Strabismus (Squint) – a guide for parents
Strabismus, commonly called squint, is an
important condition in childhood. Unrecognised and untreated
it may result in permanently reduced vision in the affected eye
(lazy eye or amblyopia). Rarely squint can become manifest
due to an inherited defect or disease process within the eye.
A squinting eye can also develop as a result of a neurological
defect.
True misalignment of the eyes is uncommon in
childhood affecting less than 4% children. However squint is
significantly more common in children with cerebral palsy and also
in children with retinopathy and prematurity. Strabismus is
more common within families, particularly when it is related to
long sightedness.
Squint in children under one year of age is
even less common, affecting only 1% of infants. However we see many
referrals with a diagnosis of possible squint in this age group and
these children are generally found to have the condition of ‘pseudo
squint’.
Surgery squint is certainly not necessary in
the majority of the children we see for squint problems. Children,
for example, with divergent squint or those in whom squint is
related to long-sightedness can be managed without surgery.
Only about 6% of children referred with a possible diagnosis of
squint would eventually require surgery.
The most important part of management of
squint is in fact occlusion therapy (eye patching) and vision can
still be maintained in the squinting eye during the period of
occlusion therapy. However in certain types of squint there
is a loss of pre-existing binocular function and in these cases
surgery is an important part of the management.
Pseudo Squint
The commonest referral we see is for the
condition of ‘pseudo suqint’ which may be noticed in infants up to
the age of 9 months. The eye appears misaligned as a
convergent squint caused by a broad bridge of the nose
(epicanthus). This is the type of squint that infants are
said to ‘grow out of’, for as the child grows the relative effect
of the broad bridge (epicanthic folds) is lost. It is not a
true squint and therefore no treatment is required. However
it is often necessary for this diagnosis to be confirmed by an
Ophthalmologist or Orthoptist.
Convergent Squint / Esotropia
Infants generally fix and focus clearly by six
weeks of age and the eyes are well aligned. Infants can be born
with a convergent squint (infantile esotropia) which is commonly
seen as an obvious large angle squint turning into the inner
corner. It is an uncommon condition. Usually the infant
will ‘swap’ vision from one eye to the other (cross fixate).
Because of this alternation the infant generally develops good
vision in each eye. Surgery is the treatment for this
condition, generally at between 9 to 18 months of age. It is very
important that the child continues to be monitored by an Orthoptist
following surgery.
Accommodative Esotropia
Some children develop a convergence squint
because they are significantly long sighted (hypermetropic). This
commonly occurs around the age of two years and can develop quite
suddenly. Due to the effort of having to focus hard, (to
overcome the long sightedness), the eyes tend to cross inwards when
looking at objects, particularly when looking at objects ‘close
up’. The squint can be straightened by wearing spectacles to
correct the long sightedness. When they remove the spectacles the
squint immediately recurs. However as the child grows older
the long-sightedness generally lessens and they may become less
dependant on the spectacle correction. In some children the
squint is only partially corrected by the glasses, and in these
children corrective surgery is also necessary.
Divergent Squint / Exotropia
Divergent squint is very uncommon in babies
and young infants and it is very important for these infants to be
assessed early, as the squint may be a sign of some other more
significant defect.
Divergent squint often develops at about the
age of 4-5. The squint is commonly only intermittent
initially. For much of the time the eyes are straight and
tend to drift out when the child is tired or ill. Commonly
children with this type of squint tend to ‘squint’ and tend to
screw up their eyes in bright sunlight. In some children the
squint becomes less intermittent and more constant requiring
corrective surgery. In a few children the squint may be
controlled by correcting a refractive error of myopia (a short
sightedness).
Vertical Squints
These are less common than horizontal squint
and are generally the result of a congenital nerve weakness.
The child may adopt an abnormal head position (head tilt) to
control the vertical muscle imbalance.
Squint Due To Eye Defect
Rarely the eyes may become misaligned, (either
convergent or divergent), due to a defect within the eye
itself. For example a unilateral cataract, congenital
cataract or retinal scar will result in the eye drifting out, as it
does not fix properly. Very rarely a tumour within the eye
may manifest as a squint.
Lazy Eye (Amblyopia)
Lazy eye develops when the normal high quality
connection between the eye and the visual cortex in the brain is
disrupted. When an infant develops a squint, rather than
experience double vision the image from the misaligned eye can be
switched off in the visual cortex. If not corrected this
eventually results in permanently reduced vision. A similar
event occurs if the focussing of each eye is markedly different
(anisometropia). This is particularly common if one eye is
particularly long sighted or astigmatic.
Occlusion Therapy (Patching)
Patching of the good eye is the treatment
advised for amblyopia, (lazy eye), or to prevent amblyopia
developing in a squinting eye. Essentially it is a technique
of ‘forcing the eye to see’ without the rivalry of the good
eye. The visual system is considered to be in a state of
‘plasticity’ up to the age of about 8 years. After this age
occlusion therapy is generally not very effective. The earlier the
treatment given, the better the visual outcome. Patching
treatment is guided and supervised by an Orthoptist. An Orthoptist
is an expert in strabismus working alongside the
Ophthalmologist.
Surgery For Squint
Surgery is generally undertaken as a day case
procedure with full anaesthetic. It is usually not necessary
to cover over the operated eye or eyes after the surgery.
Corrective surgery may be necessary to both eyes, even if the
squint is apparent only in one eye. Children tolerate the
surgery well and the eyes are sore for only a few days after the
operation. Following surgery it is very important that the
child continues to be monitored by an Orthoptist, certainly up to
the age of 8 years old.
Contributed by Mr P Kinnear, Consultant
Ophthalmologist.