The widening use of ultra-widefield (UWF^™) retinal imaging by ocular health practitioners is prompting researchers to address what appears to be a straightforward question: What’s normal? That is, now that UWF imaging is providing a more complete visualization of the peripheral retina, what is the physical extent of healthy retinal vasculature?

It’s not an academic question. On one level, baseline information about the extent and appearance of healthy retinal vasculature is an essential part of clinical practice. But more important, as practitioners use UWF to measure the extent of retinal pathology – for example, in estimating areas of ischemia in patients with diabetic retinopathy (DR) or retinal vein occlusion (RVO) – it’s critical to measure the portion of the retina that is affected. These estimates need to start with accepted measurements for normal, healthy retinal vasculature, one derived from statistically significant population studies and adjusted for any variations in age, sex, or other characteristics.

New research is giving first indications of the extent of healthy retinal vasculature as well as important population variations. This work is being enabled by recent advances in UWF image processing which allow optomap^® fa (fluorescein angiography) images to be corrected for peripheral distortion. These new processing algorithms – a part of Optos ProView^™ imaging software – create a highly accurate visualization of the retinal vasculature that can be mapped and measured.

In a paper published in 2016¹, researchers reported on a preliminary study that measured the normally perfused retinas of 31 healthy individuals aged 20 to 80.

— Those measured had a mean area of perfused retina of 977.0 mm². This compares to the total retinal area of 1133.8 mm²  established in other retinal studies².

— The mean distance from the center of the optical disc to the peripheral vascular border was measured at 20.3 mm +/- 1.5 mm. There were significant differences in mean values depending on which quadrant of the retina was measured.

— No significant differences were found between male and female eyes, or between right and left eyes.

—Older subjects – aged 60 and older – were measured with a smaller distance between the optical disc and vascular periphery as compared to younger subjects. This suggests older patients may need to be diagnosed using different baselines.

In another study³, research was done on 58 eyes of 31 patients who met the inclusion criteria. This study was an observational retrospective cohort study. Adult patients
with epiretinal membrane or choroidal nevi who underwent imaging with Optos 200 MA/
200Tx were included. Dye transit times, peripheral arteriovenous shunting, presence of
vessels crossing the horizontal raphe, right angle vessels, terminal networks, absence of
capillary detail, ground glass hyperfluorescence, peripheral drusen, and microaneurysms
were evaluated.

Results:

—Mean peripheral arterial filling time was 8.65 ± 2.54 seconds (range 3–15 seconds). One or more peripheral anomalies were noted in all patients (P , 0.01). The prevalences of findings were: arteriovenous shunting (0.00%), vessels crossing the horizontal raphe (44.83%), right angle vessels (70.69%), terminal networks (77.59%), absence of capillary detail (98.28%), ground glass hyperfluorescence (87.93%), drusen (34.48%), and microaneurysms (41.38%).

Conclusion:

—There was a high prevalence of peripheral vascular anatomic variations in eyes expected to have normal peripheral retinal vasculature. These findings may provide a reference for future studies addressing putative pathologic peripheral angiographic findings.

Studies on larger and more diverse populations will need to be undertaken before universal baselines can be established for clinical use. However, this work (1) provides a highly valuable methodological framework for future studies and, (2) demonstrates the need to examine diverse populations to assure that significant systematic differences are measured and understood. The outcome will be more refined diagnoses using ultra-widefield imaging.

Sources:

1. SaddaS,Sagong M, van Hemert J, et al. Ultra-widefield imaging of the peripheral retinal vasculature in normal subjects, Opthamology, 2016 May;123(5):1053-9. doi: 10.1016/j.ophtha.2016.01.022. Epub 2016 Feb 17.
2. Taylor E, Jennings A. Calculation of total retinal area. Br JOphthalmol1971; 55: 262–5.
3. Shah AR¹, Abbey AM, Yonekawa Y, Khandan S, Wolfe JD, Trese MT, Williams GA, Capone A Jr. Widefield Fluorescein Angiography in Patients Without Peripheral Disease: A Study of Normal Peripheral Findings. Retina. 2016 Jun;36(6):1087-92. doi: 10.1097/IAE.0000000000000878.