What is amblyopia?
Amblyopia is a condition where the vision in an eye is poor because of 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 often called a lazy eye. The visual loss in 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 vision signals that come from an eye. This visual development continues until about age 7-8 years. After this time, the visual pathways and the parts of the brain involved with vision 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 poor sight (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:
A squint is a condition where the eyes do not look together in the same direction. Whilst one eye looks straight ahead, the other eye turns to point inwards, outwards, upwards or downwards. As the eyes are not straight (aligned) they focus on different things. The result is that the brain ignores the signals from one of the eyes to avoid seeing double. This means that only 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 normal. In these cases, the eye that is used to focus changes from time to time. Consequently, the visual pathways develop from both eyes. However, in many cases of squint, one eye remains the dominant, focusing eye. The other turned (squinting) eye is not used to focus, and the brain ignores the signals from this eye. This nondominant eye then fails to develop the normal visual pathways in childhood and amblyopia develops. (See separate leaflet called ‘Squint (Childhood)’ for more information.)
Refractive errors – particularly anisometropia
Refractive errors are eyesight problems. Refractive errors include: short sight (myopia), long sight (hypermetropia) and astigmatism. (See separate leaflets called ‘Short Sight – Myopia’, ‘Long Sight – Hypermetropia’ and ‘Astigmatism’ for more information.)
Refractive errors are problems 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 or similar. A situation called anisometropia, can occur where there is a difference of refraction between the two eyes. In anisometropia, one eye may be myopic (short-sighted), and the other hypermetropic (long-sighted). If this difference is large, the brain cannot understand the images coming from both eyes, and will choose to ignore the signals coming from one eye. Usually the brain selects the eye with the better refractive error in preference. The other eye (often the most long-sighted one) then becomes amblyopic.
Refractive errors can usually be corrected with glasses. Prescription lenses change how the lens of the eye focuses light. Unless vision is tested, a parent may not realise their child has a refractive error. This is particularly the case if the child has anisometropia. One eye might have good enough vision to get by with and, without anyone realising, amblyopia may develop in the eye not being used.
Other disorders that prevent clear vision
Any disorder in a young child that prevents good vision can lead to amblyopia as the brain fails to learn and develop the visual pathways. This is known as stimulus deprivation amblyopia. 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. Even a droopy eyelid can cause amblyopia if it covers enough of the eye to prevent it seeing properly. (See separate leaflet called ‘Cataracts’ for more information.)
How common is amblyopia?
About 1 in 25 children develop some degree of amblyopia. Amblyopia is the most common condition treated by paediatric (children’s) ophthalmologists (eye surgeons) and orthoptists (non-doctor professionals who treat eye movement and vision problems).
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. Children with a known squint are monitored carefully to see if amblyopia develops.
Children in the UK are usually offered a routine preschool or school-entry vision check. One of the main reasons for this is to detect amblyopia whilst 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. Orthoptists are specially trained to assess and manage children with squint and amblyopia. If necessary, an orthoptist will refer a child to an ophthalmologist 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 enough out of one eye to get by, it is always best to have two fully functioning eyes.
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. 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 it is likely to restore vision.
What are the treatments for amblyopia?
- Correcting any underlying eye disorder, such as strabismus (squint), or correcting refractive errors (for example, long or short sight).
- ‘Training’ the amblyopic eye to work properly, so that vision can develop correctly.
Correcting the underlying eye disorders
Refractive errors such as short or long sight can be corrected with glasses. Cataracts can be treated with an operation. Improvement in eyesight after being fitted with glasses for a refractive error can take 4 -6 months.
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 most common treatment for amblyopia is eye patching. This is where the good eye is covered with an eye patch, forcing the amblyopic (lazy) eye to see. Eye patches are soft, with sticky edges that fix them to the skin surrounding the eyelids. Eye patching is also called occlusion.
The length of treatment with an eye patch is dependent on the age of the child and the severity of the amblyopia. Treatment is continued until either the vision is normal, or until no further improvement is found. Usually you would be followed up every three months. If the vision is normal or stable for six months, use of the eye patch may be tailed off. If your child has had cataracts, full-time eye patching may be advised until the age of seven years. Short breaks would be built into this time, to prevent the good, patched eye from becoming amblyopic due to disuse. It may take from several weeks to several months for eye patching to be successful. On average, patches may be worn for between 2 and 6 hours per day.
Your child will be followed up, usually until about eight years of age. This is to make sure that the treated eye is still being used properly, and does not become amblyopic again. Sometimes, further patch treatment (maintenance treatment) is needed before the vision pathways in the brain are fixed and cannot be changed.
Other treatments for amblyopia include eye drops and glasses. Occasionally, eye drops are used to blur the vision in the good eye instead of an eye patch. Eye drops can be useful when a child refuses to wear a patch. Once drops are put in a child’s eye, the child can’t change the blurring of vision, it simply wears off after time. You might have to put the drops in your child’s eye each day, but sometimes it can be done just at weekends. Some people find it difficult to hold their child and put drops in the eye. With practice, both you and your child can get used to using eye drops. From a cosmetic viewpoint, using eye drops is less obvious than an eye patch. The eye drops used to blur the vision usually contain a medicine called atropine. This can occasionally cause side effects such as eye irritation, flushing (reddening) of the skin, a fast heartbeat (tachycardia) and hyperactivity.
Another option is to be fitted with glasses that prevent the good eye from seeing clearly. Usually, one lens of the glasses will be frosted so that it can’t be seen through. Obviously, you have to be able to persuade your young child to keep the glasses on. One problem with this method is that the child can often look around the lens, defeating the object of preventing the eye from seeing. Very occasionally, special contact lenses can be used for the same job – to blur the vision in the good eye. Contact lenses can be difficult to use in young children, and careful hand washing when handling the lenses is essential to prevent eye infections.
Vision therapy can be used as a treatment to maintain the good work achieved by eye patching. This involves playing visually demanding games with a child to work the affected eye even harder – like eye training. Your child should do close-up activities when wearing a patch or using other amblyopia treatments. Activities such as drawing and colouring, reading and school work are detailed and work the eye well.
Note: some people wrongly think that eye patching is a treatment to correct the appearance of a squint. Eye patching and other treatments for amblyopia aim to improve vision, and do not correct the appearance of a squint.
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.
About 1 in 4 children develop a recurrence of amblyopia on stopping treatment. This risk is higher if patching is stopped abruptly, and is the reason for careful monitoring. If the problem returns. further treatment is usually needed.
It is very important to follow the advice given to you by an orthoptist or ophthalmologist about patching (or other amblyopia treatments) carefully. The most common reason for a treatment failure is because the patch has not been worn correctly for long enough. As your child grows older, the vision pathways will become fully formed, and impossible to change, so early patching is essential.
Hard work in persisting with treatment can give the long-term benefit of good vision. It can, however, be very difficult to persuade a young child or toddler to wear an eye patch. The patch itself might be annoying, and they are likely to try and remove it themselves. Effectively, you are temporarily making their sight worse whilst they are wearing the patch – you have covered their good eye which sees well and are forcing them to use the amblyopic eye. This is impossible for a young child to understand. Rewards, such as stickers or star charts, can be used to encourage them to wear their patch. It is usually easier to patch a baby’s eye, as they are less able to remove it. If it is impossible for your child to wear the patch properly, your ophthalmologist or orthoptist may suggest using drops or another method to make the amblyopic eye work.
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