Glaucoma is a disease in which there is a progressive loss of visual field and a characteristic alteration in the appearance of the optic nerve head.1 While the exact pathogenesis of this condition remains unclear, a significantly increased intraocular pressure (IOP) has been clearly shown to lead to damage to the optic nerve head,2 and current treatment for glaucoma consists almost entirely of interventions which lower IOP. However, some patients with glaucomatous damage have an IOP within the normal range and in some cases of glaucoma reduction of IOP to normal values does not prevent the progression of the disease, indicating that factors other than an increased IOP may be involved in the development of this condition.3 A vascular role in the pathogenesis of glaucoma, as well as in other ocular diseases, has been suggested by the association between many systemic vascular diseases (including hypertension, migraine, diabetes, and peripheral vascular disease) and the presence of glaucoma. The systemic microcirculation and ocular blood flow, which are essential for the normal function of the optic nerve head, are both impaired in these disorders. Vascular factors may be of particular importance in normal pressure or low tension glaucoma which accounts for approximately one third of all glaucoma cases and in which IOP is normal. The presence of optic disc haemorrhages among patients with low tension glaucoma, indicating ischaemic optic microinfarction or vascular insufficiency to the optic nerve head, provides further support for a vascular role in the aetiology of this type of glaucoma.4-6

Clarification of the vascular alterations during disease progression in the eye is complicated by the limited information available concerning the normal physiological control of ocular blood vessels and their role in maintaining the normal function of the eye. While the eye is one of the most …