Diseases of the Eye
Besides refractive and other focusing errors that cause decreased vision, many diseases can temporarily or permanently affect sight.
Since the eyelids, and the rest of the facial skin, have more lifetime sun exposure than other parts of the body, they are at increased risk of developing cancer. Skin cancer can often be treated by removing the lesion surgically.
The conjunctiva can become inflamed or infected from numerous diseases. This condition is called conjunctivitis. The most common causes of conjunctivitis include allergies, dry eyes, and viral infection (pinkeye). Bacterial and other types of infections can also cause pinkeye. Allergic reactions, dry eye, and chemical exposure can lead to inflamed conjunctiva.
Since the cornea helps focus images into our eye, maintaining clarity is essential. Both infection and injury can produce scars that leave the cornea cloudy. Clouded corneas reduce the amount of light that enters the eye, resulting in decreasing stimulation of the photoreceptors and dim vision. Scarring can also affect the shape of the cornea, producing astigmatism. Severe corneal scarring, or keratoconus, is one of the reasons for a corneal transplant operation.
If the tear system is not functioning properly, dry eye may develop. In some instances, it can be debilitating. Dry eye can be caused by numerous rheumatologic and auto-immune diseases, such as Sjögren’s syndrome, rheumatoid arthritis, and systemic lupus erythematosus. It has also been linked to many medications. Dry eye can even occur without an obvious cause. Rarely, ocular dryness is severe enough to cause permanent damage to the cornea. Dry eyes can be treated with artificial tear drops or prescription medications such as Restasis® or by unplugging of the tear drainage system.
The first internal structure in the eye is the anterior chamber, where aqueous humor bathes the backside of the cornea, the iris, and the front of the lens. If the aqueous humor is either produced too quickly or drained too slowly, pressure inside the eye can become elevated. This is one usual component of a disease process called glaucoma, which can cause irreversible vision loss. In glaucoma, the elevated pressure presses on the optic nerve fibers, reducing their blood flow, which may lead to cell damage or death. Because of the arrangements of nerve fibers in the optic nerve, glaucoma primarily produces loss of peripheral (side) vision. In advanced cases, it can also impair central vision. Vision loss due to glaucoma is permanent. Treatment includes prescription medications, laser treatments, and surgery. Animal research suggests that Ginkgo biloba supplementation may be helpful in glaucoma prevention and treatment (Hirooka 2004). Research has also implicated oxidative stress as a causal factor in glaucoma. Evidence now suggests that increased concentrations of reactive oxygen species play an important role in the development of glaucoma (Izzotti 2006).
Oxidative stress has also been implicated in the development of cataracts (Zoric 2005). A cataract occurs when the lens becomes cloudy as a result of normal aging. The only cure for cataracts is surgery, although one study has suggested that antioxidant therapy may help reduce the risk of developing them (Zoric 2005). Cataract surgery is very common and typically restores excellent vision in most people.
Behind the lens is the vitreous cavity. The vitreous humor can pull away from the back of the eye and form floaters, which are small condensations that appear as black spots or cobwebs in one’s vision. The vitreous humor can also form a retinal hole or tear when it pulls away from the back of the eye. Either of these conditions can lead to retinal detachment, in which the retina falls away from the back of the eye. Retinal detachments are medical emergencies. A sensation of a curtain being pulled across the field of vision is a danger sign for retinal detachment. Retinal detachments are usually repaired with surgery, although the surgery is not always successful. Retinal holes or tears can often be treated with a laser to help prevent retinal detachment from worsening. Successful treatment of retinal detachment depends on early detection.
Many diseases, both exclusive to the eye and systemic, can cause serious damage to the retina. Macular degeneration is probably the most feared of all ophthalmic diseases. It occurs when the macula (which is responsible for central vision) becomes diseased, distorting central vision. Macular degeneration is a significant cause of blindness (Seddon 2004).
Treatments for macular degeneration include laser treatments, surgery, and injection of prescription medications into the eye. One of the greatest nutritional breakthroughs in the treatment of macular degeneration was the release of the Age-Related Eye Disease Study, in which researchers found that vitamins C and E, beta-carotene, and zinc (plus copper) can reduce progression in certain types of macular degeneration (Age-Related Eye Disease Study Research Group 2001). Newer research has also implicated oxidative stress in macular degeneration. It is believed that age-related elevations in oxidative products in the eye may contribute to the development of macular degeneration (Nowak 2005; van Leeuwen 2005). Inflammation has also been identified as a possible aggravating factor in the development of macular degeneration.
The health of the eye is also affected by systemic disease (eg, diabetes, atherosclerosis, and hypertension), which can cause significant damage to the retina. In diabetic retinopathy, the small blood vessels of the retina become diseased and can leak blood products. Also, they do not carry enough oxygen, so some areas of the retina may become oxygen deficient. The eye may respond by forming more blood vessels (neovascularization), which are very fragile. In many cases, the blood vessels can bleed directly into the back of the eye. In people with high blood pressure, the retinal blood vessels can change shape and become unhealthy. Atherosclerotic disease can affect the small blood vessels of the eye and cause reduced blood flow.