The Blood Supply of the Optic Nerve Head and the Evaluation of it — Myth and Reality

Abstract

Evidence has gradually emerged that there is vascular insufficiency in the optic nerve head (ONH) in both anterior ischemic optic neuropathy (AION) and glaucomatous optic neuropathy (GON); thus both represent ischemic disorders of the ONH. Together these diseases constitute a major cause of blindness or seriously impaired vision in man. Consequently there has recently been great interest in the ONH circulation in health and disease and in how to evaluate it. Many studies of the subject have been published, with conflicting interpretations and claims. The basis of the inconsistent information seems to be confusion on some fundamental issues concerning the ONH circulation itself. The objective of this paper is to differentiate myths and misconceptions from reality about the ONH blood supply; to elucidate the reasons for disagreement on the blood supply of the ONH; and to evaluate the reliability and validity of various methods currently used to measure ONH blood flow.

Introduction

During the past quarter of a century, evidence has emerged that ischemic disorders of the optic nerve head (ONH) in man collectively constitute one of the major causes of blindness or seriously impaired vision (Hayreh, 1996). For example, anterior ischemic optic neuropathy (AION) has come to be recognized as a common, severe, visually disabling disease, particularly in the middle-aged and elderly (Hayreh (1974a), Hayreh (1974b), Hayreh (1975b), Hayreh (1981), Hayreh (1989a); Hayreh et al., 1994b). Also, there is ample evidence now that vascular insufficiency of the ONH plays an important role in the pathogenesis of glaucomatous optic neuropathy (GON) (Hayreh (1978b), Hayreh (1994)). Therefore, to understand the pathogenesis and management of these important diseases, it is imperative to have a good understanding of the highly complex nature of the in vivo ONH circulation in health and disease. It is interesting to note that in 1954 François and Neetens described the subject of blood supply of the ONH as of “only moderate interest to research workers”. However, at that time the entity of AION was unknown and GON was considered to be essentially mechanical, due to raised intraocular pressure (IOP). With the realization of the high prevalence of ischemic disorders of the ONH, the blood supply of the ONH and evaluation of blood flow in the ONH have evolved into a topic of great interest for researchers working in both basic and clinical fields, and a voluminous literature has accumulated, generating light on some aspects and merely heat on others. It is essential to separate myth from reality on these topics, so that the interpretation of findings about ONH circulation and its role in ONH ischemic disorders are put in their proper perspective. To do that, it is essential first to discuss briefly the blood supply of the ONH and then to review in the light of that information the various methods currently being used to measure the blood flow in the ONH. The findings in lower species such as rabbits, rats, etc., may not be relevant to man because their vascular patterns are quite different; in view of that, this review does not deal with studies on ONH blood supply in lower animals.

(For a complete bibliography and detailed review of previously published studies in the literature on the subject, please refer to the author's previous publications from 1958 onwards.)