Overview of Glaucoma
The term "glaucoma" refers to a group of conditions that exhibit a gradual loss of vision, often without symptoms. In glaucoma, vision loss is caused by damage to the optic nerve, which transmits light signals from the retina to the brain, where they are translated into images.
Normally, the amount of aqueous humor being produced is equal to the amount that is draining out. Enough aqueous is present to exert enough pressure to keep the eye properly formed. Pressure builds up in the eye if the aqueous humor cannot properly drain. Increased intraocular pressure (IOP) may ultimately damage the optic nerve.
Other factors besides intraocular pressure appear to contribute to glaucoma. Some people with normal pressure may experience vision loss from glaucoma, and many people with high IOP (sometimes called ocular hypertension) do not develop glaucoma. However, the higher the IOP, the more likely optic nerve damage will occur.
Incidence & Prevalence of Glaucoma
Worldwide, there are an estimated 65 million cases of glaucoma. There are 3 million cases in the United States, but only one-half of those have been diagnosed. About 2 percent of people between the ages of 40 and 50, and 8 percent of those over 70, have elevated intraocular pressure in one or both eyes.
There are 120,000 people in the United States who are blind as a result of glaucoma, which accounts for 9-12 percent of all cases of blindness. It is the second leading cause of permanent vision loss and the leading cause of preventable blindness. Open angle glaucoma accounts for 19 percent of blindness in African Americans and 6 percent in Caucasians. Glaucoma is 6 to 8 times more common in African Americans than Caucasians, and they are more likely to become blind from it.
Asians and Eskimos have a higher prevalence of primary angle-closure glaucoma than other ethnic and racial groups. This type also is more common in women, the elderly, people with myopia, and those with a family history of the condition.
Normal-tension glaucoma is more prevalent in people of Japanese ancestry and in those with a history of systemic heart disease. Family history of this disorder also increases the risk.
Types of Glaucoma
There are many different types of glaucoma and the two major types are primary open-angle and angle- closure.
Primary open-angle glaucoma
Primary open-angle glaucoma accounts for 60-70 percent of glaucoma cases in the United States. In open-angle glaucoma, the aqueous humor is unable to drain out of the eye. For unknown reasons, the trabecular meshwork (i.e., eye's filtration area) does not function normally, the pressure in the eye increases, and the optic nerve is damaged.
Most people do not experience symptoms until their vision is compromised and extensive damage to the optic nerve has been done. Peripheral vision is affected before central vision.
Angle-closure glaucoma, also known as narrow-angle glaucoma, accounts for fewer than 10 percent of cases. This type results from an abnormality in eye structure. In most cases, the iris occludes (blocks) the trabecular meshwork, preventing drainage of aqueous humor and raising intraocular pressure.
If the drainage channel is completely blocked, IOP rises suddenly, causing acute angle-closure glaucoma. Symptoms may be severe and include extreme eye pain, nausea, blurred vision, and halos around lights. Acute angle-closure glaucoma is a medical emergency that must be treated by an ophthalmologist immediately. Permanent vision loss can occur within days.
This is an inherited type of open angle glaucoma that most commonly affects myopic men in their 20s or 30s. Myopia (nearsightedness) causes the eyes to have a concave iris, creating a wide angle. This causes the color (pigment) layer of the eye, the iris, to rub off onto the lens, where it can shed into the aqueous humor and the trabecular meshwork. The pigment can clog the pores of the trabecular meshwork, which prevents adequate aqueous humor drainage and increases IOP.
Normal-tension glaucoma is also known as low-tension, or normal-pressure, glaucoma. In this type, "normal" IOP is too high for the individuals optic nerve, leading to damage of the nerve. This condition is rare and poorly understood. It may be that an inadequate blood supply to the optic nerve may cause the damage.
When glaucoma is diagnosed before a child's third birthday, it is considered congenital (present since birth). In approximately one-third of children, it is inherited through an autosomal recessive gene. Autosomal means that boys and girls are affected equally, and recessive indicates that both parents have the gene. The risk is 25 percent with each pregnancy that the infant will have congenital glaucoma when both parents are carriers (i.e., they do not have the condition but they can pass it to their child).