Alessia Casalucci

The retina is one of the most metabolically active tissues in the body. A well-organized ocular vascular system adapts to meet the metabolic requirements of the retina to ensure visual function. The retina is supplied mainly by the central retinal artery that travels along the inferior margin of the optic nerve sheath. It then enters the eye through the center of the optic nerve and branches to form three capillary layers. Blood drains from the capillary bed into retinal veins that eventually converge into the central retinal vein, which runs through the optic nerve into the cavernous sinus. The flow in these vessels is directly affected by the intraocular pressure (IOP) in the eye, the cerebrospinal fluid pressure (CSFP) in the optic nerve and intracranial pressure (ICP) in the cavernous sinus. Blood retinal circulation has various peculiarities, among which the fact that blood vessels are acted on by the intraocular pressure and, therefore, the transmural pressure is smaller than in other vascular beds. As a consequence, retinal vein pulsation is often observed. This visible phenomenon occurs spontaneously in 67 to 98 % of eyes and its frequency increases with age. Part of the fascination for retinal venous pulsation is that it is the only site of vascular pulsation that is more visible than arterial pulsation, and also that unlike the vein or venules elsewhere in the body that do not pulsate in time with the cardiac cycle, the retinal arterial and venous pulsation are in phase, being generated by the relations between venous pulsation pressure, intracranial and intraocular pressure. By measuring vein pulsation pressure has become the standard technique to quantify pulsation and it is very useful for non-invasive CSF pressure measurements but also as a prognostic value in predicting long-term outcomes for glaucoma, venous occlusion and diabetic retinopathy. Absence of retinal vein pulsation is thought to be associated to large values of the intracranial pressure but the link between retinal circulation and cerebrospinal pressure is still poorly understood from the mechanical point of view.

Aim of this research is to construct a detailed geometric multiscale model for the retinal circulation, involving arteries, veins and microvasculature. We also expect to investigate the complex interaction between retinal circulation and cerebrospinal pressure and the factors causing the retinal venous pulsation.

The topology of arterial and venous network is extracted using the Automated Retinal Image Analyzer (ARIA) software in order to detect the vessels from the fundus images. The network is segmented starting from medical images included in the RITE database which is established based on the public available Digital Retinal Images for Vessel Extraction (DRIVE) database. Rite contains 40 sets of images each of which comprises a fundus photograph, a vessel reference standard and an Arteries/Veins (A/V) reference standard. The constructed model is coupled to an existing global multiscale closed-loop human circulation model developed at the University of Trento by Müller and Toro.

The present model reproduces blood flow and pressure wave propagation in the retinal network. Moreover, it constitutes a first step towards the study of the relationship between IOP and ICP. This model represents a framework that can be used to study retinal vein pulsation and various pathological conditions that regard the interaction of the eye vasculature with other fluids (such as glaucoma).