Local Determinants

Core Messages

Local determinants of ocular blood ßow include physical factors like arterial pressure and intraocular pressure, which determine the perfusion pressure across the ocular circulations, and active mechanisms inherent to ocular blood vessels and their interaction with their immediate environment. Based on evidence from more accessible circulations, the possible active mechanisms include:

¥Myogenic local control

¥Metabolic local control

¥Flow-mediated vasodilation

¥Flow control by intercellular conduction There is evidence of local control in the

ocular circulations, but which local mechanisms are responsible and how they interact is still under investigation.

11.1Introduction

The local determinants of blood ßow through the ocular circulations include both passive and active mechanisms. The most obvious

J.W. Kiel, Ph.D., FARVO

Department of Ophthalmology, University of Texas Health Science Center, 7703 Floyd Curl Drive MS 6230, San Antonio, TX 78229-3900, USA

e-mail: kiel@uthscsa.edu

passive mechanism is the compressing force exerted by the intraocular pressure (IOP) on the intraocular blood vessels, particularly the veins. By contrast, the active Òlocal controlÓ mechanisms in the ocular circulations are more difÞcult to deÞne. Based on other tissues with more accessible circulations, the possible local control mechanisms include vascular responses linked to nearby tissue metabolism (e.g., reactive hyperemia, functional hyperemia, and autoregulation), transmural pressure (e.g., myogenic response, reactive hyperemia, and autoregulation), shear stress (ßow-dependent vasodilation), and intercellular conduction. The relative contributions of these active mechanisms in the ocular circulations are hard to deÞne given the complex vascular organization and relative inaccessibility. Ocular blood ßow measurement is technically challenging, and discrete perturbations that elicit unambiguous responses characteristic of a particular mechanism in a single vascular bed are difÞcult to achieve.Eliminatingconfoundingneurohumoral inputs is a further, though not insurmountable, challenge. For these reasons, we can often infer that a blood ßow response in an ocular circulation is locally mediated, but the relative contributions of the underlying local control mechanisms are ill-deÞned. Nonetheless, there is evidence of local control behavior in the prelaminar optic nerve, choroid, retina, ciliary body, and iris.