STRABISMUS

What is Strabismus?
Squint is the common name for ‘strabismus’ which is the medical term used to describe eyes that are not pointing in the same direction. A squint can be convergent (esotropia), divergent (exotropia) or vertical.

The squint may be present all or only part of the time, in only one eye or alternating between the two eyes. It has been estimated that four in every 100 adults suffer from this condition. Adult squints are of three main types: non-paralytic, paralytic and restrictive. Non-paralytic squints are usually a longstanding from childhood. The most common pattern is that an eye that was straight after childhood squint surgery later drifts out and causes concern over its appearance.

In a paralytic squint an eye does not move normally because one or more eye muscles are weak or paralysed. This problem may have developed as a result of other health problems, such as damage to cranial nerves, following head injury or as a complication of diabetes or stroke. Such people will, most likely, suffer from troublesome double vision.

Investigations may be required to discover the underlying cause. In a restrictive squint one or both eye do not move fully because of scarring or tethering of one or more muscles. A common example of this occurs in thyroid eye disease.

Symptoms
The most obvious sign of a squint is one eye that does not look straight ahead but turns inwards, outwards, upwards or downwards. Minor squints may be less obvious.

Adult Squint
Symptoms of adult squint problems include fatigue, double vision, difficulty with near vision and loss of stereo vision. To compensate for this, some individuals will adopt an abnormal head position. Many adults with squint are concerned about the appearance of their eyes and the impact this has on social relationships and work.

Babies and young children
It is quite normal for the eyes of newborn babies to ‘cross’ occasionally, particularly when they are tired. Speak to your GP if you notice this happening to your child after the age of three months.

If your child looks at you with one eye closed or with their head turned to one side, it may mean that they are experiencing double vision and could be a sign that they have a squint.

Lazy eye
If a squint is left untreated, lazy eye (amblyopia) can develop. The vision in the affected eye gradually deteriorates because the brain ignores the weaker message being sent from that eye. It is not possible to correct damage once it has occurred, which is why it is so important to treat a squint as soon as possible.

Causes
The cause of a squint is not always known. Squints sometimes run in families and a baby can be born with the condition (congenital squint).

Squints are also sometimes the result of childhood illnesses or other sight-related problems, such as long-sightedness (hypermetropia).

Congenital squints
A child is either born with a squint or develops one during the first six months of life.

Sometimes other family members have a squint, which suggests it can be genetic.

In most cases of congenital squint, the eye turns inwards (congenital esotropia). It is also possible (although rarer) for the eye to turn outwards (congenital exotropia).

Refractive errors
Squints are sometimes caused by the eye’s inability to focus the light that passes through the lens. This is known as a refractive error, and is also the cause of conditions such as short-sightedness (myopia), long-sightedness (hypermetropia) and astigmatism (where the cornea at the front of the eye is unevenly curved).

If a child has a refractive error, their eye may turn inwards as it attempts to focus. Squints caused by refractive errors usually develop in children who are two or older and tend to be most common in children who are long-sighted.

Other causes
Most squints are congenital or caused by refractive errors. Occasionally, squints can be the result of:

childhood illnesses, such as viral infections like measles,
some genetic conditions, such as Noonan syndrome, or
a brain condition, such as hydrocephalus where there is too much cerebrospinal fluid (CSF) in the ventricles (cavities) of the brain.

Diognesis
A squint can cause vision problems, such as a lazy eye (amblyopia), double vision or blurred vision. The earlier a squint is identified and diagnosed, the more chance there is of successfully treating it and restoring vision to normal.

Routine eye checks are carried out at birth and again six to eight weeks later. Children are also given a routine eye check just before starting primary school.

Your child will have a number of different tests which can help diagnose a squint and assess their level of vision. The type of tests that your child will have depends on their age, but may include:

looking at a light,
matching letters and pictures,
reading a letter chart, and
looking at visual targets, such as a toy, at different distances, first with one eye covered and then the other.
If a squint is suspected, optician will refer your child to an ophthalmologist for further examination. Your child will have their eyes tested to determine whether they need glasses. Before the test is carried out, your child’s pupils will be dilated (expanded) using eye drops.

