Many practitioners approach the subject of squint (strabismus)
with great trepidation, sometimes with justification.
However, if
it is approached systematically, much of the myth and mystery
can be dispelled.
What is a squint?
The word is used in many different ways.
It is often used to
describe the narrowing of the gap between the upper and lower
eyelids (interpalpebral fissure), usually carried out by patients to
create a pinhole effect.
This reduces the consequences of any
refractive error, and improves the clarity of the image.
However,
the true definition of squint is that one of the eyes is not
directed towards the object under scrutiny.
Note that if the eyes
converge for close work, this does not indicate a squint. Why is a squint important?
A squint may show that the acuity of the eye is impaired
because of ocular disease.
The eyes are kept straight by the
drive to keep the image of the object being viewed in the
centre of the macular area, where highest definition and colour
vision is located.
The tone in the extraocular muscles is
constantly being readjusted to maintain this fixation.
If the
vision is impaired in one or both eyes this constant
readjustment cannot occur and one eye may wander.
The squint is an important sign, as the cause of impaired
vision may be eminently treatable, such as a cataract or a
refractive error.
It is especially important in a child because, unlike the vision of an adult, a child’s vision may be irreversibly
impaired if treatment is not given in time.
The visual pathways
in the brain that receive information from an abnormal eye fail
to develop normally.
The resulting depressed cortical function
leads to amblyopia, commonly called a “lazy” eye.
The child
does not usually complain that the sight of one eye is poor.
A pair of glasses to correct a refractive error may prevent a
permanently impaired acuity.
A squint may itself cause amblyopia in a child. Misalignment
of the eyes may be the primary problem, with resulting double
vision.
Young children do not normally complain of double
vision.
In a young child the vision of one eye may be suppressed
to avoid this diplopia and the visual pathways then fail to
develop properly.
This leads to amblyopia of the eye that is
otherwise organically sound.
A squint may be a sign of a life threatening condition.
Squint is a common presentation in a child with a
retinoblastoma.
The resulting squint is non-paralytic and
therefore the angle of deviation is the same, irrespective of the
direction of gaze.
The eye deviates because vision is impaired
and this may occur in any eye with visual impairment.
A squint can also be caused by a sixth nerve palsy resulting
from a tumour causing raised intracranial pressure.
In this case
the squint will be paralytic and the angle of squint will vary
depending on the direction of gaze. Patients with myasthenia
gravis may present with a squint and diplopia.
Clinical detection and assessment
Adults may complain of deviation of the eyes or of diplopia.
For children, parents usually notice either one or both eyes turning
in or out, or there may be a family history of squint.
Children
may also be referred from vision screening clinics.
History
A family history of squint is a strong risk factor in the
development of squint, and if there is any doubt the child
should be referred.
Children with disorders of the central
nervous system such as cerebral palsy have a higher incidence
of squint.
Squint is more common in preterm infants.
Problems
during birth and retarded development also increase the
likelihood of a squint.
The parents’ visual problems should be
ascertained, particularly large refractive errors.
The earlier the age of onset, the more likely it is that an
operation will be needed.
A constant squint has a worse visual
prognosis than one that is intermittent.
Examination
Check the visual acuity
If the visual acuity does not correct with glasses or a pinhole,
ocular disease or amblyopia must be suspected.
This is
particularly important in children, as the amblyopia or ocular
problems must be treated immediately if sight is to be preserved.
Visual acuity in infants is difficult to assess.
A history from the
parents is useful to find out whether the baby looks at them and
at objects.
However, if only one eye is affected the visual problem
may not be apparent.
If the sight is poor in only one eye, covering
the good eye may make the child try to push the cover away.
Look at the position of the patient’s eyes
Large squints will be obvious. Wide epicanthic folds may give
the impression of a squint (pseudosquint), but children with
wide epicanthic folds may still have true squints.
Look at the corneal reflections of a bright light held in front of the eyes
Note the position of the reflections; they should be
symmetrical.
This test gives a rough estimate of the angle of any
deviation.
Cover test
Two types of cover test help to reveal a squint, especially if it is
small and the examiner is unsure about the position of the
corneal reflections.
● In the cover and uncover test, one eye is covered and the
other eye is observed.
If the uncovered eye moves to fix on
the object there is a squint that is present all the time a manifest squint.
The test should then be carried out on the
other eye.
A problem arises when the vision in the squinting
eye is reduced, and the eye may not be able to take up
fixation.
This emphasises the need to test the vision of any
patient with squint.
If the cover and uncover test is normal
(indicating no manifest squint) the alternate cover test
should be done.
● In the alternate cover test, the occluder is moved to and fro
between the eyes.
If the eye that has been uncovered moves,
then there is a latent squint.
