Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2589_Библиотеки_им_академика_М_И_Перельмана

ow from the heart. The medium and small arter­ies, called muscular arteries, have more smooth muscle cells than elastin because these arteries have contractile properties, which help regulate the blood supply to the target organ. There are many functional and anatomical differences in the elastic and muscular arteries, and the readers are referred to an extensive discussion on the sub­ject [1]. Muscular arteries also have internal and external elastic lamina on either side of the tunica media. The veins distinctly lack the elastic lam­ina and very few smooth muscle cells but have unidirectional valves, especially in the leg veins, as they carry blood against gravity.

Our current understanding of the central retinal artery (CRA) anatomy is based on extensive dissections and studies by Dr. S.S. Hayreh [3]. The CRA, often the rst branch of the ophthal­mic artery, is the primary source of blood sup­ply to the inner retina. It rst traverses in orbit and enters the optic nerve dural sheath to run in the vaginal space before penetrating the optic nerve about 10mm behind the globe to emerge as a single artery in the centre of the optic nerve head. As it emerges from the optic nerve head, it divides into a superior and an inferior divi­sion, subdividing further to supply all four quadrants of the retina. The retinal arterial sys­tem is end-arterial, and thus occlusion of any branch of the arteriolar system will lead to ret­ina ischaemia in the arteriole’s supply area. The CRA is responsible for the supply to the inner retina up to the outer limit of the inner nuclear layer and accounts for only 20% of the blood supply of the retina, the outer retina meeting its oxygen and micronutrient requirements from the choroid, which is fed by the posterior cili­ary arteries. The posterior ciliary arteries are also branches of the ophthalmic artery. In nearly one third of the eyes, bilateral in nearly 15% of people, there may be an additional small artery, the cilioretinal artery, a branch of a posterior ciliary artery or the peripapillary choroid that supplies a variable area of the entire thickness of the macula. Rarely, a minor arterial branch may arise from the CRA in the optic nerve head before it emerges into the eye and resembles a cilioretinal artery. The origin of the cilioretinal artery is best demonstrated on the fundus uorescein angiography as the dye lls the cilioretinal artery synchronously with the choroid a short while before the dye reaches the CRA [4]. In the retina, only the CRA is structurally a muscular artery, and like all such arteries, the purpose of the CRA is to distribute blood to the retina. These arteries are pulsatile; even in the retina, the CRA pulsates synchro­nously with the cardiac rhythm. Pulsations of

Moreover, the retinal arterial system is auto­regulated to maintain a constant blood ow to the inner retina. It can maintain perfusion of the inner retina even if the intraocular pressure rises to 40–50mm of Hg [5]. The autoregulation of blood ow at the level of the neuro-glial-vascular unit controls the blood supply to meet the metabolic requirements of the neural elements in the inner retina. The fractal geometry of the retinal vessels ensures a uniform blood ow and supply and removal of the metabolites from the inner retina.

Quantifying the various aspects of retinal ves­sel geometry, including their fractal dimensions, is now possible using digital images or digitizing them. The fractal dimension, a measure of the complexity of the retinal vessels, has been calcu­lated as 1.7 [8, 9]. Patients with Alzheimer’s dis­ease have been found to show decreased arteriolar and venular fractal dimensions [10] and, in future, may become an important biomarker to predict the onset of Alzheimer’s disease.