Strabismus

A squint occurs when one eye is straight and the other eye turns away from the straight position.

A horizontal squint is convergent when the eye turns inwards (esotropia) or divergent when the eye turns outwards (exotropia). A vertical squint is when one eye turns up or down.

Squints can occur at any age. They can be constant, that is, present all day, or they can be intermittent and therefore occur at different times during the day; especially when the child is tired or unwell.

Rarely squints can indicate neurological disease and tumours of the brain or eye. These can be treated if detected early and all children with squints must be examined by an Ophthalmologist.

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 is already trained to receive images from both eyes and cannot ignore the image from the turned eye.

Good vision develops during childhood when both eyes have normal alignment. Strabismus may cause reduced vision, or amblyopia, in the weaker eye.

One method of treating amblyopia is by patching the “good” eye to strengthen and improve vision in the weaker eye. If amblyopia is detected in the first few years of life, treatment is usually successful.

If treatment is delayed until later, amblyopia usually becomes permanent. As a rule, the earlier amblyopia is treated, the better the visual result.

The exact cause of strabismus is not fully understood.

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
• Brain tumours

A cataract or eye injury that affects vision can also cause strabismus.

Babies and young children cannot say if they cannot see well with one or both eyes. Older children occasionally may complain of double vision (diplopia) after the onset of a squint. Some children with a squint can adopt an abnormal head posture; others tend to close one eye in the sun.

Esotropia, where the eye turns inward, is the most common type of strabismus in infants.

Young children with esotropia do not use their eyes together. 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 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, 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.

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.

When strabismus surgery is performed on children, a general anaesthetic is required. Local anaesthetic is an option for adults.

Recovery time is rapid. Patients are usually able to resume their normal activities with a few days.

After surgery, glasses or prisms may be useful. In many cases, further surgery may be needed at a later stage to keep the yes straight.

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 eye misalignment. It is not, however, a substitute for glasses or amblyopia therapy.

Botox TM, a new drug approved by the US Food and Drug Administration for limited use, 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.

Myth: “All babies are squint”.

Fact: Not true. Although the eyes are initially uncoordinated, control of eye movements is achieved by approximately 3 months of age.

Myth: “Children under the age of two years cannot be examined”

Fact: No child is ever too young to be examined for a squint. If squinting is evident, then early appropriate action can be taken.

Myth: “Even if a squint is present, no harm will occur if it is left alone; the child will grow out of it”.

Fact: Harm will occur. The child can suffer gross loss of vision (amblyopia) in the squinting eye and this may become permanent if neglected. He/she may even lose the chance of achieving binocular vision.

Myth: “As long as the squint is only a tiny one, it really does not matter”.

Fact: A small squint is as likely to result in amblyopia and defective binocular vision as a large squint. Therefore, it must be referred as quickly as possible for correction.

Myth: “Squints are never serious”.

Fact: Squints may be a warning sign of serious neurological disease, brain tumours or eye tumours. All children with a squint must be examined by an ophthalmologist.

Myth: “Squints can be straightened at any age and delay won’t matter”.

Fact: Although surgery can be performed at any age, it may only improve the cosmetic appearance. Early surgery (after appropriate amblyopia treatment and spectacles, if needed) can restore full co-ordination of both eyes and lead to binocular vision.

Myth: “A squint operation means the child can leave his/her glasses off afterwards”.

Fact: Surgery will only alter the position of the eyes. It does not correct poor vision from refractive errors nor squints which need spectacles for control.

Myth: “Once the squint is straightened, the treatment is complete”.

Fact: Until the child is 8 years old, the eyes must be regularly checked, as amblyopia can re-occur and new deviations can develop.