Primary open angle glaucoma (POAG) develops due to the interaction of multiple factors some of which are known and others that are as yet undiscovered. Some of the known risk factors for glaucoma include intraocular pressure (IOP), age, genes, and race. POAG is the most prevalent type of glaucoma worldwide and is a leading cause of preventable, irreversible blindness worldwide (as opposed to cataracts which cause reversible blindness).
In the anterior chamber angle there is a sieve-like structure called the trabecular meshwork (TM) that is responsible for draining the fluid from within the eye. When the trabecular meshwork malfunctions fluid drainage slows down. This causes fluid, and therefore pressure, to build up within the eye. The increased IOP is believed to be the cause of the optic nerve damage in POAG.
Video | Primary Open Angle Glaucoma
Primary open angle glaucoma can occur over a wide spectrum of IOP. In some cases the IOP is within the normal range. This variant of POAG is called normal tension glaucoma (NTG), normal pressure glaucoma (NPG), low tension glaucoma (LTG), or low pressure glaucoma (LPG). In these cases the optic nerve damage is thought to be caused by an increased disc susceptibility to slight increases in pressure, disease of the blood vessels causing decreased blood flow to the optic nerve, or a combination of these and other unknown factors. Normal tension glaucoma patients often have a history of diseases involving the blood vessels such as diabetes, hypertension, migraine, atherosclerosis, ischemic heart disease, and stroke.
The symptoms of POAG in the early and moderate stages of optic nerve damage are usually so subtle that the vast majority of patients don’t notice them. These can include poor dark adaptation or night vision, frequent bumping into objects at the sides, and difficulty tracking fast-moving objects (e.g. golf ball or tennis ball). It is only when the POAG is at a more advanced stage that the patients are likely to notice poor peripheral vision and sometimes even poor central vision.
The diagnosis of POAG is made by assessing the patient’s history pertaining to risk factors, the ophthalmic examination, AND any diagnostic tests that were done (e.g. visual field test). Unlike in other diseases there is no single test or exam finding alone that can diagnose POAG. It not like diabetes mellitus, for example, where one or two blood sugar level determinations are often enough to make the diagnosis.
Medications, laser and surgery can all be used to treat POAG by lowering IOP. Which method to use first and how to combine the various methods is a decision made based on factors affecting the individual patient such as severity of the glaucoma, age, lifestyle, preference, economic status and the likelihood of progression to visual impairment. Most doctors prefer to use eye drops as their first choice when treating POAG but circumstances vary from patient to patient and the doctor may sometimes decide to use laser or surgery as the initial treatment. Aside from eye treatments, POAG patients who are smokers should stop smoking and those who are hypertensive or diabetic should strive for better control of their medical conditions.
Primary open angle glaucoma patients should not be worried about losing their sight overnight. Because of the very slow progression of optic nerve damage it takes years or even decades of no treatment or inadequate treatment for the glaucoma to reach the point where sight is completely and irreversibly lost. Only a small percentage of POAG patients become bilaterally, completely blind although many become visually impaired. Early detection, appropriate treatment, and patient compliance with treatment can slow down the rate of progression of damage so that it equals the normal rate of optic nerve deterioration due to aging.
POAG patients need to undergo periodic monitoring of their condition at the doctor’s clinic. The eye doctor looks for changes in the optic nerve and in other parts of the eye and checks the intraocular pressure. The eye doctor may occasionally request for a new visual field test to be able to compare the new test results with the older results.
People with first degree relations with POAG, the elderly, people who are myopic (near-sighted), those with diabetes or hypertension, and those with any other risk factors for glaucoma should have their eyes checked. Even if an initial screening shows no signs of glaucoma the screening should be repeated periodically as the person ages because of the increased risk of POAG with increased age.References:
- Goldberg I. How common is glaucoma worldwide? In: Glaucoma in the 21st Century. London, UK. 2000, Mosby International Ltd.
- Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, 2nd Edition. St. Louis, Missouri, USA, 1996, Mosby-Year Book, Inc.
- Epstein DL, Allingham RR, Schuman JS (Eds). Chandler and Grant’s Glaucoma, 4th Edition. Baltimore, Maryland, USA, 1997, Williams & Wilkins.
- South East Asian Glaucoma Interest Group. Asia-Pacific Glaucoma Guidelines. Sydney, Australia, 2003-2004, SEAGIG.
- European Glaucoma Society. Terminology and Guidelines for Glaucoma 2nd Ed. Savona, Italy, 2003, EGS.