The Influence of Ocular Refraction on Retinal Vascular Geometric Parameters

Abstract

Abstract: : Purpose: Quantitative measurements of the retinal vasculature have been shown to correlate with risk factors for cardiovascular disease and to predict cardiovascular events. However the impact of variations in ocular optics and anatomy on these measures has not been studied extensively. This is important as both axial length and refraction vary between individuals and ethnic groups. Refraction also changes with time and is affected by cardiovascular disease, particularly diabetes. The aim of this study was to investigate whether measurements of retinal vascular geometric parameters are influenced by ocular axial length, refraction and corneal curvature. Methods: 79 subjects, 45 males and 34 females, drawn from a cohort of twin pairs were studied. Subjects were aged 14–38 years and 45 suffered from type 1 diabetes. All had normal BP levels (<140/90 mmHg) and none received antihypertensive medication. Ocular biometric measurements and retinal photography was performed. After digitisation of retinal photographs and operator–directed image analysis, vessel width, vessel length diameter ratio (LDR), bifurcation angle and expansion factor (EF=(d₁² + d₂²)/(d₀²), where d₁ and d₂ = diameters of daughter vessels and d₀ is diameter of parent vessel) were calculated. Possible associations between ocular variables and retinal parameters were sought using univariate regression in the whole cohort and after stratification into diabetic and non–diabetic individuals. Results: The range of ametropia studied was –3.31 to +3.00 D. This corresponds to at least 90 % of the biological range. Univariate analysis showed a significant negative relationship between axial length and vessel width (ß= –0.751, p<0.001), and a negative relationship between axial length and bifurcation angle (ß= –2.088, p<0.034). This was seen in the whole group and in the diabetic/non–diabetic subgroups. Expansion factor and LDR were not related to axial length. Refraction was unrelated to any retinal parameter. Corneal power was negatively correlated to axial length (ß= –1.138, p<0.000) but not to refraction. Conclusions: Variation in axial length influences estimates of vessel diameter and bifurcation angle. In contrast, dimensionless measures such as LDR and EF are not related to axial length or refraction and may prove useful measures of retinal vascular geometry.

Keywords: retina • refraction