Vascular reactivity of optic nerve head and retinal blood vessels in glaucoma--a review.

Glaucoma is characterized by loss of retinal nerve fibers, structural changes to the optic nerve, and an associated change in visual function. The major risk factor for glaucoma is an increase in intraocular pressure (IOP). However, it has been demonstrated that a subset of glaucoma patients exhibit optic neuropathy despite a normal range of IOP. It has been proposed that primary open angle glaucoma could be associated with structural abnormalities and/or functional dysregulation of the vasculature supplying the optic nerve and surrounding retinal tissue. Under normal conditions, blood flow is autoregulated, i.e., maintained at a relatively constant level, in the retina and ONH, irrespective of variation in ocular perfusion pressure. A number of factors released by the vascular endothelium, including endothelin-1 and nitric oxide, are suggested to play an important role in the regulation of local perfusion in the retina and ONH. Most work to-date has investigated homeostatic hemodynamic parameters in glaucoma, rather than the measurement of the hemodynamic response to a provocation. Future work should comprehensively assess blood flow in all the ocular vascular beds and blood vessels supplying the eye in response to standardized stimuli in order to better understand the pathophysiology of glaucomatous optic neuropathy.