Fractal analysis of the macular region in healthy eyes using swept-source optical coherence tomography angiography.

Abstract

Purpose : Fractal dimension (FD) is a quantitative metric used to describe the complexity of a structure (branching pattern of retinal vessels) as a mathematical ratio. Although the FD of the retinal circulation varies in diseased states, age-related variations are not well documented. Thus, we calculated the FD over the macular region using swept-source optical coherence tomography angiography (SS-OCTA) in a healthy wide age-range cohort.

Methods : In this cross-sectional observational study, 116 healthy subjects were screened for any given diagnosis prior to enrollment at the Medical Research Foundation, Chennai, India. Of these, 222 eyes were imaged by SS-OCTA using a Zeiss Plex Elite 9000 (6x6mm scans centered at the fovea). En face OCTA slabs of the superficial capillary plexus (SCP) and deep capillary plexus (DCP) were binarized using Image J, and the FD was calculated using the Fractalyse plug-in. FD was computed over the entire 6x6 scan area as well as within the 1mm, 3mm, and 6mm rings of the Early Treatment Diabetic Retinopathy Study (ETDRS) grid. The mean FD values within specific age categories (Table 1) were computed and the relationship between FD values and age was assessed using Pearson correlation coefficients.

Results : The FD over the entire 6x6 region was (mean ± standard deviation) 1.931 ± 0.010 for the SCP and 1.939 ± 0.014 for the DCP. Although the FD values were consistently higher in the DCP compared to the SCP in all ETDRS rings as well as the whole scan region, the difference was statistically significant for the 6mm ring (P=0.01), and the whole scan region (P=0.001). FD values, however, appeared to be weakly or poorly correlated with age (Table 2), as only the FD of the SCP in the 6mm ring (r=0.164, p 0.01), and of the DCP in the 1mm circle (r=0.135, p 0.04) showed a statistically significant relationship with age. Overall, differences in FD values in these normal eyes were small (<1%) regardless of age or region, and thus unlikely to be of clinical relevance.

Conclusions : In normal, healthy eyes, FD values appear to be relatively uniform across the macula in both the SCP and DCP with relatively little change with age. This would suggest that FD metrics may not need to be adjusted for age when assessing FD in the setting of retinal vascular disease.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.