20/20, 20/40, and other such numbers, measure visual acuity, or sharpness of vision. If a person has 20/20 visual acuity it means that when they stand 20 feet away from the chart they can see what a "normal" person can see at 20 feet. With 20/40 acuity, the person must move up to 20 feet to see what a "normal" person can see at 40 feet. Visual acuity is just one part of the vision. Other important vision parameters include peripheral vision, depth perception, refractive error, focusing ability, and eye health. A person with excellent visual acuity may well have serious vision problems that interfere with daily activities. Indeed, a person with tunnel vision may have 20/20 acuity but be legally blind. Or a person with moderate hyperopia can have severe headaches and eyestrain while having 20/20 visual acuity.
Astigmatism is created when the optical surfaces of the eye are shaped like the side of a football, curved more in one direction than another. As a result, some light rays are focused at one distance from the retina, other rays at a different distance, resulting in a blurred image.
A person with a color vision deficiency is unable to distinguish colors in the same manner as a person with "normal" color vision. Most of those with defective color vision are only partially color-blind to the colors red and green, but there are other types of color deficiency as well. Completely color-blind individuals can recognize only black, white, and shades of gray.
The image to the right is a plate for testing color deficiency.
Hyperopia occurs when an eye is optically "weak", and additional focusing is required to bring a distant object into clear focus on the retina. In mild hyperopia, additional focusing can sometimes be provided by the lens inside the eye. In higher amounts of hyperopia, the additional focusing must be provided by a "plus" corrective lens. The term "farsightedness" is actually a misnomer since the extra focusing needed by a hyperopic eye affects both distance and near vision.
Amblyopia sometimes called a "lazy eye," occurs when one eye does not develop normal sight during early childhood. When one eye develops good vision but the other does not, the eye with decreased vision is called amblyopic. (Usually, only one eye is affected by amblyopia.) This common condition should be corrected during infancy or early childhood to obtain 3-dimensional vision and prevent permanent vision loss.
Myopia is a visual defect in which distant objects appear blurred because their images are focused in front of the retina rather than on it; synonym nearsightedness. It is corrected by a concave, or "minus", lens.
Presbyopia is the inability of the eye to focus sharply on nearby objects. It is caused by the loss of elasticity of the eye's crystalline lens secondary to the aging process.
Blepharitis is an inflammation that affects the eyelid, the edge of the eyelid, and the eyelash follicles.
In Seborrheic Blepharitis skin cells shed more rapidly than normal and excess oil is secreted by glands in the eyelid. This results in inflammation and promotes bacterial growth.
In Infectious Blepharitis, bacteria cause an infection in the glands along the eyelid.
Contact Dermatitis Blepharitis is inflammation caused by eyelid contact with a toxin or allergen. For example, mascara may produce an allergic reaction, or jalapeno, which may produce a toxic reaction.
The lens inside the eye, normally transparent in youth, helps focus sharp, clear images on the retina. However, this lens clouds with age, and a cloudy lens is called a cataract. Cataracts can interfere with driving, reading, watching television, and cause glare at night or in bright sunlight. The milder symptoms of early cataracts can sometimes be alleviated by a change in one's glasses prescription. In more advanced cataracts the cloudy lens is removed and replaced by a lens implant.
Cataracts also can be secondary to trauma or radiation or related to medications (i.e., steroids) or diseases. Cataracts can be congenital. These types of cataracts are generally treated the same as age-related cataracts: surgical replacement of the cloudy lens with a lens implant.
Conjunctivitis is inflammation of the thin membrane covering the white of the eye and the inner surface of the eyelid. The inflamed conjunctiva will appear red or pink due to irritated and enlarged blood vessels.
There are a number of causes of conjunctivitis.
Bacterial conjunctivitis is generally accompanied by a discharge from the eye with crusting of the eyelashes.
Viral conjunctivitis produces a watery discharge and may accompany the common cold. True "pink-eye", infectious viral conjunctivitis, is quite contagious and may involve conjunctival hemorrhages.
Allergic conjunctivitis, as found in hay fever and other allergies, can make the eyes itchy or cause a chronic red eye.
Environmental irritants such as dust, smoke, or fumes may cause conjunctivitis.
Washcloths and towels should not be shared when suffering bacterial or viral conjunctivitis. Handwashing will also help prevent the spread of this infection.
People with diabetes are at risk of developing diabetic retinopathy, a major cause of vision loss. Because early diagnosis and timely treatment have been shown to prevent vision loss in more than 90 percent of patients, health care practice guidelines recommend an annual dilated eye exam for all people with diabetes. Studies indicate, however, that many people with diabetes do not get an annual dilated eye exam. An estimated 50 percent of patients are diagnosed too late for treatment to be effective.
Dry eye is caused by an abnormality in the thin wafer of fluids that covers, lubricates, and moistens the surface of the eyeball. This wafer is made up of a layer of oil, covering a layer of water, covering the inner layer of mucous, which lays directly on the surface of the eye. These layers are produced by tiny glands found inside the eyelids. There is also the larger lacrimal gland, which produces the tears that cause your eye to "water" when irritated.
When there is an abnormality or imbalance in the protective wafer of fluids, the eyeball's surface can dry out and become "chapped". This causes the eye to be irritated, resulting in stimulation of the lacrimal gland and excessive tearing.
Dry eye is generally a chronic problem and can be exacerbated by dry, windy conditions. Treatment of dry eye generally consists of artificial lubricants ("artificial tears") or other topical medications. In severe cases, the use of punctual plugs, which prevent tears from exiting the eyes, is sometimes helpful.
Glaucoma is an eye disease generally related to intraocular pressure.
Fluid is constantly produced inside the eye at the ciliary body [found at the base of the green arrow in the graphic] and exits the eye via the pupil and iris angle [at the point of the green arrow]. Intra-ocular pressure is determined by that fluid's production versus its drainage. In some cases the pressure is high enough to damage the optic nerve [at the point of the longer blue arrow], causing it to cup out. The amount of intraocular pressure it takes to damage an optic nerve varies from person to person. Unmanaged glaucoma can lead to debilitating or even total vision loss.
Macular degeneration is a chronic eye disease where the tissue in the macula, that part of the retina that's responsible for central vision, deteriorates. It results in loss of the acute vision necessary for reading, driving, and recognizing people's faces. Although it affects central vision, the condition does not involve peripheral vision and thus doesn't cause total blindness. In most cases the damage caused by macular degeneration can't be reversed; however, early detection may help reduce the extent of vision loss. The condition tends to develop as you get older, and is the leading cause of severe vision loss in people age 60 and older. It comes in two major varieties: Dry and Wet.
Dry macular degeneration. Macular degeneration almost always starts out as a dry form. Initially, a person may notice little or no change in vision. But as fatty-like bodies [drusen] deposit amongst the macula's light-sensing cells and mottled pigmentation of the macula develops, vision may deteriorate.
Wet macular degeneration. Almost everyone with the wet form of the disease started out with the dry form. The wet form accounts for only about 15 percent of all cases, but it is responsible for most of the severe vision loss. Wet macular degeneration develops when new, fragile blood vessels grow into the macular area of the retina from the layers below. These new vessels leak fluid or blood, which distorts the macula, causing central vision to blur. Objects may appear distorted, wavy, or crooked. Blank spots can appear in the field of vision.