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Role of the Episcleral Vasculature in Aqueous Humor Outflow Regulation
1Selbach J. M., 2Steuhl K. P.,
1Universität-Gesamthochschule Essen, Zentrum für Augenheilkunde (Essen)
2Universität-Gesamthochschule Essen, Zentrum für Augenheilkunde, Abt. für Erkrankungen des vorderen Augenabschnitts (Essen)
If and how intraocular pressure or aqueous humor outflow are regulated is still unclear. In the recent years there is increasing evidence that a nerval regulation of aqueous outflow might exist. Aqueous humor outflow via the conventional outflow route ist dependent on the pressure gradient between intraocular pressure (IOP) and the episcleral venous pressure (EVP). The role of the episcleral vascular system in aqueous humor outflow regulation is poorly understood. The exact correlation between increased EVP and IOP is difficult to predict. According to the Goldmann equation there should be a direct correlation between EVP and IOP. However, experimental studies wherein EVP was raised artificially by inverted posture or a cuff around the neck showed that in human eyes, increase in EVP also induced increase in IOP, but to a minor extent than the EVP increase. These studies indicate that EVP might be regulated and that the episcleral vasculature is not only a passive system. Chronically elevated IOP caused by high EVP can induce a glaucomatous optic nerve damage. This is true for glaucoma associated for instance with the following diseases: cavernous-carotid fistula, cavernous sinus thrombosis, endocrine ophthalmopathy, Sturge-Weber syndrome etc. In the last years, attempts have been made to measure EVP in patients with hypertension glaucoma. The results varied between no difference between normal and glaucomatous eyes and lower EVP in glaucomatous eyes also indicating that regulatory mechanisms might be involved. In addition the episcleral vessels show morphological peculiarities which distinguish them from other vascular systems. By means of endoscopic, corrosion cast and immunohistochemical methods we have investigated the episcleral vasculature of various species. Specifically, there are no capillaries, but numerous arteriovenous anastomoses and abundant interconnecting venules with circularly arranged muscle cells similar to the arterioles. Most strikingly, there is a dense vasoconstrictive and vasodilative innervation of the venules. In experimental studies we were able to demonstrate that in the rabbit episclera, changes of the vessel diameter induced by vasoactive substances such as epinephrine or sodium nitroprusside alter EVP as well as IOP. Vice versa, IOP elevation or reduction induces significant changes of flow, flow direction and diameter of the episcleral veins. It is presumed that changes in venous diameter might influence episcleral venous pressure and thereby alter aqueous humor outflow.