The limbal vasculature
Introduction
Students of the classics will be familiar with the term “limbus” as the Latin word for edge or border. Its use in an ophthalmic context to refer to the boundary between the cornea and the sclera, thus seems appropriate. Even to a cursory inspection however, it is clear that this particular boundary is not sharply demarcated and it is common to view the limbus as a zone of transition rather than a line. As Hogan et al. pointed out in their comprehensive text on the subject [1], where this zone begins and ends depends on the proclivities of the observer. A pathologist for example, might base their view on anatomical considerations while tissue morphology would be more compelling to the histologist.
For the purposes of the present discussion, it is convenient to regard the limbus as the region within which the vascular network of the conjunctiva gives way to the avascularity of the cornea. As soft contact lenses overlie this region, we would be surprised if their presence did not produce some kind of effect on the associated vessels and the intention of this article is to give insight into the mechanisms whereby such influences may manifest. We begin by reviewing the relevant anatomy.
Section snippets
Anatomy of the limbal blood supply
The ocular blood supply derives from the ophthalmic artery. According to the description of Leber [2], this gives rise to two distinct, though connected circulatory components, one deep and the other superficial. The former derives from the long posterior ciliary arteries which pass anteriorly within the sclera, cross the supra-choroidal space and enter the major circle of the iris. Both the iris and ciliary body receive blood from this intra-ocular arterial circle.
The superficial branch
Control of blood flow at the limbus
Limbal vessels form part of the microcirculation. This system is that part of the vascular network where nutrient and metabolic waste transfer between blood and tissue takes place. As this action is vital to the normal function of all cells throughout the body, the number of blood vessels involved is extremely large. Constant perfusion of this vast network would require a very large volume of blood and a correspondingly powerful pump to move it around. The body avoids this rather inelegant
Hypoxia
Perhaps the most familiar way in which a contact lens can interact with the limbal vascular network is by producing hypoxia. All contact lenses that overlie the limbus interfere with gaseous exchange to a greater or lesser degree and, as shown in Fig. 3, if oxygen transmissibility is too low, limbal hyperaemia ensues [42], [43]. The precise mechanism whereby hypoxia increases limbal perfusion has not been verified. However, endothelium mediated relaxation of human arteries in response to
Summary
The blood supply of the limbus and conjunctiva, as with other body sites, is capable of minute control in response to conditions existing within very localised regions. Contact lenses can influence local cellular conditions either directly, or otherwise, in ways that precipitate a response from the vascular system supplying the tissue. This response forms part of the mechanism for dealing with the alteration to tissue homeostasis induced by the presence of the lens.
Acknowledgements
This work was supported by the Australian Commonwealth Government through their Co-operative Research Centre Scheme.
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