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By Amber Dobler-Dixon, M.D.
Glaucoma Specialist
Glaucoma is one of the oldest described diseases. In 400 B.C. Hippocrates wrote about “glaucosis” in reference to the bluish-green hue of the affected eye, and by the mid-19th century glaucoma was used to refer to a distinct group of ocular disorders. Yet despite the early description of glaucoma, there are misconceptions. Even today, glaucoma is mistakenly thought of as high eye pressure and in the U.S. alone it is estimated that 50% of the people with glaucoma are undiagnosed. Glaucoma is asymptomatic in early or moderate disease so patients fail to seek medical care until significant vision is lost. In my practice, I see at least one new patient a week that is legally blind from glaucoma never having been diagnosed.
In order to explain glaucoma, one must have an understanding of the anatomy and function of the eye. They eye can be thought of as a complex camera. The lens and cornea focus light and an image onto the retina, which is similar to camera film. The image is then relayed from the eye to the brain by a cable, the optic nerve, which in a normal eye consists of about 1.2 – 1.5 million fibers called retinal ganglion cells. Any interruption of these retinal ganglion cell fibers of the optic nerve result in damage to vision. The term glaucoma refers to a group of diseases that have in common a characteristic damage to the optic nerve with associated visual function loss.
Although elevated intraocular (eye) pressure is one of the primary risk factors, its presence or absence does not have a role in the definition or diagnosis of glaucoma. Intraocular or eye pressure is the pressure that is exerted on the inside walls of the eye including the optic nerve and the blood vessels. Of all the people in the U.S. with glaucoma, or damage to the topic nerve, two thirds have high intraocular pressures, greater than 21mmHg, and one third have “normal” intraocular pressure, less than 21mmHg. Why some people develop damage to the optic nerve at low to normal eye pressures is due to that individual’s susceptibility of the optic nerve to any pressure. Also blood flow to the optic nerve and auto regulation of the blood vessels to the eye play a role in damage to the optic nerve at low or normal eye pressures. In contract, most people with high eye pressures never develop glaucoma and are called ocular hypertensives and they presumably have strong optic nerves at high pressures. In summary high eye pressure doesn’t necessarily mean that a person has glaucoma and low eye pressure doesn’t mean that a person doesn’t have glaucoma.
Who is at risk of developing glaucoma? Identifying risk factors is important because this information may lead to early detection of glaucoma before significant vision is lost. Risk factors include high eye pressure, a positive family history of glaucoma, advanced age, steroid use, eye trauma, thinner corneas, and Black or Hispanic race. In racial studies Blacks were 3 - 4 times more likely than Whites to have glaucoma and were 5 - 7 times more likely to go blind. The risk among Hispanic individuals appears to be intermediate between the reported values for Whites and Blacks. In patients with normal pressure glaucoma, migraine headaches, autoimmune diseases, and low blood pressure are additional risk factors.
A complete eye examination to assess the optic nerve in patients with significant risk factors and assessment or peripheral vision is the best way to diagnoses early glaucoma and prevent irreversible loss of vision. Glaucoma is asymptomatic in early to moderate disease and only when about 90% of the optic nerve is lost can a person notice loss of vision at which point he or she may be legally blind. The American Academy of Ophthalmology recommends a glaucoma screening every 2 – 4 years past the age 40, as the incidence of the disease increases with age. Because Blacks and Hispanics have an even greater risk for development of glaucoma, those between ages 20 and 39 should additionally be screened every 3 – 5 years.
Primary open-angle glaucoma, the most common type of glaucoma is the second leading cause of irreversible blindness in the U.S. overall and is the most frequent cause of blindness in Blacks. At present nearly 2.22 million Americans have glaucoma and about 120,000 of them have become bilaterally blind. With the rapidly growing U.S. population, the number of glaucoma patients is estimated to increase by 50% to 3.36 million by 2020. Glaucoma afflicts more than 67 million people worldwide, of whom about 10% or 6.6 million are estimated to be blind. Glaucoma is the leading cause of irreversible blindness worldwide.
Despite these staggering statistics, the impact of glaucoma from a public health perspective has not been fully appreciated. Relatively little information is currently available regarding the individual burden associated with the psychological effects of having a potentially blinding chronic disease.