Changes in vision may involve loss of vision or distortion of vision. People often describe either type of change as blurring of vision.

Loss of Vision

Loss of vision is a complete or nearly complete absence of sight. People with loss of vision may see nothing whatsoever, or they may be able to distinguish light from dark and may even be able to detect vague shapes. Loss of vision may involve part or all of the visual field of one or both eyes and may be sudden or gradual, temporary or permanent. People usually notice sudden loss of vision immediately. However, gradual loss may not be noticed and may not be discovered for some time—perhaps not until a car accident or other event prompts a thorough vision examination.

Complete loss of vision may occur in one or both eyes. Common causes include blockage of the blood supply to the retina, diabetes, disorders that damage the optic nerve, glaucoma, cataracts, macular degeneration, injuries, and, in certain areas of the world, infections. Occasionally, complete or partial loss of vision occurs temporarily. This temporary loss of vision can be caused by a transient ischemic attack (sometimes called a mini stroke).

Many types of loss of vision involve only part of the visual field (visual field defects). For example, a stroke or tumor that affects the left side of the brain could result in an inability to see all or part of the right side of the visual field in both eyes (affected people can still see normally on the other side in both eyes). Another type of visual field defect results in an inability to see the outside part of the visual field in either eye (affected people can still see normally in the middle part of the visual field in both eyes). This pattern of visual field loss can be caused by a problem such as a tumor or aneurysm near the pituitary gland (which lies just below the brain, behind a cross-over of the optic nerve fibers). Some people lose the ability to see things in the center of their visual field, but they retain their side or peripheral vision (where things are seen out of the corner of the eye). This type of defect can be caused by macular degeneration and certain disorders that damage the optic nerve. Smaller, irregular patches of vision may be lost as a result of disorders that damage the retina, such as diabetic retinopathy, hypertensive retinopathy, and retinal detachment. Loss of peripheral vision in all directions, so that the only remaining vision is in the middle of the field (tunnel vision), can be caused by glaucoma and certain retinal disorders (for example, retinitis pigmentosa).

When the Visual Pathways Are Damaged Nerve signals travel along the optic nerve from each eye. The two optic nerves meet at the optic chiasm. There, the optic nerve from each eye divides, and half of the nerve fibers from each side cross to the other side. Because of this arrangement, the brain receives information via both optic nerves for the left visual field and for the right visual field. Damage to an eye or the visual pathway causes different types of vision loss depending on where the damage occurs.

The cause of vision loss can often be determined by the person's symptoms and the results of an examination, which usually includes refraction, visual field testing, ophthalmoscopy, slit-lamp examination, and tonometry. Additional examination and tests are done based on what disorders doctors suspect.

The disorder causing vision loss is treated. Depending on the cause, however, there may be no effective treatment.

How and Why Blindness Develops
Anything that blocks the passage of light from the environment to the back of the eye or disrupts the transmission of nerve impulses from the back of the eye to the brain will interfere with vision. Legal blindness is defined as a visual acuity of 20/200 or worse in the better eye, even after correction with eyeglasses or contact lenses, or a visual field restricted to less than 20° in the better eye. Many people who are considered legally blind can distinguish shapes and shadows but not normal detail. Blindness can occur under the following circumstances: Light cannot reach the retina Damage to the cornea caused by infections such as trachoma, leprosy, or onchocerciasis, which results in an opaque corneal scar Damage to the cornea caused by vitamin A deficiency, which causes dry eyes (keratomalacia) and results in an opaque corneal scar Damage to the cornea caused by a severe injury that results in an opaque corneal scar A cataract, which causes loss of clarity of the lens Light rays do not focus on the retina properly Severe focusing (refraction) errors that are not fully correctible with eyeglasses or contact lenses The retina cannot sense light rays normally Detached retina Diabetes mellitus Macular degeneration Retinitis pigmentosa Inadequate blood supply to the retina, usually due to a blockage of the retinal artery or vein, which may be caused by inflammation of the blood vessel wall (such as that caused by temporal arteritis), or due to a blood clot that travels to the eye from somewhere else (such as from the carotid artery in the neck) Cytomegalovirus infection of the retina in people who have AIDS Nerve impulses from the retina are not transmitted to the brain normally Disorders affecting the optic nerve or its pathways inside the brain, such as brain tumors, strokes, infections, and multiple sclerosis Glaucoma Inflammation of the optic nerve (optic neuritis) The brain cannot interpret information sent by the eye Disorders that affect the areas of the brain that interpret visual impulses (visual cortex), such as strokes and tumors

Distortion of Vision

Distortion of vision is an inability to see clearly and correctly. This distortion may involve a poor focus due to refractive error, lack of depth perception, double vision, glare or halos, flashes of light, or floaters. It may also involve color blindness.

