[Implications of nerve control of choroidal blood flow in ocular diseases].

EDITORIAL Physiologically, one of the main functions of the choroids is to nourish (supply O 2 and glucose) the outermost layers of the retina (mainly photorecep-tors and RPE). However, the choroids seem to be perfused in a proportion which exceeds its supply needs, suggesting therefore an additional role explaining the high rate of choroidal flow. It is believed that this flow could aid in maintaining IOP as well as carry out a thermal regulation action through the following mechanisms: dissipating the heat generated during the visual phototransduction process, preventing overheating of the external retina during exposure to bright light and finally warming the intra-ocular structures which may lose heat while exposed to extreme environmental conditions. In recent years it has been evidenced that nerve control plays an important function in regulating said choroidal blood flow. The neuroregulation of the uveal flow is governed by multiple mechanisms. This regulation would be carried out directly through peri-vascular inner-vation, which would allow a balance between vessel constriction and dilatation necessary for maintaining blood flow, whereas the indirect regulation would be carried out through paravascular fibers, both through typical neurotransmitters and by neu-ropeptides released by sympathetic, parasympathe-tic and sensory terminations of the stroma, and dif-fusible factors such as nitric oxide. It has been observed that in the human choroids, both the perivascular and paravascular fibers are marked with antibodies against sympathetic system neuropeptides (Neuropeptide Y [NPY], Parasym-pathetic (Vasoactive Intestinal Peptide [VIP]) and Sensitive (Substance P [SP] and Calcitonine Gene Related Peptide [CGRP]) (1). Sympathetic stimulation causes a sharp choroidal vasoconstriction and a fall in IOP due to a reduction in the ocular blood volume (choroidal flow reductions of up to 60%). This response is mainly executed by the stimulation of α-adrenergic receptors located in the smooth muscle cells of the vessels. Sympathetic innervation causes the choroids to be under a vasoconstrictor tone, which suggests that this could protect the retina and the optic nerve head from hyper perfusion and rupture of ocular barriers (which could occur in certain circumstances such as high arterial pressure). The role of parasympathetic innervation is not so well defined as the role of the sympathetic system. However, it has been observed that the choroids respond to parasympathetic collinergic stimuli (arriving through the short ciliar nerves) by means of vasodilatation. This vasodilatation would explain the light-induced reflex increase of choroidal flow. Recently it has been proposed that the sensitive peripheral …