Relationship between the foveal avascular zone and foveal pit morphology

Abstract

Purpose: To assess the relationship between foveal pit morphology and size of the foveal avascular zone (FAZ).

Methods: Forty-two subjects were recruited. Volumetric images of the macula were obtained using spectral domain optical coherence tomography. Images of the FAZ were obtained using either a modified fundus camera or an adaptive optics scanning light ophthalmoscope. Foveal pit metrics (depth, diameter, slope, volume, and area) were automatically extracted from retinal thickness data, whereas the FAZ was manually segmented by two observers to extract estimates of FAZ diameter and area.

Results: Consistent with previous reports, the authors observed significant variation in foveal pit morphology. The average foveal pit volume was 0.081 mm(3) (range, 0.022 to 0.190 mm(3)). The size of the FAZ was also highly variable between persons, with FAZ area ranging from 0.05 to 1.05 mm(2) and FAZ diameter ranging from 0.20 to 1.08 mm. FAZ area was significantly correlated with foveal pit area, depth, and volume; deeper and broader foveal pits were associated with larger FAZs.

Conclusions: Although these results are consistent with predictions from existing models of foveal development, more work is needed to confirm the developmental link between the size of the FAZ and the degree of foveal pit excavation. In addition, more work is needed to understand the relationship between these and other anatomic features of the human foveal region, including peak cone density, rod-free zone diameter, and Henle fiber layer.

Figure 1.

Quantifying FAZ morphology using semiautomated segmentation methods. (A) FAZ image from subject JC_0342 taken with the RFI. (B) The edge of the FAZ was delineated with 40 manually selected points. (C) The coordinates were saved and processed with custom software to quantify FAZ area.

Figure 2.

Bland-Altman plot showing the agreement between FAZ area values obtained from manual segmentation data performed by two observers. The mean difference is 0.029 mm² (95% CI, 0.016–0.041 mm²), indicating a slight bias of one observer to select a larger FAZ segmentation. The limits of agreement are −0.05 mm² and 0.11 mm². For subsequent analyses, the FAZ area value used is the average of the values obtained from the two observers.

Figure 3.

Process for measuring foveal pit volume. (A) Retinal thickness maps were exported and interpolated to 512 × 512 pixels and then transformed to polar coordinates. (B) The foveal rim was defined as described in the text. An irregular surface was defined as described in the text to create a cap on the foveal pit. (C) The fill of the space between this cap and the internal limiting membrane represents the volume of the foveal pit.

Figure 4.

Comparison of FAZ imaging methods. Shown are images of the FAZ for two subjects obtained using three different imaging methods (RFI, AOSLO, FA). Estimates of FAZ area varied by <1.5% between the three methods for each subject. Scale bars, 0.10 mm.

Figure 5.

Correlation between FAZ area and foveal pit morphology. Significant correlation between FAZ area and foveal pit area (A), pit depth (B), and pit volume (C) were observed. Collectively, these data indicate that bigger FAZs are associated with larger foveal pits, which is consistent with the hypothesis that the size of the FAZ determines the degree of foveal excavation.

Figure 6.

Examples of increasing FAZ with increasing foveal pit excavation. Foveal OCT scans (A, D, G), retinal thickness maps (B, E, H), and FAZ images (C, F, I) are shown for three subjects. Data from the left eye of a patient with albinism reveals that the absence of a foveal pit is associated with the absence of a FAZ (A–C). The data from this patient were not included in the present analysis but is shown to illustrate one extreme in how the FAZ and pit are related. In two healthy persons with extreme foveal pit morphology, the positive correlation between the size of the foveal pit and the size of the FAZ is captured (D–I). Scale bar, 100 μm.