Legal blindness, defined as 20/200 vision in the better eye in the United States, affects 45 million people worldwide, according to ophthalmologist Jan Roodhooft, M.D. in an article, "Leading Causes of Blindness Worldwide" published in Bulletin de la Societe Belge d'Ophthalmologie in 2002. The causes of blindness vary according to the poverty level of an area. Some treatable causes of blindness, such as cataracts, are common in areas without good medical resources but uncommon in areas with resources to treat them easily. Unpreventable diseases are more likely to cause blindness in areas with adequate medical care.
Cataracts
Although people in developed countries think of cataracts as treatable, cataracts are responsible for half of all cases of blindness worldwide. Three different types of cataracts exist, but all involve a clouding of the clear lens in the eye. Cataracts are easily curable with surgery, but many people in developing nations do not have access to medical care for cataracts. Most cataracts develop as people age, but disease such as diabetes can accelerate cataract formation.
Trachoma
Trachoma, an eye infection caused by chlamydia trachomitis, blinds 6 million people worldwide. The disease causes repeated eye infections that cause clouding of the cornea, the clear transparent dome that covers the front of the eye. Trachoma occurs frequently in areas with poor sanitation and hygiene but is easily curable with antibiotics if treated before the cornea becomes permanently clouded.
Glaucoma
Glaucoma is third most common worldwide cause of blindness, Roodhooft states, affecting 5.2 million people. Glaucoma refers to optic nerve damage. Typically, elevated intraocular pressure in the eye compresses the optic nerve and decreases vision, but it's possible to have glaucoma with normal eye pressure. Several different diseases can cause glaucoma, which is much more common in people of African descent.
Diabetes
Diabetes causes abnormalities in the small blood vessels in the eye. The vessels may leak fluid and swell. Leaking in the central part of the vision, the macula, located in the center of the retina, causes vision loss most often. Roodhooft notes that after having diabetes for 15 years, around 2 percent of people will be blind and 10 percent will have serious vision loss. Diabetic retinopathy, disease that affects the retina, can be treated with laser or intravitreal injections of medication to decrease leakage and swelling.
Macular Degeneration
Age-related macular degeneration (AMD) is a very common cause of vision loss. The disease is not preventable, but it is treatable, although it can't be cured. AMD affects over 10 million Americans alone, according to the Massachusetts Eye and Ear Infirmary. AMD causes vision loss because abnormal blood vessels develop under the macula, causing swelling in the central part of vision. There are two stages of AMD, the dry stage, affecting around 15 percent of people with the disease, and the more serious wet form, affecting around 15 percent. When one eye develops the wet form, the other eye is likely to become wet also. Taking an AREDS formulation that contains zinc, Vitamin C and E, copper and beta carotene may, in some cases, slow or stop the progression from dry to wet AMD. AMD is treated to slow or stop vision loss by intravitreal injections of medication that stop the formation of the abnormal blood vessels.
Retinitis Pigmentosa
Retinitis pigmentosa (RP) is the leading cause of inherited blindness, affecting 1.6 million people worldwide, according to the Massachusetts Ear and Ear infirmary. RP narrows blood vessels in the eye beginning at the peripheral areas of the retina and progresses slowly toward the central part of vision. The optic nerve head may also be damaged by the disease.
Other Common Causes
Other common causes of blindness worldwide include river blindness, or onchocerciasis, caused by a parasitic worm that produces microfilariae that migrate to the eye and cause severe inflammation, trauma and Hansen's disease, also known as leprosy.