What Is Astigmatism? Astigmatism is an irregularity in the curvature (curved differently in different directions) of the cornea or lens that causes light traveling in different planes to be focused differently. For example, vertical lines may be in focus when horizontal lines are not (or vice versa). The irregularity can be in any plane, however, and is often different in each eye. A person with astigmatism (each eye should be tested separately) tends to see certain lines more boldly (that is, in better focus) than the others. Astigmatism is correctible with prescription eyeglasses or contact lenses. It often occurs together with nearsightedness or farsightedness. The diagram above is of a standard chart used to test for astigmatism in one eye at a time.

Refractive Error: Refractive error is an inability of the eye to properly focus an image onto the retina. It causes objects to appear blurred. Refractive error usually results from a mismatch between the focusing power of the cornea or lens and the length of the eye. Such a mismatch causes images to be focused slightly in front of or behind the retina. If only distant objects are blurred, the person is nearsighted (myopic). If only nearby objects are blurred, the person is farsighted (hyperopic). With myopia and hyperopia, objects tend to be blurred throughout the visual field, and the degree of blurring tends to be related to the distance between the eye and the object. Upon reaching middle age, most people—even those with previously excellent vision—develop difficulty focusing on nearby objects (presbyopia).

Astigmatism is another type of refractive error. Astigmatism results when the cornea is irregularly curved. For example, this curvature might result in horizontal lines being in focus but vertical lines being blurred. The irregular curvature can occur in any direction and can be different in each eye. Astigmatism can occur together with nearsightedness or farsightedness.

Refractive error is treated with corrective lenses (glasses or contact lenses) or refractive surgery such as laser in situ keratomileusis (LASIK). A person may need more than one type of corrective lens (for example, one to see distant objects and another to see nearby objects, as in bifocals). The blurred vision from refractive error can usually be fully corrected.

Impaired Depth Perception: Depth perception is the ability to determine the relative position of objects in space. People with impaired depth perception may have difficulty distinguishing which of two objects is closer.

The retina, being a two-dimensional surface like a piece of film in a camera, can only produce a two-dimensional image. The brain integrates the two-dimensional images from each eye to create a sense of three dimensions (stereopsis). Stereopsis allows people intuitively to perceive depth. Disorders in which the eyes do not align properly (such as strabismus) can interfere with stereopsis.

However, unlike stereopsis, some depth perception clues can be detected with only one eye. These clues include relative size, overlap, relative motion, movement parallax, and haziness and texture. For example, a car that looks larger, overlaps another, or moves faster across our field of vision is closer. Movement parallax means that when the head is shifted from side to side closer objects appear to move faster and farther across the field of vision. Objects that are farther away tend to appear hazy (because of the atmosphere), and their surface texture is less apparent. Most of these clues require knowledge of the normal size and appearance of the objects.

Double Vision (Diplopia): Double vision is seeing two images of one object. Double vision almost always occurs only when both eyes are open. This type of double vision is typically due to poor alignment of the eyes, usually caused by weakness in one or more of the muscles (or nerves) that control eye movements. This weakness results in cross-eyes (strabismus—see Eye Disorders in Children: Strabismus). Other causes include fatigue, alcohol intoxication, multiple sclerosis, and injury. The sudden appearance of double vision may indicate a serious disorder of the brain or nervous system, such as a tumor, aneurysm, or stroke. Rarely, double vision occurs when only one eye is open. Possible causes include cataracts, irregular astigmatism, displacement of the lens of the eye, and retinal detachment.

