central vessel density was correlated with foveal thickness (p < 0.001) and FMTR, (p = 0.01). Central perfusion was correlated with foveal thickness (p < 0.001) and FMTR, (p = 0.003).

Conclusion: In this study, foveal thickness, FMTR and foveal microvasculature measurements were correlated. Clinicians need to be aware that shallow foveal pits and persistent foveal microvasculature are likely to occur in optical coherence tomography angiography images. In healthy eyes from healthy children, an atypical high FMTR and a small AFAZ may be associated with incomplete foveal development. The mechanism and functional implications of this remain unknown.

Keywords: children; fovea; foveal avascular zone; foveal pit; optical coherence tomography angiography.

FIGURE 1

Panel showing some stages of the image processing for case ID 163 with axial length 20.97 mm. Top‐left: original uncorrected image with 758 × 777 pixels and presumed 3 × 3 mm. Top‐right: the image was cropped to remove the extra vertical pixels resulting in 758 × 758 pixels, and then the size of the scan was corrected resulting in a real size of 2.51 × 2.51 mm. The image was then “binarized” using the Fiji software; the red circle shows the total area where measurements of perfusion and vessel density were taken. It must be noted that the diameter of the total area was different for each participant, in this case 2.51 mm. Bottom‐Left: The image shows the central 1 mm diameter circle where central measurements of perfusion and vessel density were taken. Bottom‐Right: example of the measurement of vessel density in the central 1 mm. For this measurement images were “skeletonised” using the Fiji software resulting in fine lines corresponding to the length of the capillaries as shown in the image. The diameter of the central circle was 1 mm for all participants.

FIGURE 2

Relationships between the area of the foveal avascular zone (AFAZ) and structural measures of the macular area. (a) shows the negative correlation between foveal thickness (y‐axis) and area of the FAZ (x‐axis). (b) shows the negative correlation between the fovea‐to‐macula thickness ratio (y‐axis) and the area of the FAZ (x‐axis). A more marked change in fovea‐to‐macula ratio is expected for FAZ with smaller areas (left side of the graph b) but that is not captured by the linear regression line.

FIGURE 3

Example of images from two participants, one with a fovea‐to‐macula thickness ratio of 0.86 (a, b, c, ID 119) and one with a ratio of 0.56 (d, e, f, ID 140); both were male and born in 2010. The top images A and D shows the en‐face image of the superficial capillary plexus, the avascular zone in a) is extremely small compared to d ). In d), the foveal avascular zone was fitted automatically by the instrument; however, in (a), it was fitted manually because the instrument was unable to find the foveal avascular zone. Images b) and e) show the slab used by the AngioPlex software to compute the superficial capillary plexus statistics. Images c) and f) show a high‐resolution picture of the macular structure. In the “normal FAZ” shown in d) (fovea‐to‐macula thickness ratio 0.56) the corresponding structural image f) shows a large cleavage of the inner retinal layers at the foveola. In contrast, in the reduced FAZ given in a) (fovea‐to‐macula thickness ratio 0.86) the corresponding structural image c) shows a continuity of inner nuclear layer and ganglion cell layer over the foveolar centre., FAZ, foveal avascular zone.