Etiology
Open-angle glaucoma is a neurodegenerative process wherein retinal ganglion cells slowly degenerate. It is strongly associated with an intraocular pressure >21 mmHg, myopia, age >50 years, black or Hispanic ethnicity, and hereditary factors.[8][12][13][14][15]
Juvenile-onset open-angle glaucoma and familial normal-tension glaucoma have strong genetic components.[13][16][17] Multiple genes and single nucleotide polymorphisms are associated with open-angle glaucoma, but individual effect sizes are typically small.[13] Myocilin mutations have been described in 2% to 4% of people with primary open-angle glaucoma.[13]
Pathophysiology
There are two distinct types of glaucoma: open-angle and angle-closure. In open-angle glaucoma, the anterior chamber angle is open and unremarkable in appearance. In angle-closure glaucoma, the anterior chamber is typically shallow and the peripheral cornea and iris are apposed. While their pathophysiologies differ, both cause the death of retinal ganglion cells.
Glaucoma is commonly associated with elevated intraocular pressure (IOP), possibly by a mechanism involving deformation and stress to the lamina cribrosa that compresses retinal ganglion cell axoplasmic flow.[13] Damage from increased IOP manifests as thinning of the neuroretinal rim with an enlarged cup-to-disk ratio and loss of peripheral vision as demonstrated on automated visual field testing.[1][13]
Impaired axonal transport may contribute to retinal ganglion cell death through an insufficiency of trophic factors.[18] Hypoxic insult, subsequent to faulty or reduced blood flow to the optic nerve and retinal tissues, may lead to ganglion cell damage and death. The process of ganglion cell death may accelerate due to localized vascular insufficiency compromising key ocular structures.[19]
The flow of aqueous humor maintains the pressure balance of the eye. Fluid is secreted by the ciliary body into the posterior chamber, flows to the iris and through the pupil into the anterior chamber, and then leaves through the trabecular meshwork or uveoscleral outflow routes. In normal-tension glaucoma, the IOP may be within normal limits.
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