Strabismus: squint

Strabismus or squint is a vision condition in which a person cannot align both eyes simultaneously. The condition mainly manifests itself during childhood but can also affect adults.

The various forms of strabismus

There are various forms of strabismus:

  • In the most common form, convergent strabismus, one (sometimes both) eye turns in towards the nose.
  • In divergent strabismus the eye turns out
  • But the eye may also turn up (sursumvergens) or down (deosumvergens).

Some 2-5 % of children develop a squint, usually after the first year of life. The unusual eye position is considered ugly with the result that cross-eyed children are often bullied. Aside from the aesthetic aspect, the main reason to diagnose and address squints at an early age is to prevent that the eye becomes lazy (amblyopia).

Lazy eye

Children who develop a squint in early childhood will rarely suffer from double vision because the brain is able to suppress this double vision. If the image captured by that same eye is suppressed for a long time, the eye will not develop properly, vision deteriorates (= lazy eye) and details become less and less visible. When both eyes squint in turn, the chances of developing a lazy eye are small. However, a lazy eye does not heal without treatment and can only be successfully treated in children up to the age of about 7.

If the squint develops during the eighth year of life or in adulthood, the brain’s ability to suppress the aberrant image is lost. In that case, the squint will not result in a lazy eye because the brain functions are fully developed by then. In that case, the patient will suffer from double vision because the signals from both eyes are converted into separate images.

Strabismus: symptoms

A pronounced squint is clearly noticeable. But there are also minor eye misalignments that are hardly noticeable (micro-strabismus) and may seem less serious. However, the consequences are the same and even with micro-strabismus, the patient may end up with a lazy eye and poor visual acuity.

When the squint develops later on in life (after the age of 8), the chances of developing a lazy eye are small but the patient may suffer from double vision. The patient may squeeze one eye shut, put one hand in front of the eye or complain of double vision. Furthermore, people with a squint can present with motor problems such as stumbling, reaching for, but missing the object they are trying to pick up, difficulties estimating distances.

What are the causes of strabismus?

People see with both eyes. The brain combines the images seen by both eyes into one single image. The ability to see with both eyes develops in the first 6-7 years of a child’s life. If that development is disrupted, a squint may develop. The causes why a child develops a squint are not entirely clear but hereditary and environmental factors are believed to play a role:

  • Hereditary predisposition
  • Ophthalmic abnormalities
  • Refractive disorders (e.g. high hypermetropia), unequal eye strength
  • Medical problems around the time of birth
  • Cranial nerve disorders, the most common cause of the onset of strabismus at a later age. A malfunctioning cranial nerve might be the result of another pathology such as heart attack, diabetes, etcetera
  • Premature birth, smoking or drug use during pregnancy

How is strabismus or squinting treated?

The treatment of strabismus can be lengthy and consist of:

  • Glasses, with or without prisms
  • Exercises
  • Lazy eye treatment
  • Strabismus surgery

If strabismus is accompanied by a lazy eye, the lazy eye must be treated before any eye muscle correction is carried out. This treatment includes the following methods: covering the good eye, prescription glasses and, where indicated, eye drops.

Strabismus surgery will only be suggested at a later stage and if deemed necessary.

Strabismus surgery

In some children with a squint, the eye position will have to be corrected sooner or later. In that case, strabismus surgery or eye muscle surgery will be performed during which the eye muscles attached to the outside of the eyeball are either weakened or strengthened by moving or shortening them. This can be done to one or both eyes.

In about 80 % of cases a single surgical procedure will suffice, though sometimes a second surgical procedure will be required, e.g. in the case of a clear under or over correction. Before carrying out eye muscle surgery in older children and adults it is very important to fully investigate the risk of post-operative double vision first. Sometimes the brain is so well adapted to an existing abnormal eye position that it becomes impossible to correct a squint for cosmetic reasons without causing double vision. In such cases, surgery is not indicated.

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