Squint

Summary

• Strabismus is a squint
• About 4% of children have strabismus
• It may cause reduced vision in the weaker eye
• The exact cause is not fully understood
• Treatment includes eyeglasses, surgery or the removal of a cataract

What is strabismus?

Strabismus is a squint. The eyes are misaligned and look in different directions. One eye may look straight ahead, while the other eye turns inward, outward, upward or downward. The turned eye may straighten at times and the straight eye may turn.

Strabismus is a common condition among children. About 4% of children have strabismus. It can also occur later in life.

It occurs equally in males and females. Strabismus may run in families.

How do the eyes work together?

With normal vision, both eyes aim at the same spot. The brain then fuses the two pictures into a single three-dimensional image. This three-dimensional image gives us depth perception.

When one eye turns, two different pictures are sent to the brain. In a young child, the brain learns to ignore the image of the misaligned eye and sees only the image from the straight or better-seeing eye. The child then loses depth perception. Adults who develop strabismus often have double vision because the brain cannot ignore the image from the turned eye.

Amblyopia

Good vision develops during childhood when both eyes have normal alignment. Strabismus may cause reduced vision, or amblyopia, in the weaker eye. The brain will recognise the image of the better-seeing eye and ignore the image of the weaker or amblyopic eye.

Amblyopia can be treated by patching the "good" eye to strengthen and improve vision in the weaker eye. If treatment is delayed until later, amblyopia usually becomes permanent. As a rule, the earlier amblyopia is treated, the better the eventual vision.

What causes strabismus?

The exact cause of a squint or strabismus is not fully understood. Six eye muscles controlling eye movement are attached to the outside of each eye. In each eye, two muscles move the eye right or left. The other four muscles move it up or down and at an angle. To line up and focus both eyes on a single target, all of the muscles in each eye must be balanced and working together. In order for the eyes to move together, the muscles in both eyes must be coordinated.

The brain controls the eye muscles. Strabismus is especially common among children with disorders that affect the brain, such as cerebral palsy, Down syndrome, hydrocephalus and brain tumours. Cataracts and other causes of poor vision can also cause strabismus.

How is a squint diagnosed?

The main symptom of strabismus is an eye that is not straight. Sometimes children will squint one eye in bright sunlight or tilt their head to use their eyes together. Strabismus can be diagnosed during an eye exam.

It is recommended that all children have their vision checked by their paediatrician, family doctor or ophthalmologist (eye specialist) at or before their fourth birthday. If there is a family history of strabismus or amblyopia, an ophthalmologist can check vision even earlier.

The eyes of infants often seem to be crossed. Young children often have a wide, flat nose and a fold of skin at the inner eyelid that can make the eyes appear crossed. This appearance of strabismus may improve as the child grows. A child will not outgrow true strabismus. An ophthalmologist can tell the difference between true and false squint.

What are the different types of squint?

Esotropia

Esotropia, where the eye turns inward, is the most common type of squint in infants. In most cases, early surgery can align the eyes. During surgery for esotropia, the tension of the eye muscles in one or both eyes is adjusted.

Accommodative esotropia

Accommodative esotropia is a common form of esotropia that occurs in farsighted children two years or older. When a child is young, he or she can focus the eyes to adjust for the farsightedness, but the focusing effort (accommodation) to see clearly causes the eyes to cross.

Glasses reduce the focusing effort and can straighten the eyes. Sometimes bifocals are needed for close work. Eye drops, ointment or special lenses called prisms can also be used to straighten the eyes.

Exotropia

Exotropia, or an outward turning eye, is another common type of strabismus. This occurs most often when a child is focusing on distant objects. The exotropia may occur only from time to time, particularly when a child is daydreaming, ill or tired. Parents often notice that the child squints one eye in bright sunlight. Although glasses, exercises or prisms may reduce or help control the outward turning eye in some children, surgery is often needed.

How is strabismus treated?

Treatment for strabismus aims to preserve vision, straighten the eyes and restore binocular (two-eyed) vision. After a complete eye examination, the ophthalmologist will recommend appropriate treatment. In some cases, eyeglasses can be prescribed for your child. Other treatments may involve surgery to correct the unbalanced eye muscles or to remove a cataract. Covering or patching the strong eye to improve amblyopia is often necessary.

Surgery

The eyeball is never removed from the socket during any kind of eye surgery. The ophthalmologist makes a small incision in the tissue covering the eye to reach the eye muscles. Certain muscles are repositioned during the surgery, depending on which direction the eye is turning. It may be necessary to perform surgery on one or both eyes.

A general anaesthetic is usually required. Recovery time is rapid. People are usually able to resume their normal activities within a few days. Often more than one operation is necessary to straighten the eyes and then to keep them straight. After surgery, glasses or prisms may be needed.

For children with constant strabismus, early surgery offers the best chance for the eyes to work well together. In general, it is easier for children to undergo such surgery before school age. As with any surgery, eye muscle surgery has certain risks. These include infection, bleeding, excessive scarring and other rare complications that can lead to loss of vision. Strabismus surgery is usually a safe and effective treatment for strabismus. It is not, however, a substitute for glasses or amblyopia therapy.

An alternative to surgery

Botox (TM), a drug, is an alternative to eye muscle surgery for some individuals. An injection of this drug into an eye muscle temporarily relaxes the muscle, allowing the opposite muscle to tighten and straighten the eye. Although the effects of the drug wear off after several weeks, in some cases, the misalignment may be permanently corrected.

Reviewed by Dr L.C. Boezaart, M.B.Ch.B, M.Prax.Med, M.Med (Ophth).