Optics And Refraction

Strabismus

Strabismus refers to misaligned eyes. If the eyes turn inward (crossed), it is called esotropia. If the eyes turn outward (wall-eyed), it is called exotropia. Or, one eye can be higher than the other which is called hypertropia (for the higher eye) or hypotropia (for the lower eye). Strabismus can be subtle or obvious, intermittent (occurring occasionally), or constant. It can affect one eye only or shift between the eyes.

Strabismus usually begins in infancy or childhood. Some toddlers have accommodative esotropia. Their eyes cross because they need glasses for farsightedness. But most cases of strabismus do not have a well-understood cause. It seems to develop because the eye muscles are uncoordinated and do not move the eyes together. Acquired strabismus can occasionally occur because of a problem in the brain, an injury to the eye socket, or thyroid eye disease.

When young children develop strabismus, they typically have mild symptoms. They may hold their heads to one side if they can use their eyes together in that position. Or, they may close or cover one eye when it deviates, especially at first. Adults, on the other hand, have more symptoms when they develop strabismus. They have double vision (see a second image) and may lose depth perception. At all ages, strabismus is disturbing. Studies show school children with significant strabismus have self-image problems.

Amblyopia, or lazy eye, is closely related to strabismus. Children learn to suppress double vision so effectively that the deviating eye gradually loses vision. It may be necessary to patch the good eye and wear glasses before treating the strabismus. Amblyopia does not occur when alternate eyes deviate, and adults do not develop amblyopia.

Strabismus is often treated by surgically adjusting the tension on the eye muscles. The goal of surgery is to get the eyes close enough to perfectly straight that it is hard to see any residual deviation. Surgery usually improves the conditions though the results are rarely perfect. Results are usually better in young children. Surgery can be done with local anesthesia in some adults, but requires general anesthesia in children, usually as an outpatient. Prisms and Botox injections of the eye muscles are alternatives to surgery in some cases. Eye exercises are rarely effective.

Sunglasses

Sunglasses are popular for comfort and fashion, but now there is medical evidence supporting the use of sunglasses to protect the long-term health of the eyes.

More than a dozen studies have shown that spending hours in the sun without proper eye protection can increase the chances of developing age-related eye diseases like cataracts and macular degeneration. Ophthalmologists now recommend wearing UV-absorbent sunglasses and brimmed hats when in the sun long enough to get a suntan or sunburn.

People mistakenly confuse the ability of sunglasses to block UV light with the color and darkness of the lenses. In truth, UV protection comes from a chemical coating applied to the surface of the lens. Shop for sunglasses that absorb 99 or 100% of all ultraviolet (UV) light. Some lens manufacturers’ labels say “UV absorption up to 400 nm.” This is the same thing as 100% UV absorption.

In addition to UV light, sunlight also has low levels of infrared rays. Infrared wavelengths are invisible and produce heat. The eye seems to tolerate infrared well. Research has not shown a connection between eye disease and infrared light ray exposure.

Polarized lenses cut reflected glare, like sunlight bouncing off water, pavement, or snow. Sunglasses with polarized lenses are popular and useful for fishing, driving, and skiing. Polarization has nothing to do with UV light absorption, but many polarized lenses are now made with a UV-blocking substance.

Wraparound glasses are shaped to keep light from shining around the frames and into the eyes. Studies have shown that enough UV rays enter around ordinary eyeglass frames to reduce the benefits of protective lenses. Large-framed, close-fitting wraparound sunglasses protect the eyes from all angles. Wraparound sunglasses should be considered by commercial fishermen, mountain climbers, skiers, or anyone who spends time at high altitudes or on the water.

Tinted Contacts

Many types of tinted contact lenses are available. They can enhance and even change the color of one’s eyes for cosmetic purposes, for costumes, or provide special effects for the movie industry.

Tinted contacts can make light eyes more blue, green or hazel. They can alter the color of the eyes, such as making brown eyes blue.

Tinted lenses have been used in the movies since 1939. In the movie “Ghostbusters,” actors playing gargoyles wore red contact lenses. Reptile lenses were crafted for the commander in “Star Trek” and white contact lenses were used for the Hulk in “The Incredible Hulk.” Recently, these costume lenses have become available to the general public.

Tinted contacts may also be used to disguise or improve the appearance of an abnormal eye. They can be used to conceal corneal scars, irregular pupils and to hide shrunken, unsightly eyes. Sometimes tinting a lens can make the lens easier for a person with poor vision to handle. These tints are more subtle handling tints.

Contact lenses for the general public, including those with no correction, are considered medical devices. They must undergo clearance for safety by the Food and Drug Administration (FDA). Color additives used by the manufacturers of costume contact lenses must also be approved for use. Additives in unapproved lenses may be toxic.

Purchase only tinted contacts prescribed by an ophthalmologist, and never share lenses with someone else

Visual Field Test

The visual field is the entire area one can see. It includes central and peripheral (side) vision. A visual field test can detect problems with vision in any part of the visual field. Changes in the visual field may be difficult to notice since both eyes are generally used at the same time. One eye can sometimes compensate for some vision loss in the other. A problem may not be detected until each eye is tested separately.

The visual field test provides information that no other test can. It is used to detect many diseases, such as glaucoma or retinitis pigmentosa, which affect the eye, optic nerve, and brain. It can also help diagnose brain tumors, strokes, and other conditions. Visual field testing helps diagnose the disease and can follow the progress of the disease and its treatment.

During a visual field test, one eye is temporarily patched while the other eye is being tested. You are asked to look straight ahead at a fixed spot and watch for targets to appear in your field of vision.

There are two kinds of visual field tests. One method uses moving targets. Targets are moved from outside the visual field (where you can’t see them) toward the center of your vision. When you see them, you press a button. The test can be done using a dark screen on a wall (called tangent screen testing) or using a large bowl-shaped instrument (called Goldmann testing).

The other testing method uses small fixed targets that appear briefly as bright or dim lights (called computerized static perimetry). You sit in a chair facing either a bowl-shaped instrument or a computer screen and indicate when you see the targets appear.