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Squint

What is it?

A squint is where the eyes are out of alignment with each other. The most common squints are convergent (cross eyed) or divergent. One or both eyes can be involved. Mostly the misalignment is in the horizontal plane (side to side). Sometimes the eyes are also vertically out of alignment (one eye looking above the other). Vertical squints can occur on their own, but this is much rarer.

If the same eye is always out of alignment it often becomes “lazy”, that is it does not see as well as the other eye. The term lazy does not refer to the eye movement, only to the fact that one eye does not see as well as the other eye (see lazy eye). Convergent squints are more likely to be associated with a lazy eye.

There are other types of squint which are quite rare. The most common are discussed here.

What causes it?

Nobody really knows. Its thought to be a problem with the eye movement control system. There is probably a hereditary factor as squints tend to run in families.

Convergent and divergent squints are different conditions and have different treatments. Because of this they will be explained separately.

Convergent squint

Most children with a convergent squint (cross eyed) are also abnormally longsighted (see refractive error). That means they have to work harder to see things close up. Adults over a certain age cannot do this and longsighted adults have blurred vision when trying to look close up. Longsighted children however can do this, but they have to work their internal eye muscles (to change the shape of the eye lens) hard to do so. When anyone uses their eyes to focus close up a very complex reflex system is initiated. This essentially involves both eyes rolling inwards so that they both point at the near object as well as the internal muscles of the eye increasing its focus. The rolling in is brought about by one of the external “in pulling” muscles of each eye contracting and rolling the eye inwards. The change in focus is called accommodation and the rolling in of the eyes is called convergence. It’s very complex and requires a fine degree of control by the eye movement parts of the brain.

In a child with a convergent squint this process somehow malfunctions. When focusing on a close object the rolling in mechanism over works and the eyes become crossed. Usually this occurs intermittently but after a while one or both eyes become permanently crossed, especially when looking close up. Eventually the child will be cross eyed looking at both near and distant objects. Almost always the squint is worse when looking close up and when the child is tired or ill. 

Initially both eyes do the squinting but eventually it settles into just one eye. If one eye always does the squinting it is likely that there will be changes occurring in the visual brain whereby the input from that eye is ignored. If this continues the eye does not see as well and becomes “lazy”, (see lazy eye). A lazy eye means reduced vision, it is nothing to do with the movement of the eye. If the squinting eye is even more longsighted than the other eye, this will also cause changes in the visual brain making the laziness worse.

How is it treated?

Any child suspected of having a squint needs to be assessed for several things. They need to have the presence of a squint confirmed. Their vision has to be checked to see if they have a lazy eye and they need a glasses test. Very rarely there are other serious conditions that can cause a squint so all squinting children are also seen by an eye surgeon (ophthalmologist). A paediatric eye surgeon can do an initial assessment for all of these.

Once serious disease has been ruled out, the most important thing is to give glasses if required (they usually are) and treat the lazy eye if there is one. Most children with a squint never need surgery. For those that do, it’s done at a later stage when the lazy eye has been treated.

Glasses
Glasses are pivotal in the management of convergent squint. Most of these children are longsighted, correcting this with glasses means they don’t have to work hard to look close up. As a result the excessive rolling in of the eye is much reduced and the child is seen to squint much less. In many children, once they are wearing their glasses, the squint is hardly noticeable. Once they remove their glasses however, the squint returns (often looking larger at first which can be alarming!). Squinting children need to wear their glasses all the time.

If the child is even more longsighted in the squinting eye and the eye is lazy, wearing the glasses also helps treat the lazy eye. In these cases the glasses are doing two jobs, controlling the squint and helping treat the lazy eye. Most children whose glasses help control all or part of a noticeable squint, will always need some form of correction for their long sight. If they don’t want to wear glasses when they are older, then contact lenses will do the same job.

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Patching
If the squinting eye is lazy, this needs to be treated (see lazy eye) with a combination of glasses and patching of the good eye (or alternatives to patching). Its important to treat the lazy eye early because the older the child is the more difficult and prolonged the treatment becomes. After a

 

certain age patching does not work. As the vision improves in the lazy eye the squint can start occurring in both eyes again. This can be worrying to parents but should be regarded as a good sign.

Even with exemplary treatment of the lazy eye from an early stage, the vision does not always become normal. The aim is to get the vision as good as possible and keep it there. Most people have good vision in both eyes and the eyes work together to give 3D or stereo vision. Most children with a convergent squint have little if any 3D vision. It’s only rarely that treatment of the lazy eye gives the child normal 3D vision. That said in our society this is not a major handicap and they can still function normally, driving is not a problem. A few jobs are not open to them such as the armed forces or the fire brigade.

Surgery
Once a lazy eye has been either ruled out or treated, the question of surgery arises. If a child has a squint that is still obvious whilst wearing their glasses, surgery should be considered. What the squint looks like without the glasses is not important. The aim of squint surgery is to make the eyes appear straight whilst wearing the glasses.  Getting rid of the glasses is not the aim.

Surgery cannot correct the actual cause of the squint. All it does is alter the way the normal external eye muscles respond to the abnormal instructions they receive. In most cases this involves weakening the in-pulling muscle on each eye. That is, both eyes are operated on even if one eye is the predominantly squinting eye.

