I write concerning the article by Lim and associates about the relation of retinal vascular caliber with optic disc and macular structure. They found that larger cup-to-disc ratios and thinner optic disc (OD) rims and retinal nerve fiber layer (RNFL) measurements were associated with narrower retinal vessels. They concluded that retinal vessel caliber varies systematically with morphologic differences in the optic nerve head, retina, and macula. In this letter, I offer some rational explanations for the observed relationships.
Retinal vasculature is influenced by a variety of internal and external mechanical forces such as blood pressure, intraocular pressure, and nerve fiber crowding within the optic disc. In principle, intraocular pressure can affect retinal vessels by its direct pressure as well as the tensile stress conveyed via the ocular wall structures. Because retinal vessels are embedded within optic disc rims and inner retina, any factors that increase the tensile stress of these structures can increase the mechanical stress on the vessels, and consequently can decrease the blood flow and vascular diameter.
Tensile stress of the wall of a sphere (σ) is calculated by the formula: σ = pr /2 t , where p is pressure, r is the inner radius of the sphere, and t is the wall thickness; with σ directly related to both the pressure and inner radius of the sphere and indirectly to the wall thickness. On their path, retinal vessels may be affected by mechanical forces when emerging from the optic disc as well as within the neural retina. The above-mentioned formula predicts that in the optic cup hemisphere, vascular tension would rise by enlarging the optic cup and thinning the optic disc rim (both can be translated into increased cup-to-disc ratio). In addition, in the ocular sphere, the vascular tension would rise by increasing axial length and decreasing RNFL thickness, provided that the intraocular pressure and other contributing factors remain constant.
Several studies corroborate the validity of these assumptions. Accordingly, it has been shown that increasing axial length and cup-to-disc ratio and decreasing RNFL and optic disc rim thickness are associated with thinner retinal vasculature. The formula also predicts that glaucomatous eyes with decreased RNFL thickness should have thinner vessels. This assumption may challenge the vascular theory of glaucoma; the thinner vessels may be the effect rather than the cause of the condition.
Another important issue to be discussed is the more vulnerable veins being influenced by biomechanical forces compared with arteries. Retinal arteries are more resistant to deformation than retinal veins; as a result, the magnitude of association of venular diameter to optic disc and RNFL parameters consistently was greater than arteriolar diameter. The reported “lack of association of cup-to-disc ratio with arteriolar diameter” may be rooted in the limitations in resolution of devices used for measurements as well as the relatively small sample size.
In summary, the above-mentioned formula explains many findings regarding the relation of retinal vascular diameter with ocular morphologic differences in the optic nerve head and retina. However, another important concern is the role of nerve fiber crowding within optic disc, because its increase in smaller discs, cups, or both was associated with smaller vessels.