Evaluating the Quantitative Foveal Avascular Zone and Retino-Choroidal Vessel Density Using Optical Coherence Tomography Angiography in a Healthy Indian Population

Objective: The foveal avascular zone (FAZ) is the round capillary-free zone within the macula and is supplied only by a single-layered parafoveal capillary arcade. This study aimed to evaluate the quantitative FAZ and retino-choroidal vessel density (VD) using optical coherence tomography angiography (OCTA) in a healthy Indian population. Methods: This was a cross-sectional observational study that was conducted for evaluating the quantitative FAZ and retino-choroidal VD of 200 eyes of 100 healthy Indian subjects, including 62 males and 38 females (age range 17-50 years) having the best-corrected visual acuity (BCVA) of logMAR 0 (20/20; 6/6) and spherical equivalent refractive error of not more than 1 D. The subjects were examined using OCTA automated software on spectral-domain OCT (SD-OCT; Nidek RS 3000 Advance 2; Nidek, Inc., Fremont, CA) on a 3 x 3 mm OCTA macular scan centred on the fovea. The FAZ size, perimeter and circularity index, VD in superficial, deep, and outer retina (OR), outer retinal chorio-capillaries (ORCC), chorio-capillaries (CC) and choroid (C) were analysed in the circular and quadrant-segmented zones. A correlation was found between the FAZ size, perimeter and circularity, and VD in retino-choroidal layers, and between BCVA, age, central foveal thickness (CFT) and sub-foveal choroidal thickness (SFCT), and OCTA parameters. Results: The FAZ and surrounding vascular arcades were intact in all eyes, showing either a vertical or horizontal oval-shaped symmetrical formation without gaps, holes or interruption of the capillary network. The mean value of CFT was 237.5±26.0 microns and SFCT was 269.6±53.0 microns. The mean FAZ area was 0.42±0.23 mm(2), FAZ perimeter was 3.3±1.0 mm and FAZ circularity index was 0.46±0.1. The mean VD in superficial capillary plexus (SCP) was 23.87±10.66, in deep capillary plexus (DCP) was 16.03±9.90, in OR was 13.22± 12.27, in ORCC was 39.74±14.32, in CC was 37.02±16.43 and in choroid was 37.43±16.76. The increasing order of VD in different retino-choroidal layers was OR<DCP<SCP<CC<C<ORCC. The FAZ area and perimeter had a statistically significant (SS) negative correlation with the VD in SCP, DCP and OR, while the FAZ perimeter additionally had a SS negative correlation with the VD in the choroid. The FAZ circularity had a SS positive correlation with VD in SCP, DCP and OR. There was no SS correlation between the age of subjects and OCTA parameters. The BCVA had a SS correlation with only one parameter, i.e. ORCC foveal (r=-0.15; p=0.03). There was a SS negative correlation between CFT and FAZ area and perimeter, and a SS positive correlation between the CFT and VD in SCP, DCP and OR. There was a SS positive correlation between the SFCT and VD in different layers including SCP, DCP, ORCC, CC and choroid. Conclusions: VD is the highest in the ORCC and lowest in the OR segment. A better VD in SCP, DCP and OR is associated with a reduced FAZ area/FAZ perimeter and an increased FAZ circularity index. A lower VD in the ORCC is associated with worse BCVA. In a healthy subject, a relatively thicker foveal region is likely to have better vascularity of SCP, DCP and OR and a smaller FAZ area/FAZ perimeter. In the absence of diseases, relatively thicker sub-foveal choroid is expected to have better vascularity of SCP, DCP, ORCC, CC and choroid.