Abstract and Introduction

AIM: To assess the macular capillary networks and foveal avascular zone (FAZ) with swept-source optical coherence tomography angiography in healthy eyes.

METHODS: This cross-sectional, prospective, observational study enrolled 222 eyes of 116 healthy participants with no ocular or systemic disease. SS-OCTA images were captured using the PLEX Elite 9000 (Carl Zeiss Meditec Inc., Dublin, CA, USA) with a 6 × 6mm pattern centered on the foveal center. Vessel length density (VLD), perfusion density (PD), and FAZ parameters were analyzed using the manufacturer’s automated software.

RESULTS: A significant negative correlation was observed between age and average VLD in the superficial plexus, and average PD in both the superficial plexus and the whole retina. A significant positive correlation between age and foveal avascular zone perimeter and area was also noted. Age-wise comparisons showed a trend for an increase in VLD and PD until 40 years of age, with a subsequent decrease in the older age in the macular region. The central subfield showed a decrease in the vessel density measurements in the 21–40 age group. FAZ area and perimeter were the mirror inverse of the central subfield vessel density measurements with a numerically greater area and perimeter in the 21–40 age group compared to the 0–20 and 41–60 age groups. FAZ circularity was significantly reduced after 40 years of age.

CONCLUSION: Age-related changes in the vessel density and FAZ parameters in the healthy macula are complex and vary with the macular location. These results carry significance when interpreting the data from diseased eyes.

Introduction

Optical coherence tomography angiography (OCTA) has significantly enhanced our knowledge of the retinal and inner choroidal microcirculation. OCTA, which utilizes decorrelation signals from OCT scans obtained over time, allows the circulation to be visualized in a depth-resolved volumetric fashion ^[1,2], offering significant advantages over conventional dye-based fundus fluorescein angiography and indocyanine green angiography ^[3]. While OCTA cannot currently demonstrate leakage which can provide insight into the integrity of the blood-retinal barrier, the absence of obscuring leakage provides a clearer visualization of the circulation, particularly the deeper layers. The deeper capillary layers can be specifically impacted in various retinal vascular diseases including diabetic retinopathy and retinal vascular occlusive disease ^[4-8]. A specific phenotype of selective occlusion of the deeper circulation, termed pcentral acute middle maculopathy (PAMM), has also been described ^[9].

Recently, OCTA has become available on swept-source (SS) OCT devices, which feature a tunable laser with a longer deeper-penetrating wavelength (1050 nm) and faster scanning speed. These devices offer some advantages over spectral-domain (SD) OCT devices for evaluation of the choriocapillaris and inner choroid ^[10].

OCTA has provided novel insights into the organization and physiology of the normal healthy vasculature. These include a detailed description of the number of layers in which the vascular plexuses are arranged, regional variations between the macular and peripapillary region, a hybrid model of series and pllel organization of the microvascular blood flow, with an apparent directional flow between the superficial (SCP) and deep capillary plexuses (DCP) and preferential venous outflow at the level of the DCP ^[11-14].

Since the macula is amongst the metabolically most active tissues of the body, it is affected by hypoxic conditions early on in many disease states ^[15-17]. It is supplied by multiple surrounding capillary plexuses for its high oxygen demand, and therefore, depends upon the health of these capillary beds for its proper integrity and function ^[18]. Understanding the normal circulation is a critical prerequisite for understanding the impact of diseases. Characterizing the variation in the normal circulation related to age, ethnicity, and retinal region is essential. Not surprisingly, as with other vascular systems, changes have been reported to occur in the retinal microvasculature with age ^[19,20]. The alterations occurring in the foveal avascular zone (FAZ) and surrounding vessels with age in healthy eyes have been described in the past ^[21-35]. Though the relationships are not perfectly consistent, generally, a decreased perifoveal vessel density and increased FAZ size have been reported with advancing age ^[23-32]. The perimeter of the FAZ is supplied by both the SCP and DCP which form a ring of interconnecting capillaries at the margin of the FAZ ^[36,37]. The FAZ zone is known to be sensitive to ischemic events, and the shape and size of the normal FAZ correlate well with foveal morphology ^[33]. Recently, researchers have reported early evidence of alterations in the FAZ circularity in patients with diabetes as compared with healthy eyes ^[38-40]. One important reason for the lack of consistent results in all such studies may be related to the age of the study cohort included for analysis. Thus, precise information regarding age-related alterations in the FAZ and surrounding retinal vasculature becomes a prerequisite to understand the development in health and alterations in diseased states ^[41]. To address this need, we assessed normal healthy eyes over a wide age range using SS OCTA device which permits dense scanning of the macular circulation to unravel the complex variations in these structures as the age advances.