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Open Angle Glaucoma (also call Primary Open Angle Glaucoma or 'POAG') is a generic term for glaucomas which exclude angle closure glaucoma (Closed Angle Glaucoma or CAG), and Normal Tension Glaucoma (NTG). Even some glaucomas of known etiology like pigmentary glaucoma and pseudoexfoliation glaucoma (XFG) might be considered subsets of POAG by some clinicians. However, POAG itself is poorly understood. Eye pressure is the single most important diagnostic and treatment factor in glaucoma, and it is generally accepted that normal intraocular pressure runs in the range of 10-21 mm Hg. If pressures are found consistently higher than this range, but there is no visible optic nerve damage or Visual Field abnormalities, then the diagnosis is usually Ocular Hypertension (OHT) - which is sometimes also termed 'glaucoma suspect.' The difference between OHT and true glaucoma, is the finding of optic nerve deterioration or visual field defects. Primary open angle glaucoma has the following general characteristics:
- It is characterized by open anterior angles (called iridocorneal angles), which distinguishes it from narrow angle glaucoma (though the two may coexist).
- POAG presents 'cupping' or excavation of the optic nerve disk and atrophy of the optic nerve.There is also retinal nerve fiber thinning on such tests as the OCT scan - Optical Coherence Tomography. Often this test will show problems before they are manifested on visual field exams.
- There are defects seen in the results of the visual field test.
- The intraocular pressure is elevated above the normal range (i.e., above 21 mm Hg) in the presence of above optic nerve pathology.
Symptoms of Open Angle Glaucoma
Open Angle Glaucoma, Risks, and Demographics
- Over two and a half million people in the US have POAG,and over one half the people with glaucoma have not been properly diagnosed as having the disease.
- In some studies, the average eye pressure in blacks is higher than in whites, and blacks are 6 times more likely to have optic nerve damage than whites, and 3-4 times more likely to develop glaucoma
- A recent 4 year study showed that blacks with OHT had a 3-4 times greater likelihood of then developing glaucoma. Studies reveal that blacks have thinner corneas (CCT) and this is thought to be a risk factor in glaucoma which may have a correlation with the increased risk noted.
- There are reports of a difference in the rates of glaucoma and OHT between men and women, but because there are also conflicting reports, a firm conclusion has not been established.
- Women who have passed menopause have a greater risk of glaucoma and/or higher eye pressures. Studies seem to indicate that estrogen may have a protective effect on glaucoma, and is one reason why post menopausal women are at higher risk.
- Although there are types of glaucoma which occur earlier, age plays a big factor in the risk of glaucoma and a great percentage of POAG cases occur during or after the seventh decade of life. There are also additional complicating factors such as cataracts and the risk of retinal vein occlusion (with higher eye pressure) in older age.
- Near sightedness (or myopia) is considered a notable risk factor for glaucoma.
- As with many diseases, heredity plays a role.
- Pigmentary Glaucoma and Pseudo Exfoliation glaucoma are discussed separately and have risk and demographic factors of their own. Many cases of POAG are thought to be under-diagnosed Pseudo Exfoliation glaucoma.
- A recent study reveals that US glaucoma cases increased 22% over past decade.
Causes of Open Angle Glaucoma
The causes of glaucoma usually involve many variables, making POAG so complex. However, elevated intraocular pressure is the single greatest risk factor. High eye pressure can damage the optic nerve and other relevant structures via a number of different mechanisms. For example, Occular Perfusion Pressure (OPP) is considered an important factor in the risk of glaucomic optic nerve damage according to many studies.
“The pathophysiology of primary open-angle glaucoma remains unknown,” said Farnaz Memarzadeh, M.D., Doheny Eye Center, University of Southern California, Los Angeles. She acknowledged that elevated IOP remains an important risk factor for glaucoma but pointed out that “other factors, particularly those affecting perfusion to the optic nerve, may play a role.”
“Ocular perfusion pressure is a delicate balance between IOP and blood pressure,” said Donald Budenz, M.D., Bascom Palmer Eye Institute, University of Miami. “Lower ocular perfusion pressure is associated with an increased risk of the development of open-angle glaucoma as well as its progression.” He pointed to several major studies, including the Baltimore Eye Survey, the Barbados Eye Study, the Egna-Neumarkt Study, and Projecto Ver, all of which identified low OPP as a risk factor for developing glaucoma. “In the Baltimore Eye Survey, glaucoma was six-fold more common in eyes with low versus high diastolic perfusion pressure.”
We see then that high IOP will result in lower OPP and thus increased glaucoma risk. However, it should not be assumed that because a higher blood pressure results in a higher OPP that high blood pressure is desirable. Indeed it is not, because hypertension has also been identified as increasing the risk for glaucoma by a presumably different mechanism than hypotension. So, there is consensus that elevated IOP is a risk factor in glaucoma; What has not been completely resolved and agreed upon is the issue of whether elevated IOP causes more deleterious effects via a mechanical cause to eye structures, or whether it exerts its effects mostly through ischemia, oxidative stress and inflammation.
Basics of high IOP with POAG
Other Causes of POAG