EYES FRONT!

Strabismus

A squint, or strabismus, is a condition in which the eyes do not work together to look in the same direction at the same time.Whilst one eye may look straight ahead, the other eye points outwards, inwards, upwards or downwards.

HOW DO THE EYE MUSCLES WORK?

The movement of each eye is controlled by muscles that pull the eye in specific directions, outwards, inwards, upwards and downwards. Additional sets of muscles help to stabilise the eye movements, particularly for looking downwards and inwards, or for upward and outward movements.

With normal binocular vision, both eyes look and focus on the same spot. The brain combines the signals from the two eyes to form a three-dimensional image. A squint develops when the eye muscles do not move together in a balanced way, the two eyes focus on different spots, and the brain receives two different images.

In some cases of squint, the eyes will take turns to focus, resulting in an alternating squint. As both eyes are doing some focusing, the visual pathways to the brain develop from both eyes. However, in many cases of squint, one eye will remain the dominant, focusing eye. The weaker (squinting) eye is not used to focus, and the brain ignores the signals from this eye, in order to avoid double vision. This lack of use means that the non-dominant eye fails to develop the normal visual pathways in childhood and visual loss, or amblyopia, develops. The child effectively only sees with one eye, resulting in poor depth perception and an inability to see properly in three dimensions.

WHAT CAUSES STRABISMUS?

Squints are common and affect about 1 in 20 children, and usually the cause is unknown. Most squints develop in early childhood, the crucial time when the brain learns to see. Sometimes squints develop in older children, or in adults. Some squints are related to severe refractive errors, i.e. shortsightedness, farsightedness or astigmatism.

Strabismus may be associated with other disorders in both children and adults. In children, these include cerebral palsy, traumatic brain injury and certain genetic disorders. Strabismus that develops in adults can be caused by stroke, brain injury and certain neurological conditions.

A family history of strabismus may be a risk factor. Any other disease that causes vision loss may also cause strabismus.

WHAT ARE THE SYMPTOMS?

Symptoms of strabismus may be present all the time or they may occur intermittently. The most obvious sign is that the eyes appear to be pointed in different directions. There are, however, more subtle signs, including uncoordinated eye movements, tilting the head to one side, squinting with one eye, complaints by the child of double vision, and loss of depth perception. The eye that turns off-target may be the same eye at all times or the eyes may alternate.

It is quite common to notice a brief squint in children when they are tired or daydreaming. Babies occasionally cross their eyes, and this is quite normal, especially when they are tired. A squint seen in a newborn baby is likely to resolve if it is intermittent, and has usually disappeared by 4 months of age.A baby with a constant fixed squint, or with an intermittent squint that seems to be worsening from the age of 2 months, should be assessed by an eye care specialist. The earlier strabismus is detected, the better the long-term prognosis.

HOW IS A SQUINT DIAGNOSED?

It is important to diagnose a squint and amblyopia as early as possible, as it is unlikely that the baby will outgrow it. Routine checks to detect eye problems in babies and children are usually done at the newborn examination.

Various tests can be done, even for babies. An examination to find a squint can involve covering and uncovering each eye in turn, to determine which eye has the squintand how it moves.To rule out other underlying reasons for a squint, a light may be shone into the eyes to check that the pupils constrict with light and dilate when the light is removed, and an ophthalmoscope may be used to examine the retinaat the back of the eye. Very occasionally, a scan of the eye or the brain may be needed.

HOW IS A SQUINT TREATED?

In most cases the problem can be corrected if identified and treated early.

In order to improve vision, the weakened muscles in the affected eye or eyes must be put to work. Several treatments may be used alone or in combination, depending on the type, severity, and cause of strabismus. These include covering or patching the stronger eye in order to strengthen the weaker eye, glasses or contact lenses,and/or surgery to straighten and re-align the eye muscles. Occasionally, medication is injected into the eye to relax the contracted muscles and make it easier for the eyes to focus. Even if the squint is only in one eye, it may be necessary to operate on both eyes to balance themuscles.

Adults with mild strabismus that comes and goes may do well with glasses and eye muscle exercises to help keep the eyes straight. More severe forms will require surgery to straighten the eyes. If strabismus has occurred because of vision loss, the vision loss will need to be corrected before strabismus surgery can be successful.

WHAT ASSOCIATED PROBLEMS COULD THERE BE WITH A SQUINT?

A squint is the most common cause of amblyopia, commonly called lazy eye, a condition in which the weaker eye fails to develop normal visual pathways to the brain. In effect, amblyopia is a problem with development of vision in the brain rather than a problem with the eye itself.

Visual development continues until the age of 7 - 8 years, after which the visual areas of the brain are fully formed and cannot change. If amblyopia is not treated by then, the visual impairment is usually permanent.The main treatment for amblyopia is to restrict the use of the good eye by patching or covering it, forcing the affected eye to work. If this is done early enough in childhood, the vision will usually improve, often up to a normal level. Occasionally, to blur the vision in the good eye, eye drops or glasses are used instead of a patch.

The child needs to be followed up regularly to make sure that the treated eye is still being used properly and does not become amblyopic again. 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. Activities that involve a lot of detail, such as drawing and reading could also be beneficial.

In some cases, vision problems may remain even after surgery, and the child may still need to wear glasses. Some children may have reading problems at school due to compromised depth perception, or their ability to play sport may be affected.

Adults may find driving and other tasks involving depth perception difficult.

DRUGS AND YOUR EYES
WORLD RETINA WEEK: 21 to 27 September