The operation involves finding the muscle to be weakened, taking it off its normal attachment to the eye and moving it further back. This effectively lengthens the muscle and weakens it. Doing this to both in-pulling muscles reduces the rolling in force, thus straightening the eyes. The eyes are not taken out to do this. A general anaesthetic is required and in Derby these operations are performed as day cases. The requirements of surgery can vary and in these cases the surgeon will discuss this with the parents

The main risks of squint surgery are under correction (the child is still cross eyed) and over correction (the eyes diverge). These occur in about 20% of all cases, as a result you can expect about one in five children having surgery will need a second operation. The surgery is surprisingly pain free and the child is usually only off school for two or three days.

Divergent Squint

This is where the eyes are “outgoing”. In most cases these are not present all the time but intermittent. They are more noticeable when the child is tired or daydreaming. One puzzling association is the tendency of these children to close one eye when they go out into bright sunlight. Many children can bring the squint back under control if they are asked. They tend to be more obvious when the child is looking into the distance though some can be worse when looking close up.

Because most of the time the eyes are straight, lazy eye (reduced vision) is rare. For the same reason the two eyes are mainly working together and as a result the child has normal 3D or stereo vision.

On the whole these children do not have a refractive error (glasses requirement), of those that do the majority are short sighted.

 How is it treated?

Like all children with any kind of squint, they will have their squint and vision assessed. They will also have a glasses test (refraction) and be examined by an eye surgeon to rule out other very rare diseases that can also cause squints.

The majority of children with these squints do not require treatment. Unfortunately, it is not possible to predict those who will, so initially all these children are monitored. Because lazy eye is rare in intermittent divergent squints, there is no need to rush into treatment. On the whole surgery, if required, is better done at a later age compared to those with a convergent squint.

If the squint is occurring relatively few times and is not felt to be getting worse, no treatment is required.

Treatment should be considered for those children whose squint is, or starts to become, frequent and obvious. This can be a difficult decision with some parents very much noticing relatively small infrequent squints, whilst others are happy to accept larger and more frequent ones.

There are some factors that would predispose to treatment being given. If the squint is so frequent that it affects the child’s ability to see 3D (stereo) vision, then treatment should be considered. Occasionally, only one eye starts to diverge instead of both and as a result this eye may become lazy. This is rare but if it occurs patching would be required (see lazy eye). The vast majority however have treatment for cosmetic reasons.


 

 

 

 

Surgery usually involves weakening the external eye muscle that rotates the eye outwards on each eye. That is both eyes are operated on in most cases.
 Rarer types of these squints do better if only one eye is operated on, here the out-pulling muscle is weakened and the in-pulling muscle tightened.

Weakening an external eye muscle involves taking it off its normal attachment to the eye and moving it further back. This effectively lengthens the muscle and weakens it. Doing this to both out-pulling muscles reduces the rolling out force, thus straightening the eyes.

Tightening the muscle is similar except the muscle is shortened by removing a piece of it and stitching it back together.

The eyes are not taken out to do any of this. A general anaesthetic is required and in Derby these operations are performed as day cases.

The main risks of squint surgery are under correction (the child’s eyes still diverge) and over correction (the eyes become crossed). It’s not unusual for the eyes to be a little crossed in the first few weeks after divergent squint surgery. The child may notice some double vision. In most cases this settles down.

Overall, significant over or under-corrections  occur in about 20% of all cases, as a result you can expect about one in five children having surgery will need a second operation.

The surgery is surprisingly pain free and the child is usually only off school for two or three days. An annoying feature of intermittent divergent squints is that they can recur even after successful surgery.

In recent years an alternative to surgery in the form of glasses has been shown to be effective.

If an older adult with normal vision is given short sight glasses, the focusing power of their eye is reduced and their vision blurred. A child has a very strong internal focusing mechanism that wears out in adulthood. If given the same glasses, a normal (none short-sighted) child can increase the focusing power of their eye to overcome the weakening effect of the glasses. Whenever a child focuses however, a complex reflex is also invoked that causes the in-pulling external eye muscle to contract and roll the eyes inwards on both sides. Therefore whilst overcoming the focus weakening effect of the glasses, the eyes will rotate inwards. Whilst this would be pointless in normal children, in those with an intermittent divergent squint this can help control the tendency for the eyes to diverge.

This is why short sighted children with this type of squint benefit from wearing their glasses, not only does it improve their distance vision but helps to control their squint as well (see above).

It also seems to work in a proportion of children with a divergent squint who are not actually short sighted. This is called minus lens therapy. Ideally glasses will help control the squint and ultimately the visual system somehow compensates for the squint enabling the glasses to be discarded. This may take a number of years. Some children remain reliant on the glasses and eventually require surgery. Experts remain unsure as to which is the best treatment. In reality both have advantages and disadvantages and parents will need to discuss the options with their eye care team.

Pseudo squint.

Very often young children can appear to have a squint but in fact be normal. The most common situation is where the child appears to be cross-eyed but actually is not. This is because very young children and babies have a broad bridge to their nose. This covers the inside white of the eye and makes the child look cross-eyed. As the face grows the bridge of the nose thins out and this appearance disappears. However if a squint is suspected, it is always worth having the child examined.

Useful link for squint and lazy eye:   www.lazyeyesite.org

 

Childrens eye problems

Blepharitis
/
Chalazion
/
Conjunctivitis
/
Lazy Eye
/
Refractive Errors
/
Squint
/
Sticky Eye

 

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