Description

Primary open-angle glaucoma (POAG) is a chronic, progressive optic neuropathy which causes loss of the optic nerve rim and retinal nerve fiber layer (RNFL) with associated visual field defects. POAG is associated with increased intraocular pressure (IOP). Normal IOP is 10 to 21 mm Hg. However, early glaucoma can be present even with normal IOP, which can make early detection very difficult.

Pregnancy Considerations
Prostaglandins should be avoided during pregnancy in the treatment of POAG.

Epidemiology

Prevalence

• Prevalence in persons >40 years of age is ~3.5%.
• POAG is the second leading cause of blindness and the leading cause of blindness of people of African descent in the United States.

Etiology and Pathophysiology

• Aqueous humor is produced by the ciliary epithelium of the ciliary body and is secreted into the posterior chamber of the eye. The purpose of aqueous humor is to maintain the shape and function of the eye as well as provide nourishment. Aqueous humor then flows through the pupil and enters the anterior chamber to be drained by the trabecular meshwork (TM) in the iridocorneal angle of the eye. It then drains into the Schlemm canal and passes into the episcleral venous system. 5–10% of the total aqueous humor outflow leaves via the uveoscleral pathway.
• IOP is maintained by a balance of aqueous humor production in the ciliary body and drainage by the TM.
• Impaired aqueous humor outflow through the TM leads to greater resistance in the aqueous humor drainage system and causes an increase in IOP.
• IOP varies with blood pressure and respirations as well as throughout the day with lower IOP observed in the evenings.
• Increase in IOP above normal values over time can damage the optic nerve, eventually leading to vision loss. Typically, peripheral vision is lost before central vision, which allows POAG to go undiagnosed for longer.

Genetics

• TMCO1 genotype has been found to increase the risk of developing glaucoma among non-Hispanic whites.
• The myocilin (MYOC) gene was the first gene associated with POAG.

Risk Factors

• Myopia
• Diabetes mellitus, hypothyroidism, hypertension
• African descent
• Positive family history
• Prolonged use of corticosteroids
• Systemic calcium channel blockers
• High coffee consumption
• Prior history of pars plana vitrectomy
• Obstructive sleep apnea

General Prevention

Higher dietary nitrate and green leafy vegetable intake has been associated with a lower POAG risk. Evidence suggests that nitrate, a precursor of nitric oxide, is beneficial for blood circulation.