A squint develops when the eye muscles do not work in a balanced way and the eyes do not move together correctly.
Are there different types of squint?
A squint may be classified and described in different ways. For example:
- By the direction of the squinting (turning) eye:
- An eye that turns inwards is called an esotropia.
- An eye that turns outwards is called an exotropia.
- An eye that turns upwards is called a hypertropia.
- An eye that turns downwards is called a hypotropia.
- Whether the squint is present all the time (constant), or comes and goes (intermittent).
- Whether the affected eye turns when the eyes are open and being
used (manifest squint) or whether the eye turns only when it is covered
or shut (latent squint) but looks fine when the eyes are open.
- Whether the severity (angle) of the squint is the same in all directions or not:
-
A concomitant squint means that the angle (degree) of the squint is
always the same in every direction that you look. That is, the two eyes
move well, all the muscles are working, but the two eyes are always out
of alignment by the same amount no matter which way you look.
- An incomitant squint means that the angle of squint can vary. For
example, when you look to the left, there may be no squint and the eyes
are aligned. But when you look to the right, one eye may not move as
far and the eyes ar then not aligned.
- By age of onset. Most squints develop sometime in the
first three years of life. Some develop in older children and adults.
Squints that develop in children usually have different causes to
squints that develop in adults.
- By the cause.
- In many cases of childhood squint the reason why a squint develops is not known.
- In some cases of childhood squint (and most cases of adult squint)
the squint occurs because of a disorder of the eye, the eye muscles,
the brain or the nerves.
The rest of this leaflet only deals with squint in children.
What are the types and causes of squint in children?
About 1 in 20 children over the age of three months have a squint (or have had one treated). Causes include the following:
Congenital squints of unknown cause
Congenital
squint means that the child is born with a squint, or it develops
within six months of age. In most cases, the cause is not known. (The
eye muscles are not balanced but the reason for this is not known.)
In
most cases one eye turns inward. This is called congenital esotropia
(sometimes called infantile esotropia). This common type of squint
tends to run in some families so there is some genetic component to
this type of squint. However, many children with congenital esotropia
have no other family members affected. In some cases the eye turns
outward (congenital exotropia). Less commonly, a squint of unknown
cause may result in an upward or downward turn of the eye.
Squint related to refractive errors
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. When the child with a refractive
error tries to focus to see clearly, an eye may turn. This type of
squint tends to develop in children who are two years or older, in
particular in children with long sight. The squint is most commonly
inward looking (an esotropia).
Other causes
Most children with a squint
have one of the above types of squint and are otherwise healthy. In
some cases, a squint is one feature of a more generalised genetic or
brain condition. For example, a squint may develop in some children
with cerebral palsy, Noonan syndrome, hydrocephalus, brain injury, and
various other conditions.
What problems can be caused by a squint in a child?
Amblyopia
Amblyopia is sometimes called a
'lazy eye'. It 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. The visual loss
of amblyopia cannot be corrected by wearing glasses. However, it is
usually treatable (see below). But note: if it is not treated before
the age of about seven years, the visual impairment usually remains
permanent.
To understand how amblyopia occurs, it is helpful to
understand how vision develops. 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.
A squint is the
common cause of amblyopia (but there are other causes). 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. The turned eye then fails to develop the normal visual pathways in
childhood and amblyopia develops in this eye.
How the squint looks
A squint can be a
cosmetic problem. Many older children and adults who did not have their
squint treated as a child have a reduced self-esteem because of the way
their squint looks.
Impaired binocular vision
With normal eyes,
both eyes look and focus on the same spot (binocular vision). The brain
combines the signals from the two eyes to form a three dimensional
image. If you have a squint, the two eyes focus on different spots. In
children with squint this does not usually cause 'double vision'. As
described above in the amblyopia section, in children the brain quickly
learns to ignore the signals and images coming from the turned eye. The
child then only sees with one eye. This means the child does not have a
good sense of 'depth' when looking at objects. That is, he or she
cannot see properly in three dimensions.
(Adults who develop a squint often have double vision as their developed brain cannot ignore the images from one eye.)
How is a squint diagnosed and assessed?
It is important to diagnose a squint (and amblyopia) as early as
possible. Routine checks to detect eye problems in babies and children
are usually done at the new-born examination and at the 6-8 week
review. There is also a routine pre-school or school-entry vision check.
Some
newborn babies have a mild squint that soon goes. However, any squint
that is present after the age of three months is usually permanent
unless treated. So, even if your child has had routine eye checks, tell
your doctor if you suspect that a squint has developed.
A baby
or child with a suspected squint 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.
What are the treatments for squint?
Treatment typically involves the following:
- Treating amblyopia (visual loss) if this is present.
- Wearing glasses to correct any refractive error, if this is present.
- Surgery is often needed to correct the appearance of the squint itself, and may help to restore binocular vision in some cases.
Treating amblyopia
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. 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.
Note: some people think that eye-patching is a
treatment to correct the appearance of a squint. It is not. Eye
patching and other treatments for amblyopia aim to improve vision and
do not correct the appearance of a squint.
Correcting refractive errors
In a child has
a refractive error (long sight, etc) then glasses will be prescribed.
This improves vision in the eye and may also straighten the eye if the
refractive error was the cause of the squint.
Surgery
In many cases an operation is
advised to make the eyes as straight as possible. The main aim of
surgery is to improve the appearance of the eyes. In some cases,
surgery may also improve or restore binocular vision. The exact
operation depends on the type and severity of the squint. For example,
it may involve moving back slightly the place where a muscle attaches
to the eyeball. Or, one of the muscles that moves an eye may be
shortened. Or, a combination of these techniques done to different
muscles.
Botulinum toxin
Botulinum toxin stops muscle
cells from working. It is used for a variety of conditions where it is
helpful to weaken one or more muscles. In recent years, injections of
botulinum toxin directly into eye muscles has been used as a treatment
for certain types of squint. In particular, for squints that turn
inward. This treatment may become more popular as an alternative to
surgery - but only for certain types of squint.
What is the outlook (prognosis)?
For amblyopia (if it develops)
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.
For improving the appearance of the eyes
Squint
surgery usually greatly improves the straightness of the eyes. However,
sometimes after an operation the eyes are not perfectly straight. In
some cases, two or more operations are needed to get the eyes straight.
Also, several years after successful surgery, in some cases the squint
may gradually return again. If this occurs, another operation is an
option to re-straighten the eyes.
References
© EMIS and PiP 2008 Updated: 19 Feb 2008
