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  • Squint And Amblyopia
    Squint And Amblyopia

Squint And Amblyopia

UPDATE ON SQUINT AND AMBLYOPIA

At birth a baby's vision is very immature, with an acuity of around 3/60. Visual experience in infancy and childhood has a major influence on the development of the visual pathways, particularly the visual cortex. During the critical periods of visual development visual acuity develops as do other modalities such as contrast sensitivity, colour, pattern and motion perception, and binocular function with stereopsis. Disturbance to normal visual development will affect the maturation of these different visual functions, with associated structural changes in the visual pathways and cortex, resulting in amblyopia—i.e. reduced acuity in one or sometimes both eyes without any ocular lesion. Either these children lose binocular function as a result of a squint or they acquire a squint because of poor development of binocular function.

The commonest causes for amblyopia are

  • Squint (STRABISMIC AMBLYOPIA)
  • Blurring of the image in one eye due to unequal refractive power in the two eyes (ANISOMETROPIC AMBLYOPIA).

Squint and amblyopia are common conditions: about one in fifty children have a squint, and up to 5% of the population have an amblyopic or lazy eye. In view of their lifelong impact on visual function and physical appearance, with consequences for education, jobs and psychological wellbeing, good management offers substantial long-term benefits. Recent work favors early diagnosis and treatment, and there has been increasing effort to treat children as soon as possible. There is also renewed interest in the treatment of adults.

AMBLYOPIA

Disability due to amblyopia

Patients with amblyopia may be debarred from undertaking certain jobs because they fail the required visual standards (in particular they may not be able to hold a class II professional driving licence) and additionally they may be at risk of visual handicap if they should damage or lose the vision of the fellow eye.


It is necessary to emphasize the importance of providing children with good vision in each eye, and also to undermine the long-held belief that the amblyopic eye will improve if the non-amblyopic eye is damaged.

Screening and treatment of amblyopia

The mainstay of amblyopic treatment remains refractive correction with spectacles, and occlusion therapy. Treatment with spectacles alone may be enough to improve vision in some patients with late-onset amblyopia. Compliance with treatment is the major factor in response.


Drug treatment for amblyopia has been tried, with levodopa and carbidopa. In the small number of trials reported, usually including patching as well, vision has improved modestly.

Assessment of vision defects in children with amblyopia

The accurate assessment of vision in children is particularly important in the detection of amblyopia and in the assessment of response to treatment. The standard method of assessing vision in older children and adults has been the Snellen-based letter chart, but this is not appropriate for very young children. For these, picture-based tests or single letter tests are used. However, it is important to realize that these can give an apparently better acuity than a test with a line of letters, because of the effect of crowding. This means that a child who is apparently doing well with a single letter test can appear to deteriorate when he or she starts with a linear test—which can be very disappointing to parents unless the reason is carefully explained. Because the differences between lines in the Snellen chart are unequal, the research standard for testing vision in adults is a logMAR based chart. Similar logMAR based test cards have now been developed for children. They seem to be much more sensitive for detecting interocular acuity differences, and are gaining popularity for detecting and managing children with amblyopia.


Treatment of amblyopia

Glasses will be ordered if there is any significant long or short sightedness or astigmatism.


Occlusion

The usual treatment for amblyopia is to wear a patch over the good eye that will, in turn, stimulate the poorer sighted eye. If your child wears glasses the patch is usually worn on the face with glasses on top.


Occlusion does not replace the need for glasses nor does it eliminate any squint.

present. The poorer the vision the longer the patch will need to be worn. If the amblyopia is left untreated the vision will be permanently impaired. Unfortunately, children do not always understand why they need to patch their good eye and so the treatment can sometimes be difficult. Give lots of praise when the patch is worn well and be ready to distract their attention to prevent the patch being removed.

Atropine Occlusion

Occasionally atropine drops/ ointment may be used in the good eye instead. This blurs the vision in the good eye and again encourages the weaker eye to work harder. This treatment is only used after patching has been tried. Glasses will still need to be worn. It is only suitable with certain levels of vision. Your orthoptist will advise you whether this is an option with your child. You orthoptist is very experienced in dealing with children who have amblyopia and will be available to advise you about carrying out the treatment at home as effectively as possible.


Complications

Occasionally children are allergic to the patches. If the skin around the eye becomes sore you should mention it to your orthoptist who may be able to suggest other patches.


In older children there is a very small risk of them developing double vision. If this happens you should stop the patching immediately and contact the orthoptic department. The orthoptist will monitor the risk of this happening and will stop any treatment if the risk becomes too high.