Test eye movements in all directions of gaze
If there is a paralytic squint, the degree of deviation will vary
with the direction of gaze. An adult will often say that the
separation of the images varies and that it increases in the
direction of action of the weakened muscles.
Examination of the eye with a pupil dilating agent (mydriatic) and a
ciliary muscle relaxing agent (cycloplegic)
Any overt abnormalities of the eye should be noted.
Dilating
the pupil allows you to check for retinal disease, such as a
retinoblastoma, and the cycloplegic allows a check for any
refractive error.
Adequate examination of the peripheral
fundus and refraction require dilation of the pupil and special
equipment.
Cataracts and other opacities in the media, and the
white reflex suggestive of retinoblastoma, may be checked
without dilating the pupil, by observing the red reflex.
Management
Paralytic squints
Paralytic squints usually occur in adults.
Underlying conditions
such as raised intracranial pressure; compressive lesions; and
diseases such as diabetes, hypertension, myasthenia gravis, and
dysthyroid eye disease should be excluded.
If diplopia is a problem, one eye may need to be occluded
temporarily, for example, by a patch stuck to the patient’s glasses.
Alternatively, temporary prisms may be stuck on to the glasses to
eliminate the diplopia. An operation on the ocular muscles may
be indicated if the squint stabilises.
If an operation on the
muscles is either inappropriate or proves inadequate, permanent
prisms may be incorporated into the glasses’ prescription.
Botulinum toxin is a recent addition to the diagnostic and
therapeutic options in squint management.
When injected into
an extraocular muscle (under electromyographic control), the
toxin produces a temporary reversible paralysis of the muscle.
This technique can be used to alter extraocular muscle balance
and correct squint, and it can be used to help predict the
outcome of extraocular muscle squint surgery.
Non-paralytic squints
Non-paralytic squints usually occur in children.
If the squint is
caused by disease in the eye that is causing reduced vision and
subsequent deviation of the eye (for example, cataract) this
needs to be treated.
Treatments for non-paralytic squints are
described below.
Spectacles
There are two main indications for prescribing glasses for
children.
● A child who is hypermetropic (longsighted) and has a
convergent squint.
Normally when the ciliary muscle
contracts the lens becomes more globular to allow the eye to
focus on close objects (accommodation).
This is linked to
convergence so that both eyes can fix on the close object.
If the child is hypermetropic the ciliary muscle has to contract
strongly for the child to be able to focus on a near object.
This excessive accommodation may cause overconvergence so
that a squint occurs.
This type of squint is called an
accommodative convergent squint.
The use of hypermetropic
glasses in this case relaxes the ciliary muscles and removes
the drive to overconverge.
● A child who has a refractive error, particularly if this is
unilateral.
Because of the refractive error the image on the
retina will be indistinct.
The visual pathways will then not
develop properly (resulting in amblyopia).
Children with a
refractive error may not develop a squint until the vision is
poor in one eye, which emphasises the need to check the
visual acuity.
Glasses may prevent a child from developing
severe visual loss in an otherwise “normal” eye hence the
need to refract every child with a squint or impaired vision.
Occlusion
This is the well known patching of one eye to encourage the
development of the visual pathway of the “bad” eye.
If the
development of one pathway has been retarded by a squint or
refractive error this pathway can be stimulated if the “good” eye is patched.
However, this can only be done for a limited period,
and there is a danger of the good eye itself becoming
amblyopic.
Most clinicians feel that after the age of about
seven, occlusion therapy is unlikely to be helpful.
In the
meantime, the underlying problem must, of course, be
corrected.
The vision of the good eye may also be “blurred” with drops
such as atropine.
Although there is much debate about the
value of occlusion therapy, this therapy is useful for many
children with specific types of amblyopia.
Orthoptic treatment
A series of visual exercises may encourage the simultaneous
use of both eyes.
Surgery
The ocular muscles can be repositioned to straighten the eyes.
Glasses are prescribed and occlusion performed before surgery,
because an eye is more likely to stay straight if the vision is
good.
In adults “adjustable” surgery can be carried out.
The
muscle position is adjusted by altering the tension on the
sutures postoperatively.
Botulinum toxin
Very small amounts of botulinum toxin can be injected into
overacting muscles to paralyse them for a few months.
The
treatment can then be repeated.
It can also permit the
assessment of the effect of prospective surgery before
permanent surgery is carried out.
In the older child
The effectiveness of treatment in reversing amblyopia decreases
as the child gets older.
Once the child is about 8 or 9 years old
the visual system is no longer flexible and amblyopia cannot be
reversed. However, the child may still need glasses to correct
any refractive error, and an operation may be required if the
squint poses a cosmetic problem.
