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Open-Angle Glaucoma

Open-Angle Glaucoma

The term POAG stands for Primary Open Angle Glaucoma and it is the most frequently diagnosed form of glaucoma. However, terms can sometimes get confusing as it may involve an overlap with other forms of glaucoma. We will clarify the term in the following article and describe the basics of POAG.

 

 

Introduction

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

The fact that POAG very seldom presents itself with any symptoms until there is irreparable nerve loss is why screening and eye exams are critically important. This is even more important for high risk populations like blacks and those with other health conditions. Eye pressures normally have to be very high before there are any visual, ocular, or pain symptoms, and by the time there are noticeable visual acuity losses from glaucoma, the nerve loss is usually extensive.There can also be collateral damage like retinal vein occlusion from high IOP and associated cardiovascular risks.
 

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

It is beyond the scope of this article to delve into the complex physiology of eye pressure regulation. However, the basic factors have to do with the production of aqueous fluid from the ciliary body, and the drainage of this fluid via the trabecular meshwork and related ‘drainage’ structures in the front of the eye. When the production of aqueous fluid (inflow) exceeds the functioning capability of the drain (outflow), the result is elevated IOP. That is why current eye pressure lowering therapies target the drainage mechanism or seek to reduce aqueous production (i.e., beta and alpha based eye drops).

 

Other Causes of POAG

 
Beyond the above association of glaucoma and IOP, there is new interest in the subject of neuroprotection because it is believed that there are other factors which affect the eyes of those with glaucoma and cause the optic nerve and related structures to be vulnerable to degenerative changes via oxidative stress, ischemia, circulatory dysfunction etc. This is most obvious considering the many who have OHT with very high eye pressures over a long period of time, and yet show no sign of optic nerve damage. Sally and Bailey are good examples of this within our own FitEyes community.
 
One thing that separates POAG from normal pressure glaucoma is that factors like obesity, high blood pressure, thyroid dysfunction, and poor general health have been shown to be risk factors. This does not diminish the fact that heredity plays a large part, but we believe these conditions may be compounding in their effect. Finally, we at FitEyes believe that stress is an important and often overlooked factor in the risk and progression of glaucoma and elevated eye pressure. It is worthwhile to review the article Stress and Eye Pressure to understand the impact stress can have on both elevated intraocular pressure and other glaucoma factors like inflammation and cardiovascular components . It is well known that stress plays a negative role in almost all diseases, and glaucoma is no exception!
 
The reader might find the following free online book version helpful in the understanding of newer ideas on treating glaucoma; The Mystery of Glaucoma.
 
Additional insights reveal the connection between glaucoma and the brain

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