Strabismus

What is Strabismus?

Strabismus is a misalignment of the eyes where the two eyes are pointed in different directions. Though it is a common condition which affects about 4% of children, it may appear later in life. The misalignment may be permanent and always noticeable, or it may come and go appearing normal at times and abnormal at others. One eye may be directed straight ahead while the other is turned inward (esotropia), outward (exotropia), upward (hypertropia) or downward (hypotropia). In other cases, the turned eye may straighten at times and the straight eye may turn.

Eye Muscles

There are six eye muscles attached to the outside of each eye which control its movement. In each eye, two muscles move the eye right or left; there other four muscles move the eye up or down and control tilting movements. In order to line up and focus both eyes on a target, all eye muscles of each eye must be balanced and working together with the corresponding muscles of the other eye. When the eye muscles do not work together, misalignment of the eyes, or strabismus, results.

The eyes are designed to focus images clearly on the retina and then to relay them to the brain. If both eyes are lined up on the same target, the visual portion of the brain can fuse the two pictures into a single, 3-dimensional image. This creates depth perception and binocular vision, which helps the eyes work together to transmit one “picture” to the brain. When one eye turns, as in strabismus, two different pictures are sent to the brain. In the young child the brain learns to ignore the image from the misaligned eye and see only the image from the straight or better seeing eye. This causes loss of depth perception. Adults who develop strabismus usually have double vision because their brain is already trained to receive images from both eyes and cannot ignore the image from the turned eye.

Normal alignment of both eyes during childhood allows good vision to develop in each eye. Abnormal alignment as in strabismus may cause reduced vision or amblyopia. Amblyopia occurs in approximately one half of children with strabismus. The brain will recognize the image of the better seeing eye and ignore the image of the weaker eye. Amblyopia can often be reversed by patching the preferred eye in order to strengthen and improve the vision of the weaker one. If amblyopia is detected within the first few years of life, treatment is often successful. If adequate treatment is delayed until later, amblyopia generally becomes permanent. As a rule, the earlier amblyopia is treated, the better the visual result.

Causes and Symptoms

Strabismus is cause by misaligned eye muscles. However, the exact reason for the misalignment of the eyes leading to strabismus is not fully understood. It is known that strabismus runs in some families. However, in many patients, there are no relatives with the problem. The condition occurs equally in males and females.

The brain controls the eye muscles. This explains why children with cerebral palsy, Down’s Syndrome, and hydrocephalus often have strabismus. Even a brain tumor may cause strabismus. If the vision of one eye is cloudy because of a cataract or injury, then the eye can frequently develop strabismus.

The primary symptom of strabismus is an eye that is not straight. Sometimes a youngster will squint one eye in bright light. Faulty depth perception may be present. Some children turn or tilt their heads in a specific direction in order to use their eyes together.

Parents often get the false impression that a child may “outgrow” the problem. Though fatigue or illness may worsen strabismus, children do not outgrow strabismus. Once a child has a suspected turning of an eye, an examination by an ophthalmologist is necessary to determine the cause and to begin treatment.

Detection and Diagnosis

A child should be examined by the family doctor, pediatrician or ophthalmologist (a medical eye doctor) during infancy and preschool in order to detect any potential eye problem, particularly if a relative has had strabismus or amblyopia. Even the most observant parent may not discover strabismus without a doctor’s help. It is often difficult to determine the difference between eyes that appear crossed (pseudostrabismus) and true strabismus. Young children usually have a wide nasal bridge, flat nose and redundant fold of skin at the inner eyelid that tends to hide the eye during side gaze and cause concern about strabismus. An ophthalmologist can readily distinguish this from true strabismus.

It is never too early to have a child’s eyes examined. Fortunately, an ophthalmologist can test even a newborn’s eyes. If the eye examination is delayed until the child enters school, it may be too late to properly correct strabismus and amblyopia. Occasionally, a misaligned eye may be cause by a cataract or tumor in the eye. It is important to know about such conditions as early as possible so both the underlying condition and the resulting strabismus can be corrected.

Treatment

The goals of treatment are to preserve vision, straighten the eyes and restore binocular vision. Treatment of strabismus depends upon the exact cause of the misaligned eyes. It can be directed towards unbalanced muscles, cataract removal, or other conditions which are causing the eyes to turn. After a complete eye exam, including a detailed study of the inner parts of the eyes, an ophthalmologist can recommend appropriate medical, optical or surgical therapy. Covering or patching the good eye to force use of the amblyopic eye may be necessary to ensure equal vision.

Infantile Esotropia

The most common type of strabismus in infants is esotropia, where and eye turns in. Infants born with esotropia will not learn to use their two eyes together and may lose vision in the weaker eye. In most cases, early surgery is needed to align the eyes in an effort to obtain binocular vision and prevent permanent vision loss. The aim of the surgery is to adjust the muscle tension on one or both eyes, on order to pull the eyes straight. (Please see section on strabismus surgery.)

Accommodative Esotropia

Another common form of esotropia that occurs in children usually after the age of one or two is caused by a need for glasses. These children are farsighted. They have the ability to focus the eyes enough to adjust for the farsightedness, which allows them to see well for both distance and near. Some children excessively cross their eyes when they focus, which cause the eyes to turn in. Wearing glasses in equal strength to their farsightedness reduces the need to focus and straightens their eyes. Sometimes the addition of bifocals is necessary to further reduce the need to focus when looking up close. Occasionally eye drops and special lenses called prisms can be used to focus the eyes properly.

Exotropia

Exotropia or an outward turning of an eye is another common type of strabismus. Most commonly this occurs when a child is focusing at distant objects. Often the exotropia will occur intermittently particularly when the child is daydreaming, ill or tired. Parents often notice that the child squints one eye in the bright sunlight. Although glasses and prism therapy may reduce the amount of outward turning in some patients, surgery is usually needed.

Strabismus Surgery

Strabismus surgery is usually a safe and effective treatment but is not a substitute for glasses or amblyopia therapy. During surgery the eyeball is not removed from the socket. The selection of the muscles to be operated is dependent on the type of strabismus in each individual’s case. More than one surgery is often necessary to get the eyes straight. Please see our section on strabismus surgery for more information.

Who can treat strabismus?

An ophthalmologist is the medical doctor who is educated trained and licensed to provide total eye care of the eyes including the diagnosis and treatment of strabismus. Total eye care includes performing comprehensive medical eye examinations, prescribing corrective lenses, diagnosing disease and disorders of the eye, and using the appropriate medical and surgical procedures necessary for their treatment. An ophthalmologist should be consulted if a child has a family history of amblyopia or strabismus, if the eyes do not appear straight, if there is decreased vision in one of both eyes or if there is any evidence of cataract, glaucoma or any other medical eye problem