Activities and patching

Doing detailed work is a good idea when your child is wearing the patch. Activities could include reading, drawing, colouring, dot-to-dots or any other activity that requires concentration. In school age children it can often be a good idea for your child to wear the patch at school, as this is when they do their most concentrated work.


It is important to remember that your child is relying on the poorer eye to see. Also, when one eye is patched the child has no 3D vision and may struggle to judge distances.

Treatment of adults with amblyopia

The success of amblyopia treatment declines steadily with age. For most amblyopes it becomes ineffective around the age of eight, although some straight-eyed anisometropes respond well to patching at much later ages.

CONCLUSION

Treatment of strabismus and amblyopia in childhood reduces the risk of later visual and employment disadvantage and maintains a more stable ocular alignment. Treatment of amblyopia in adulthood is not yet a possibility. There will be an increasing trend to early squint surgery in young children and an increase in demand for strabismus surgery in elderly patients.

What is a squint?

Squint is the term used when the two eyes are not pointing in the same direction.

An eye may turn in (convergent squint) or turn out (divergent squint). Occasionally one eye may be higher or lower than the other (vertical squint). The squint may be constant (present at all times) or occur only intermittently. 'Strabismus' is another word for ‘squint’.

Why do squints develop?

We don’t always know why children develop squints. It can run in families and there is a bigger chance of a child developing a squint if their Mum or Dad had a squint as a child. We know that premature babies are more likely to develop a squint and babies who have been very ill at an early age, especially if there has been damage to the brain. In the past, illnesses such as measles and mumps seemed to cause squints, but with vaccination these conditions are luckily very rare now.

There are a number of other reasons why a squint may develop

  • Poor vision in one eye can stop the brain being able to use the eyes together.
  • The most common cause of poor vision in one eye in childhood is the the need for glasses. One eye may be more long-sighted (hypermetropic) or more short-sighted (myopic) or the front of the eye may be an abnormal shape (astigmatism)
  • Rarely, something may be wrong with the eye itself such as a cataract (cloudy lens), or a scar on the front of the eye. Very rarely, one eye may not have developed properly and have a problem with the retina or optic nerve
  • Being long-sighted (or, less commonly, short-sighted) in both eyes can cause the eyes to develop a squint. Generally, long-sightedness causes a convergent squint (eyes turning in). This is because long-sighted children have to make more effort to focus, and this causes the eyes to turn in.
  • Abnormal development or damage to the nerves or muscles that control movement of the eye. There are six muscles attached to each eye that are responsible for controlling the position and movement of the eye. The brain controls these muscles by sending signals down wires called nerves. In some children the muscles or nerves do not develop properly and this leads to the eyes not moving properly or not being lined up as they should be. The nerves controlling eye movements are delicate and can be damaged particularly by severe head injury. In adults, a stroke can cause damage to the nerves or parts of the brain responsible for eye movements and very rarely a brain tumour can cause a squint and double vision.

Why is diagnosis important?

If a squint develops, it is important to find out what has caused the squint as it may be necessary to treat the underlying cause.

In children it is important to get a squint sorted out soon because having a squint can affect development of the vision permanently. If an adult develops a squint they get double vision because each eye is seeing a different picture. If a young child develops a squint they tend not to get double vision. This is because the brain is still developing and it can turn off the vision from the squinting eye. Although this is good because it means they don’t get double vision, if the squint is not sorted out the vision will be permanently switched off in that eye by the brain. The poor vision in a squinting eye is what we mean by the term lazy eye. Another name for the lazy vision in a squinting eye is amblyopia, a Greek word which literally means “blunted sight”.


Can squints be treated?

The short answer is “yes” and you can find out more about treatment in the sections on child and adult squint. However, treatment is not the same as cure. Most of the common problems with having a squint such as the cosmetic appearance and double vision can often be made better with various treatments. But it may not be possible to get rid of the squint completely, and it may not be possible to get rid of double vision. Sometimes, even after surgery, the eyes are not perfectly aligned, and there is a limit to how much surgery can be performed. Surprisingly, age is rarely a limiting factor and just because an adult has had a squint since childhood does not mean it can’t be treated.

Adult Squint Surgery

A large squint can be a cosmetic problem. It can also cause double vision which can be very disabling. Double vision may also mean that you cannot drive.

Can squints be treated?

A full assessment of your eye and vision is performed and the cause of the squint is assessed.

  • If the squint comes and goes then it may be possible to watch and see if it gets any worse.
  • Sometimes it is possible to treat the squint with glasses or exercises.
  • If these do not work then it may be that we need to consider doing an operation to move the muscles which control the eye to a better position to straighten the eyes up.

