Physiology of perfusion as it relates to the optic nerve head.

Blood flow in the optic nerve has been demonstrated to be autoregulated, and, thus, within certain limits, to be independent of the local perfusion pressure. As in the brain, a close coupling of neuronal activity and optic nerve head blood flow has been demonstrated. A number of regulatory systems and factors participate in the regulation of vascular tone in various organs, including the optic nerve. Metabolic and myogenic mechanisms keep local perfusion constant or adapted to the local metabolic needs. Such mechanisms seem to be involved in the regulation of optic nerve blood flow as well. In contrast, neuronal blood flow regulation is of minor importance in the optic nerve. Many of the regulatory modalities induce a response of vascular smooth muscle cells through stimulation of factors produced by the endothelial cell layer. Indeed, endothelial factors are of utmost importance in the regulation of optic nerve blood flow. The facts that there is a basal formation of nitric oxide, which leads to an active dilation of the ocular vasculature, and that endothelin-1 decreases blood flow to the anterior optic nerve in a dose-dependent manner suggest that alterations in these regulatory mechanisms might be relevant for optic nerve blood flow alterations as they relate to glaucomatous optic neuropathy. It is hoped that a detailed knowledge of blood flow regulation in the optic nerve might initiate new treatment modalities in optic neuropathies that are hemodynamic and vascular in nature.