Their retina (the light-sensitive membrane at the back of the eye) and the optic nerve (the nerve that carries visual messages from the retina to the brain) will also be examined to make sure that there are no other problems with them.

Treatment
It is very important that a squint is treated as soon as possible after being detected. If it is not treated, vision problems, such as those caused by a lazy eye (amblyopia), are likely to get worse or could become permanent. Treatment is most effective in very young children. Types of treatment.

Several types of treatment are available for squints:

glasses,
eye patch,
botulinum toxin injection,
eye drops, and
corrective surgery.

Glasses
If your child is long-sighted (hypermetropia), they may be prescribed glasses which can often correct both the vision problem and the squint.

Eye patch
A patch may need to be worn over the ‘good eye’ to encourage the eye with the squint to work harder and train it to work properly.

Botulinum toxin injection
Botulinum toxin (botox) is injected into one of the muscles on the eye’s surface. This may be recommended if a squint develops suddenly and no underlying cause can be found.

In children, a botox injection will usually be given under general anaesthetic. The injection temporarily weakens the injected muscle, allowing the eyes to realign.

Eye drops and eye exercises
In some cases, it may be possible to treat a squint using special eye drops or eye exercises.

Surgery
If none of the above treatments work, surgery may be needed.

Surgery has two main benefits:

it can improve the alignment of the eyes, and
it can get the eyes working together (binocular vision).

The procedure
Surgery to correct a squint involves moving the muscles attached to the outside of the eye to a new position. It may be necessary to operate on both eyes to ‘balance’ them effectively, even if the squint is only in one eye.

As corrective squint surgery usually takes less than an hour to perform, the procedure is often carried out as a day case. The operation is usually performed general anaesthetic (where the patient is asleep).

You may be able to accompany your child to the operating theatre and stay with them until they have been given the anaesthetic. A nurse will be with your child throughout the procedure.

During the operation, your child’s eye will be kept open using an instrument called a lid speculum. The ophthalmologist will detach one part of the muscle that is connected to your child’s eye and will either move it backwards to weaken the pulling effect or shorten it to increase the pulling effect. Once the correction has been made, the muscles will be sewn back into place using dissolvable stitches.

There is very little chance that your child’s eyesight will be damaged during the operation because the part of the eye responsible for focusing is not touched. For a short time after the operation, your child will need to use eye drops. The nurse will show you how to use them before you leave the hospital.

Risks of surgery
There are risks associated with any kind of surgery. Before your child’s operation, ask the ophthalmologist to discuss the possible risks with you.

Some of the risks of eye surgery are:

Your child may need to have further surgery to correct their squint, particularly if it is a major squint.
The eye can sometimes remain red for a long time after the operation. This is rare, but may be due to scar tissue forming on the surface of the eye.
After the operation, your child may have double vision. This usually resolves itself after a week or so.
The squint may reoccur. Each child reacts differently to surgery and sometimes a further operation may be needed.
One of the eye muscles can slip after the operation, although this is very rare. This makes the eye point inwards or outwards (depending on which muscle has slipped) and can impair movement. If this happens, your child may need to have another operation.
In very rare cases, the inside of the eye can become damaged during surgery.
There is a risk of an infection developing after the operation. This is the case with any type of surgery. Your child will be given eye drops or ointment to reduce the risk.

Recovery
After having corrective eye surgery, your child may have a sore eye for a few days. The pain can be treated using simple painkillers, such as paracetamol. Children under the age of 16 should not be given aspirin.

Your child will not have to wear a patch or bandage and can return to daily activities, such as reading, as soon as they feel able to. The aim is to get the eyes working normally as quickly as possible.

Advice following corrective eye surgery
Follow the advice below when your child gets home after an eye operation.

Your child may not feel like eating but they should drink water at regular intervals.
Your child may complain of double vision. This usually resolves itself over a few weeks.
The stitches can take up to six weeks to dissolve and your child may feel like there is a bit of grit in their eye. Encourage them not to rub it.
Avoid any soap or shampoo getting into their eye for two weeks.
Your child will need to go back to the eye specialist for regular follow-up visits.
If your child wore glasses before the operation, it is likely that they will have to continue to wear them. Your child should be able to return to school or nursery after about four days.

They should avoid swimming for at least a month after the operation and most other sports for two weeks.