I need an operation so what happens next?

The angle of the squint needs to be measured and the movement of each affected muscle needs to be assessed. The amount the eyes are working together is also assessed. Your general health is discussed with one of our specially trained nurses and the details of the operation are discussed with one of the doctors.

You will then be asked to sign a form to say that you understand the type of operation which is going to be performed.

Are there any risks?

Although most squint operations successfully straighten the eyes all surgery carries risks and squint surgery is no different. There can be some bruising on the eye which can make it look red.

  • Under or overcorrection of the squint
  • Double vision
  • Loss of vision - very rare, the vision in the operated eye can be made worse. This may be caused by bleeding inside the eye or infection.

If the eye is not quite straight after the operation it may settle by the three month appointment if it does not then we may need to do a further operation to fine tune the first outcome.

Adjustable sutures?

In some adults who have squint surgery it may be more accurate to use adjustable stitches. The doctor will tell you if this is necessary. The operation is performed while you are asleep and the stitch is tied in a bow at the end of the operation. You are then woken up and the eyes measured, if the eyes are not straight we can tighten up the muscle position using some drops to numb the eye. This makes the operation more accurate especially for people who have already had a squint operation.

What happens on the day of surgery?

You will be seen by the anaesthetist who will discuss with you the anaesthetic. This will be a general anaesthetic so you will be asleep. One or two eyes are operated on and the muscles will be either shortened or moved to a new position to straighten the eyes. It may not be exactly right after one operation and sometimes a second operation is needed.

What happens after surgery?

You can go home once you have woken up properly, this is usually 2-4 hours after the operation. The eye will feel gritty for around 3-5 days following surgery. The eye may well look pink after surgery for up to 2-3 weeks and you will be asked to take eye drops for this length of time. A clinic appointment will have been made for a few weeks after the operation to check things are healing well. The exact position of the eye will be fully assessed at 3 months after the operation.

Child Squint Surgery

Pre-assessment

This is an appointment prior to surgery. The angle of the squint will be re-measured and you will have the opportunity to discuss the surgery involved with the surgeon.

Day of surgery

Child must not eat or drink anything from midnight before the operation. On arrival your child will be seen by the anaesthetist who will check their general health and make themselves familiar to the child. One adult is allowed with the child while they are given the anaesthetic. The operation takes between ¾ and 1 ½ hours depending on the surgery involved. Your child will be woken up in an area called recovery. Once awake your child will be returned to you on the ward and will be offered something to eat and drink. After 2 hours if everything is fine, you can take your child home. The operation is not painful, but can be uncomfortable so paracetamol and ibuprofen are good painkillers to have at home. Your child’s eyes will be uncomfortable and sore immediately after surgery and they will want to keep them closed for a few hours. The whites of the eyes will be pink for 2 to 3 weeks – the stitches dissolve on their own.

Post-operative follow-up

You will be given eye drops to put in which will help to settle the swelling as quickly as possible, though important they are not essential if causing a lot of distress.Your child is not allowed to swim for two weeks to reduce the risk of infection. They will be seen in the eye clinic two weeks after the operation to check the eye is settling down and again at three months to check on the final position of the eyes.

Glasses wear

Why wear glasses?

Glasses may be prescribed for several reasons:

  • To correct a refractive error, that makes the eyes unable to focus clearly
  • To alter squints with glasses
  • If a young child is moderately long sighted the effort to focus and see clearly at close range i.e. reading, may make one eye turn in and squint. This happens because the muscles needed to focus the lens in the eye are linked to the muscles that turn the eye in towards the nose.

    Wearing the correct glasses allow the eye muscles to relax and the eye “straighten”. When both eyes are seeing clearly the child’s vision can develop normally.

  • To relieve symptoms such as eye strain and headaches
  • Children often say that they can see as well without their glasses as with them. This may well be true. Young children can over-exert the muscles in the eye and see clearly, but this puts a lot of stress on these muscles and can lead to eyestrain or headaches. Wearing glasses allows the muscles to relax and enables both eyes to see clearly so that the child’s vision can develop properly.

Will my child always need glasses?

This depends on the reason for wearing them.

  • A small amount of long sight will need correcting while the eyes are developing but after, around the age of 10, it may be possible to manage without glasses.
  • Myopia (short sight) tends to increase with age. This means a child will usually continue to wear glasses.
  • Astigmatism can only be corrected by wearing glasses.
  • Glasses may also continue to be needed where children have squints that are improved by wearing glasses or where surgery is not an option.
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