Amblyopia

What is amblyopia?

Amblyopia is a condition where the vision in an eye is poor and it is caused by lack of use of the eye in early childhood. In most cases, only one eye is affected, but it sometimes affects both eyes. Amblyopia is sometimes called a 'lazy eye'. The visual loss of amblyopia cannot be corrected by wearing glasses.

Understanding the development of vision

Newborn babies can see. However, as they grow, the visual pathways continue to develop from the eye to the brain and within the brain. The brain 'learns' how to interpret the signals that come from an eye. This visual development continues until about age 7-8 years. After this time, the visual pathways and the 'seeing' parts of the brain are fully formed and cannot change.

If, for any reason, a young child cannot use one or both eyes normally, then vision is not learnt properly. This results in impaired vision (poor visual acuity) called amblyopia. The amblyopia develops in addition to whatever else is affecting the eye. In effect, amblyopia is a developmental problem of the brain rather than a problem within the eye itself. Even if the other eye problem is treated, the visual impairment from amblyopia usually remains permanent unless it is treated before the age of about seven years.

What disorders can cause amblyopia?

Various eye disorders can cause amblyopia. The three main causes are as follows:

Squint (strabismus)
A squint is a condition where the eyes do not look in the same direction. That is, when one eye looks straight ahead the other eye is pointing inwards, outward, up or down. As the eyes are not straight (aligned) they focus on different points or objects. Because of this, the brain ignores the signals coming from one of the eyes to avoid seeing double. So, just one eye is used to focus on objects. Most cases of squint occur in early childhood - the critical time when the brain is learning to see.

In some cases of squint, the vision in each eye remains fine. In these cases the eye that is used to focus switches from time to time. Therefore, the visual pathways develop from each eye. In many cases of squint, one eye remains the dominant focusing eye. The other 'turned' eye is not used to focus and the brain ignores the signals from this eye. This non-dominant eye then fails to develop the normal visual pathways in childhood and amblyopia develops.

Refractive errors - particularly anisometropia
Refractive errors include: short sight (myopia), long sight (hypermetropia) and astigmatism. These are conditions that are due to poor focusing of light through the lens in the eye. If you have a refractive error in one eye, often the other eye is the same. However, a situation called anisometropia means that there is a difference of refraction in the two eyes.

For example, a child may be much more longsighted on one eye compared to the other. In this situation, the brain tends to ignore the signals coming from the eye with the worst refractive error (in this case, the eye that is most longsighted). Amblyopia may then develop in this eye.

Refractive errors can usually be corrected with glasses. However, unless vision is tested, a parent or guardian may not realise that their child has a refractive error. In particular, if the child has anisometropia and one eye is 'not bad' with good enough vision to to get by with. Without anyone realising, amblyopia may be developing in the eye not being used.

Other disorders that prevents clear vision
Any disorder in a young child that prevents good vision can lead to amblyopia as the brain fails to learn the visual pathways. For example, a cataract in a lens of an eye or a scarred cornea stops light getting to the back of the eye. This is why it is important to remove a cataract in a child as early as possible.

How common is amblyopia?

About 1 in 25 children develop some degree of amblyopia.

How is amblyopia diagnosed and assessed?

Amblyopia can be diagnosed by examining the eyes and testing vision. Different techniques are used to test vision, depending on the age of the child. Also, in particular, children with squint are monitored carefully to see if amblyopia develops.

Children in the UK are usually offered a routine pre-school or school-entry vision check. One of the main reasons for this is to detect amblyopia whist it is still treatable. However, even if your child has had an eye check in the past, tell your doctor if you suspect that vision in one or both eyes has become poor.

A baby or child with a suspected amblyopia is usually referred to an orthoptist. An orthoptist is a health professional who is specially trained to assess and manage children with squint and amblyopia. If necessary, an orthoptist will refer a child to an ophthalmologist (eye surgeon) for further assessment and treatment.

Why is amblyopia important?

If you have permanent amblyopia you do not see properly out of one eye. The severity of visual impairment can vary. Although you can see well out of one eye, enough to get by, it is always best to have two fully functioning eyes.

For example, even with mild amblyopia you may not have a good sense of 'depth' when looking at objects (you cannot see properly in three dimensions). You cannot do some jobs if you have good vision in only one eye. Also, if you only have good vision in one eye, you risk severe sight problems if you have an injury or disease of the good eye later in life. So, treatment is usually always advised if treatment is likely to restore vision.

What are the treatments for amblyopia?

Treatments include:

• Where possible, to treat or correct any underlying eye disorder, AND
• Making the the affected eye work so that vision can develop properly.

Correcting the underlying eye disorders
For example: refractive errors such as short or long sight can be corrected with glasses; cataracts can be removed; etc.

Making the the affected eye work
The main treatment for amblyopia is to restrict the use of the good eye. This then forces the affected eye to work. If this is done early enough in childhood, the vision will usually improve, often up to a normal level. In effect, the visual development of the affected eye catches up. The common way this is done is to put a patch over the good eye for a time. This is called 'eye patching'.

The length of time an eye patch is used, and how long it should be used each day can vary. It depends on the age of the child and how bad the amblyopia has become. Basically, treatment is continued until vision is normal, or until there is no further improvement with continued treatment. This may be for several weeks or for several months. The child is then monitored every now and then up to the age of about eight to check that the eye remains 'in use' and is not becoming amblyopic again. Sometimes spells of 'maintenance' treatment are needed up to the age of eight when the vision pathways are mature and fixed.

Sometimes eye drops or special glasses are used to blur the good eye instead of using an eye patch. Again, this forces the child to use the amblyopic eye to see.

Vision therapy is also sometimes used. In particular, as maintenance treatment. For example, playing 'visually demanding' games with a child which require a lot of use of the affected eye.

What is the outlook (prognosis)?

As a rule, the younger the child is treated, the quicker the improvement in vision is likely to be, and the better the chance of restoring full normal vision. If treatment is started before the age of about 6-7 years then it is often possible to restore normal vision. If treatment is started in older children then some improvement in vision may still occur but full normal vision is unlikely to be achieved.

The most important point is that you must follow the advice given by an orthoptist or eye surgeon about patching (or similar methods to force the affected eye to work). The most common reason why eye patching does not work as well as it might is because treatment is not followed as instructed. Wearing an eye patch for much of the day for several weeks or months may be irritating to some children. However, do try to encourage your child to persist with treatment for the long-term benefit of good vision.

© EMIS and PIP 2006   Updated: July 2006