Strabismus

What is Strabismus?

Strabismus is a condition in which the eyes are not aligned with each other — one eye turns in a different direction from the other.

Under normal circumstances, the six muscles that control eye movement work together to point both eyes in the same direction. Patients with strabismus have difficulty controlling their eye movements and cannot maintain normal ocular alignment (eye position).

Strabismus can be categorised by the direction in which the misaligned eye turns:

• Inward turning (esotropia)
• Outward turning (exotropia)
• Upward turning (hypertropia)
• Downward turning (hypotropia)

Other factors that help determine the cause and treatment of strabismus include:

• Did the problem come on suddenly or over time?
• Was it present in the first 6 months of life, or did it occur later on?
• Does it always affect the same eye, or does it switch between eyes?
• Is the degree of turning small, moderate, or large?
• Is it always present, or only part of the time?
• Is there a family history of strabismus?

What are the types of strabismus?

There are several forms of strabismus. The three most common are:

• Infantile esotropia: This condition is characterised by a large inward turn of both eyes in infants, usually beginning before the age of six months. There is usually no significant hypermetropia, and glasses do not correct the strabismus. Treatment for this type of strabismus is surgery on the muscles of one or both eyes to correct the alignment.
• Accommodative esotropia: This often occurs where there is uncorrected long-sightedness and a genetic predisposition (family history) for the eyes to turn in. Because the ability to focus is linked to where the eyes are pointing, the extra focusing effort needed to keep distant objects clear may cause the eyes to turn inwards. Symptoms include double vision, closing or covering one eye when looking at something close, and tilting or turning the head. This type of strabismus typically begins in the first few years of life. It is usually treated with glasses, but may also require eye patching and/or surgery on the muscles of one or both eyes.
• Intermittent exotropia: In this type of strabismus, one eye fixates (concentrates) on a target while the other points outwards. Symptoms may include double vision, headaches, difficulty reading, eye strain, and closing one eye when looking at distant objects or in bright light. Patients may have no symptoms at all, even though the ocular deviation is noticed by others. Intermittent exotropia can occur at any age. Treatment may involve glasses, patching, eye exercises and/or surgery on the muscles of one or both eyes.

Adults can also develop strabismus. Most commonly, ocular misalignment in adults is due to stroke, but it can also result from physical trauma, or from childhood strabismus that was never treated or has relapsed or progressed. Adult strabismus can be managed in a variety of ways, including observation, patching, prisms in glasses and strabismus surgery.

How common is strabismus?

An estimated 4% of the population has strabismus.

What causes strabismus?

Most strabismus results from an abnormality of the neuromuscular control of eye movement, and our understanding of these control centres in the brain continues to evolve. Less commonly, there is a problem with the eye muscle itself. Strabismus is often inherited: about 30 per cent of children with strabismus have a family member with a similar problem.

Other conditions associated with strabismus include:

• Uncorrected refractive errors
• Poor eyesight in one eye
• Cerebral palsy
• Down syndrome (20-60% of these patients are affected)
• Hydrocephalus (a congenital condition in which fluid accumulates in the brain)
• Brain tumours
• Stroke (the leading cause of strabismus in adults)
• Head injuries, which can damage the area of the brain responsible for controlling eye movement, the nerves that control eye movement, or the eye muscles
• Neurological problems
• Thyroid eye disease

When do the symptoms of strabismus appear?

By 3 to 4 months of age, an infant’s eyes should be able to focus on small objects and should be straight and well aligned. A 6-month-old infant should be able to focus on objects both near and far.

Strabismus usually appears in infants and young children, most often by the time a child is 3 years old. However, older children and even adults can develop it. The sudden appearance of strabismus, especially with double vision, in an older child or adult could indicate a more serious neurological disorder — if this happens, call your ophthalmologist immediately.

A condition called pseudostrabismus (false strabismus) can make it look as though a baby has crossed eyes when in fact both eyes are pointing in the same direction. Pseudostrabismus can be caused by extra skin covering the inner corners of the eyes and/or a flat nasal bridge.

How is strabismus diagnosed?

Anyone older than four months of age who appears to have strabismus should undergo a full ophthalmological examination by a paediatric ophthalmologist, with extra time spent examining how the eyes focus and move. The examination may include the following:

• Patient history (to establish the patient’s symptoms, family history, general health problems, medications used and any other possible causes of the symptoms)
• Visual acuity (reading letters from an eye chart, or observing the visual behaviour of young children)
• Refraction (checking the eyes with a series of corrective lenses to measure how they focus light).
• Alignment and focus tests
• Examination after dilation of the pupils to determine the health of internal eye structures

How is strabismus treated?

Treatment options include the following:

• Eyeglasses or contact lenses: Used in patients with uncorrected refractive errors. With corrective lenses, the eyes need less focusing effort and may remain straight.
• Prismatic lenses: Special lenses that bend the light entering the eye, reducing the amount of turning the eye has to do to look at objects.
• Orthoptic exercises (eye exercises): These can help in certain types of strabismus, especially convergence insufficiency (a form of exotropia).
• Medications: Eye drops or ointments. Injections of botulinum toxin type A (such as Botox) can also weaken an overactive eye muscle. These treatments may be used together with surgery, or in place of it, depending on the patient’s situation.
• Patching: Used to treat amblyopia (lazy eye) when it occurs alongside strabismus.
• Eye muscle surgery: Surgery changes the length or position of the eye muscles so that the eyes are properly aligned. It is performed under general anaesthesia using dissolvable sutures. Adults are sometimes offered strabismus surgery with adjustable sutures, allowing the muscle positions to be fine-tuned after the operation.

What can happen if strabismus is not treated?

It is sometimes assumed that children will grow out of strabismus, or that it will improve on its own. In fact, it can get worse if left untreated.

If the eyes are not properly aligned, the following may result:

• Amblyopia (lazy eye) — permanently poor eyesight in the eye with strabismus. When the eyes look in different directions, the brain receives two images. To avoid double vision, the brain may ignore the image from the turned eye, so that vision in this eye fails to develop properly.
• Blurred vision, which can affect performance at school and work, and the enjoyment of hobbies and leisure activities
• Eye strain
• Fatigue
• Headaches
• Double vision
• Poor three-dimensional (3-D) vision
• Low self-esteem (from embarrassment about the eye’s appearance)

Failing to diagnose strabismus can also mean that a serious underlying problem (such as a brain tumour causing the condition) is overlooked.

What can be expected after treatment for strabismus?

The patient will need follow-up visits with the paediatric ophthalmologist to check the response to treatment and to make any necessary adjustments.

For children with strabismus, catching the condition in time and treating it properly can result in excellent vision and depth perception, and protects against loss of vision.