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08 December 2010

Strabismus (squint)

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.

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Description

  • Strabismus is the medical term for a squint
  • About 2-4% of children have strabismus
  • If the onset is in early childhood it may result in reduced vision in the affected eye
  • There are many causes for squint, some of which are not fully understood
  • Treatment may include eyeglasses, occlusion of the non-squinting eye, and surgery  

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 in children. About 2-4% 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 very 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 is unable to ignore the image from the turned eye.

Amblyopia

Good vision develops during childhood when both eyes have normal alignment and focus. Strabismus may cause reduced vision, or amblyopia, in the squinting eye. The brain will recognise the image of the stronger 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. Amblyopia may become permanent if treatment is delayed. As a rule, the earlier amblyopia is treated, the better the eventual vision.

What causes strabismus?

The exact cause of all squints or strabismus is not fully understood. Six eye muscles that control eye movement are attached to the outside of each eye. In each eye, two muscles move the eye horizontally. The other four muscles move it up, down or obliquely. To line up and focus both eyes on a single target, all of the muscles in each eye must be balanced and working together - the muscles in both eyes must be co-ordinated.

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

How is a squint diagnosed?

The main symptom of strabismus is that one eye does not appear aligned with the fellow eye. Sometimes children will close one eye in bright sunlight or tilt their head to use their eyes together. Strabismus can be diagnosed during an eye examination.

It is recommended that all newborn babies undergo an eye examination by theirpaediatrician, and have a repeat examination at four to six months.  Children should have their vision checked by theirpaediatrician, family doctor, optometrist,orthoptist, ophthalmologist (eye specialist), or at a pre-school screening, at or before their fourth birthday. If there is a family history of strabismus or amblyopia, an ophthalmologist should assess their 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 on 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 a true and a 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 acquired esotropia that occurs in far-sighted children who are slightly older. When a child is young, he or she can focus the eyes to adjust for the far-sightedness, but the effort in focusing (accommodation) can cause the eyes to cross. Glasses reduce the focusing effort and can straighten the eyes. Very rarely bifocal eyeglasses are needed for close work. Eye drops or special lenses called prisms can also be used in the management of this disorder.

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 closes 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 or to restore vision, straighten the eyes and develop binocular (two-eyed) vision. After an eye examination, including the application of pupil-dilating eye drops, the ophthalmologist will recommend appropriate treatment. In some cases, eyeglasses may be prescribed.  Covering or patching the strong eye to improve amblyopia is often necessary.  Other treatments may involve surgery to correct the unbalanced eye muscles or, very rarely, to remove a cataract.

Surgery
The eyeball is never removed from the socket during any kind of strabismus 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 the direction the eye is turning. It may be necessary to perform surgery on one or both eyes.

A generalanaestheticis usually required. Recovery time is rapid and normal activities can normally be resumed within a few days. Occasionally more than one operation may be necessary to straighten the eyes.  After surgery, glasses or prisms may still be needed.

As with any surgery, eye muscle surgery has certain risks. These include infection, bleeding, scarring over the white of the eye 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 occlusion therapy.

An alternative to surgery
Botox (TM), a drug better known for its use in reducing facial wrinkles by relaxing facial muscles, is an alternative to eye muscle surgery for some forms of strabismus. 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, the misalignment may in some cases be permanently corrected.

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

Reviewed by Dr Andrew Ivey, MB ChB, MRCOphth, FCS(SA)/FRCS(Edinburgh, December 2010                           

Useful resources:

South African Optometric Association
Tel: 011 805 4517

South African National Council for the Blind
Tel: 012 452 3811

Retina South Africa
Tel: 011 622 4904

Ophthalmological Society of South Africa

 

 

 
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