Background/Aim: Autoregulation is the ability of a vascular bed to keep its blood flow stable despite changes in perfusion pressure.
While several studies have investigated choroidal blood flow (ChBF) regulation and the mechanisms behind it, only few data are available for the optic nerve head (ONH). The aim of the present project was to investigate whether the regulatory behavior of the ONH differs from that of the choroid. Further, the role of nitric oxide (NO) in ONH blood flow regulation during changes in ocular perfusion pressure (OPP) was assessed.
Material and Methods: The present project consisted of three studies in healthy volunteers. In the first study, choroidal and ONH blood flow in response to an experimental increase and decrease in OPP was compared.
In addition, two three-way crossover, double-masked, placebo controlled studies were performed in which subjects received an unspecific NO synthase (NOS) inhibitor (L-NMMA), an alpha-receptor agonist (phenylephrine) or placebo. During infusion of these substances, OPP was increased by isometric exercise in one study and in the other study OPP was decreased by an artificial increase in intraocular pressure (IOP).
In all studies, blood flow was assessed using laser Doppler flowmetry.
Results: The response in blood flow to provocation was less pronounced than the response in OPP, indicating for blood flow regulation in the choroid as well as in the ONH. ChBF regulated better than optic nerve head blood flow (p =0.023) during isometric exercise. ONH blood flow regulated better than ChBF (p = 0.001) during artificial IOP increase.
Administration of an NOS inhibitor significantly decreased ONH blood flow at rest (p < 0.01). In contrast, inhibition of NOS did not alter the pressure/flow relationship in the ONH during isometric exercise or experimental increase in IOP (p = 0.37 and p = 0.49, respectively).
Conclusion: ONH blood flow regulation is complex and differs from that of the choroid. During isometric exercise, the choroid regulates its blood flow better than the ONH while the opposite is the case during artificial increase in IOP. NO is an important regulator of basal vascular tone in the ONH. In contrast, it does not seem to be involved in the regulatory processes during experimental manipulation in OPP.