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 is the light-sensing structure at the back of the eye. It is a two-dimensional surface like a piece of film in a camera and 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.
Glare and Halos
Some people experience glare (star bursts) or halos around bright lights, especially when driving at night. Such symptoms are more common among older people and among people 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.
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 be focused 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.
Some people experiencebright flashes of light, flickering lights, or streaks of light. This visual sensation most commonly results from shifting of the jellylike substance that fills the eyeball (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 be concentrated on an ophthalmoscopy. A detached retina or migraine headache is treated. Otherwise, treatment may be unnecessary.
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.
Sensitivity to bright light occurs normally during extremely sunny conditions or when coming out of a dark environment into bright sunlight. Such sensitivity can also be caused by drugs used to dilate the pupils (mydriatics). However, pain resulting from bright light (photophobia) can be a symptom of a migraine headache or a number of eye disorders, for example, those that involve inflammation or infection within the front part of the eye (uveitis), a corneal disorder (such as keratitis), or an eye injury. It may also be due to meningitis (which is also typically accompanied by a severe headache and neck stiffness—see Meningitis: Acute Bacterial Meningitis).
Doctors first try to differentiate light sensitivity from photophobia. The cause of light sensitivity or photophobia can usually be determined by the person's symptoms and an eye examination. A slit-lamp examination (see Diagnosis of Eye Disorders: Slit-Lamp Examination) is particularly useful for detecting disorders that cause photophobia. Light sensitivity and photophobia can be minimized by protecting the eyes from light (for example, by wearing sunglasses). When photophobia is the result of inflammation within the eye, dilating eye drops can help to relieve pain
Itching may result from allergy and is usually accompanied by watering of the eyes (tearing). Inflammation of the eyelids (blepharitis) and dry eyes may also cause itching. Much less commonly, itching may result from infection or infestation with lice or other parasites. Abnormalities that cause itching can usually be diagnosed by a slit-lamp examination. Until the cause of itching is relieved, applying a cool washcloth may provide some relief.
The sensation of dryness of the eyes can be caused by a variety of conditions, including inadequate tear production, accelerated tear evaporation, or, less commonly, refractive surgery, vitamin A deficiency (rare in developed nations), or Sjögren syndrome. Dry eyes may also be a result of aging.
Tear production may be measured, particularly if Sjögren syndrome is suspected. Doctors may also try to determine whether tears evaporate too quickly. They place a tiny amount of yellow dye (fluorescein) in an open eye and measure how long it takes for tears to evaporate. During the day, dry eyes can be relieved with the use of eye drops that substitute for a person's tears (artificial tears). At night, an ointment can be used before bed to relieve morning dryness.
Last full review/revision August 2012 by Kathryn Colby, MD, PhD