People are evaluated to determine whether double vision occurs with only one eye open (indicating a problem in the affected eye) or when both eyes are open (indicating a problem with eye movements). If double vision occurs only with both eyes open, doctors pay special attention to how well the eye muscles move the eyeball. The person is asked to move both eyes in all directions without moving the head. The doctor may also cover one of the eyes with a red lens or use prisms to help identify weakness in the muscles that control eye movements.

Treatment is directed at the disorder causing double vision.

Glare and Halos: Some people experience glare ("star bursts") or halos around bright lights, especially when driving at night. Such symptoms are more common in older people and in those who have had certain types of refractive surgery or who have certain types of cataracts. Glare and halos can also occur in people whose pupils are widely dilated (for example, those who have been given eye drops for an examination). When the pupil is widely dilated, light is able to pass through the peripheral part of the lens of the eye, where it is bent differently from light passing through the more central parts of the lens and therefore causes glare.

An eye examination is done. Sometimes symptoms can be relieved by treating the cause (for example, a cataract). Otherwise, people should take precautionary measures, such as minimizing driving at night or after receiving eye drops for an examination and avoiding looking directly at oncoming headlights while driving.

Night Blindness: Older people frequently have difficulty seeing in low light. This is sometimes referred to as night blindness. Most commonly night blindness results from a cataract, although night blindness is a feature in certain forms of retinal degeneration, such as retinitis pigmentosa. The eyes of some older people dilate slowly and take longer to adjust to low light. An eye examination should focus on detection of cataracts and should include an ophthalmoscopy. The cause is treated. Improving household lighting, particularly in the kitchen and around steps and other areas in which falls can occur, may improve safety.

Flashing Lights: Some people experience bright flashes of light, flickering lights, or streaks of light. This visual sensation most commonly results from shifting of the jellylike substance that fills the back of the eye (vitreous humor) or less commonly from a detached retina or a migraine headache. Before a migraine headache, some people temporarily see jagged, zigzag-like, bright, shimmery lines. Flashes of light can also result from a blow to the back of the head ("seeing stars"), probably because of stimulation of the part of the brain where vision is interpreted. An eye examination should concentrate on an ophthalmoscopy. A detached retina or migraine headache is treated. Otherwise, treatment may be unnecessary.

Floaters: Floating spots (floaters) are dark specks that appear to move in front of the eye. They are fast-moving or slow-moving clumps of the microscopic fibers that make up the vitreous humor. Floaters become increasingly common with age. Floaters rarely affect vision and are generally considered normal. However, a sudden increase in the number of floaters (especially in association with flashing lights) may indicate a serious problem, such as a detached retina. A person with these symptoms should be urgently evaluated by an ophthalmologist. If a detached retina is detected, it is treated.

Color Blindness: People who have color blindness (dyschromatopsia) are unable to perceive certain colors, or they may perceive certain colors with different intensity than do people with normal color vision. For instance, in the most common form of color blindness (red-green color blindness), people are less able to distinguish dark or pastel green or red or both. Often, the changes are subtle, and many people are unaware they have color blindness. People are often tested for color blindness if someone else in the family has the disorder or if a doctor suspects a problem with the nerve that carries information from the retina to the brain (optic nerve). Color blindness cannot be treated. At traffic lights, people with red-green color blindness should be guided by cues other than the color of the light.

What Causes Color Blindness?
Color blindness (dyschromatopsia) affects how people perceive certain colors. It is usually present from birth and is nearly always due to an X-linked recessive gene, which means that almost all affected people are men. Women, who are not usually affected themselves, can pass the gene for color blindness on to their children. Most cases of color blindness are due to a relative deficiency or abnormality of one of the types of light-sensing retinal cells (photoreceptors). Red-green color blindness, the most common form, is one example. Blue-yellow color blindness, however, may be caused by optic nerve disease and is usually due to acquired rather than inherited disease. Color blindness is also sometimes due to a problem with how the brain interprets color (rather than a problem with the eyes). A person may be tested for color blindness if it is known that a family member has the abnormality. Some people may be tested because they notice they have difficulty with matching colors. Other people may be unaware of any problem until they are tested for a job or need a license (such as for piloting an airplane) that requires them to be able to distinguish colors.

Last full review/revision December 2006 by Kathryn Colby, MD, PhD