Primary Open-Angle Glaucoma

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The definition of glaucoma has changed drastically since its introduction around the time of Hippocrates in approximately 400 BC.

The word glaucoma came from ancient Greek, meaning clouded or blue-green hue, most likely describing a person with a swollen cornea or who was rapidly developing a cataract, both of which may be caused by chronic (long-term) elevated pressure inside the eye.
 
Pressure inside the eye is termed intraocular pressure. Eye pressure is measured in millimeters of mercury (mm Hg). Normal eye pressure ranges from 10-21 mm Hg. Elevated intraocular pressure is an eye pressure of greater than 21 mm Hg.

The concept of glaucoma has been refined, particularly over the last 100 years. Currently, glaucoma is defined as damage to the optic nerve that is usually caused by high pressure inside the eye. This optic nerve damage can usually be stopped but cannot be reversed by adequate lowering of intraocular pressure.

The generic term glaucoma should only be used to refer to the entire group of glaucomatous disorders, because multiple subsets of glaucomatous disease exist. A more precise term should be used to describe the glaucoma if the specific diagnosis is known.

• One subset of glaucomatous disease is primary open-angle glaucoma.
  
  
  • Primary open-angle glaucoma is described as optic nerve damage from multiple possible causes that is chronic and progresses over time, with a loss of optic nerve fibers that is characteristic of the disease.
    
    
  • In addition to the loss of optic nerve fibers, primary open-angle glaucoma is characterized by open anterior chamber angles, visual field abnormalities, and intraocular pressure that is too high for the continued health of the eye.
    
    
  • Primary open-angle glaucoma exhibits cupping (depression) and atrophy of the optic disc (the front surface of the optic nerve, which is seen in the back of the eye), in the absence of other known causes of glaucomatous disease.
    
    
  • Note that the definition of primary open-angle glaucoma is not synonymous or solely defined by the presence of elevated intraocular pressure. Increased intraocular pressure is a risk factor associated with the development of the disease; increased intraocular pressure is not the disease itself.
  
  
• People can have optic nerve damage without having elevated intraocular pressure. This condition is known as normal-tension (or low-tension) glaucoma.
  
  
• People can also have elevated pressures without signs of optic nerve damage or vision loss. They are considered to be at risk for glaucoma because of the elevated intraocular pressure. In medical terms, these people are known as glaucoma suspects or ocular hypertensives (see Ocular Hypertension and Adult Glaucoma Suspect).
  
  

Primary open-angle glaucoma is a major health concern throughout the world because of its usually silent, progressive nature and because it is one of the leading preventable causes of blindness in the world. With appropriate screening and treatment by an ophthalmologist (a medical doctor who specializes in eye care and surgery), glaucoma can usually be identified and stopped before significant vision loss occurs.

Multiple studies have been performed to estimate the prevalence of glaucomatous eye disease, including primary open-angle glaucoma and those individuals with ocular hypertension who are at risk for developing primary open-angle glaucoma.

• Over a 5-year period, several studies have shown the incidence of glaucomatous damage in people who previously had no signs of glaucoma to be about 2.6-3% for intraocular pressures of 21-25 mm Hg, 12-26% for intraocular pressures of 26-30 mm Hg, and approximately 42% for those intraocular pressures higher than 30 mm Hg.
  
  
• In the United States alone, more than 1.6 million people have significant visual impairment, and 84,000-116,000 people are blind in both eyes. Such statistics emphasize the need to identify and closely monitor those at risk of glaucomatous damage.

In addition, studies involving people in the United States found the following:

• Glaucoma is a leading cause of blindness, second only to cataract.
  
  
• Only half of the people who have glaucoma may be aware that they have the disease.
  
  
• More than 2.25 million Americans aged 40 years and older have primary open-angle glaucoma.
  
  
• Studies estimate that 3-6 million people in the United States alone, including 4-10% of the population older than 40 years, have intraocular pressures of 21 mm Hg or higher, without detectable signs of glaucomatous damage using current tests.
  
  
• Per year, roughly 0.5-1% of people with elevated intraocular pressure will develop glaucoma over a period of 5-10 years.

Worldwide, studies found the following:

• More than 3 million people are blind in both eyes from primary open-angle glaucoma.
  
  
• More than 2 million people will develop primary open-angle glaucoma each year.

The prevalence of primary open-angle glaucoma may differ between races. Some studies have found that the average intraocular pressure in blacks is higher than in whites, while other studies have found no difference. Many possible etiologies, particularly genetic, are likely for these differences. Consequently, further study needs to be conducted to clarify this issue, but some statistics regarding race are noted below.

• Glaucoma is the most common cause of blindness among people of African descent. They are more likely to develop glaucoma early in life, and they tend to have a more aggressive form of the disease.
  
  
• Blacks are considered to have a 3-4 times greater risk of developing primary open-angle glaucoma than whites. Blacks are also 6 times more likely to have optic nerve damage than whites.
  
  
• A 4-year study showed that blacks with ocular hypertension were 5 times more likely to develop glaucoma than whites. Findings from the recent Ocular Hypertension Treatment Study suggested that blacks might have thinner corneas, which could cause their eye pressures to be measured erroneously low in the office. This may be a contributing factor to the large prevalence of severe glaucoma damage in African Americans at the time of diagnosis.

Reports on the prevalence of primary open-angle glaucoma between men and women also differ.

• Although some studies have reported a significantly higher average intraocular pressure in women than in men, other studies have not shown any difference between men and women.
  
  
• Other studies have even shown males to have a higher prevalence of glaucoma than women.

Intraocular pressure slowly rises with increasing age, just as glaucoma becomes more prevalent as you get older.

• Being older than 40 years is considered to be a risk factor for the development of primary open-angle glaucoma, with up to 15% of people affected by the seventh decade of life. However, the disease itself is not limited to only middle-aged and elderly individuals.
  
  
• In people who are older than 65 years, intraocular pressure is often kept to 25 mm Hg or less because in approximately 3% of people with high eye pressure, the veins in the retina can become blocked (called a retinal vein occlusion), which could lead to vision loss.

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