A squint, or strabismus, is a condition in which the eyes do not align properly.

Strabismus can be constant or intermittent.

This usually occurs because the muscles that control the movement of the eye and the eyelid, the extraocular muscles, are not working together.

As a result, both eyes are unable to look at the same spot at the same time.

It can also happen from an injury or disorder to the brain means which impacts the eye’s ability to work together.

Constant strabismus makes binocular vision impossible, which causes a person to lose depth perception.

It is estimated to affect around 2-5% of the general population.

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There are several different types of strabismus. Image credit: teap/iStock

There are different types of strabismus. They can be described by the cause or by the way the eye turns.

The following terms describe strabismus by the positions of the eye:

  • Hypertropia: when the eye turns upwards
  • Hypotropia: when the eye turns downwards
  • Esotropia: when the eye turns inwards
  • Exotropia: when the eye turns outwards

An early diagnosis of strabismus will enable more effective treatment. In the past, it was thought that after a “critical period,” strabismus could not be treated.

While treatment up to the age of 6 years is believed to be the most effective, strabismus surgery can be performed at any time.

The sign of a squint is fairly obvious from an early age. One of the eyes does not look straight ahead. A minor squint may be less noticeable.

Infants and newborns may go cross-eyed, especially if they are tired. This does not mean that they have a squint. Parents can check with a doctor.

If a child has one eye closed, or tilts their head when looking at things, this could be a sign of double vision, and a possible squint. It is a good idea to see a doctor.

Strabismus is normally either present at birth or develops after 3 months of age.

Lazy eye

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Strabismus can lead to double vision if it returns in adulthood. Image credit: simplytheyu/iStock

Untreated, it can lead to amblyopia, or “lazy eye,” in which the brain starts ignoring input from one of the eyes.

The brain ignores one of the eyes to avoid double vision.

If there is poor vision in the affected eye, a child may benefit from wearing a patch over the eye to encourage the vision to develop.

Sometimes a squint that was treated successfully in childhood returns later in adulthood.

This may lead to double vision in the adult because, by that time, the brain has been trained to gather data from both eyes, so it cannot ignore one of them.

Strabismus can be:

  • congenital, meaning a person is born with it
  • hereditary, or running in families, suggesting a genetic link
  • the result of an injury, illness, or long-sightedness
  • due to a lesion on a cranial nerve

High amounts of hyperopic refractive error can make the affected eye turn inward, in an attempt to better focus.

Strabismus that results from refractive errors tends to emerge later on, usually around the age of 2 years or older.

Hydrocephalus can also lead to strabismus. Hydrocephalus is a condition in which too much cerebrospinal fluid builds up in and around the brain.

Some viral infections, such as measles, can cause strabismus. Other conditions that can cause it include Noonan syndrome and some other genetic conditions.

Children and babies should have routine eye checks as they develop. The American Optometric Association recommends starting eye tests at 6 months of age, or earlier if the child has a constant eye turn.

If there are signs of strabismus, the physician or optician will refer the child to an optometrist or ophthalmologist.

The eye doctor will probably use eye drops that dilate the pupils before testing is done.

The Hirschberg test, or Hirschberg corneal reflex test, is one of many tests used to assess whether the patient has strabismus.

The eye doctor shines a light in the eye and observes where the light reflects from the corneas.

If the eyes are well-aligned, the light will go to the center of both corneas. If it does not, the test can show whether the patient has exotropia, hypertropia, esotropia, or hypotropia.

Some people may have more than one tropia at the same time.

Treatment options

Prompt treatment reduces the risk of complications, such as amblyopia, or lazy eye. The younger the patient is, the more effective treatment is likely to be.

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Eye drops are one solution for certain types of squint. Image credit: kdshutterman/iStock

Treatment options include:

  • Glasses: If hypermetropia, or long-sightedness, is causing the squint, glasses can usually correct it.
  • Botulinum toxin injection, or botox: This is injected into a muscle on the surface of the eye. The doctor may recommend this treatment if no underlying cause can be identified, and if signs and symptoms appear suddenly. The botox temporarily weakens the injected muscle, and this can help the eyes to align properly.
  • Vision therapy: These may help enhance brain-eye coordination and increase muscle control to improve focus.

Surgery is only used if other treatments are not effective. It can realign the eyes and restore binocular vision. The surgeon moves the muscle that connects to the eye to a new position. Sometimes both eyes need to be operated on to get the right balance.

While surgery can sometimes straighten the eyes, a program of vision therapy may be needed to completely restore one’s vision.

Vision therapy

Though eye exercises, or vision therapy, is not a treatment for complete strabismus, they may be beneficial for those who experience strabismus intermittently. An optometrist specializing in vision therapy can recommend the most effective approach depending on the type of strabismus.

A standard type of exercise for strabismus is home-based pencil pushups (HBPP).

To do HBPP, follow these steps:

  1. Hold a pencil at arm’s length, around midway between the eyes.
  2. Look at the pencil while moving it toward the nose, and try to maintain a single image of it.
  3. Keep moving the pencil toward the nose until it no longer appears as a single image.
  4. Hold the pencil at the closest point where a single image is possible.
  5. If unable to regain a single image, start again.

A study in 176 people who performed this exercise at home for 15 minutes each day for 6 weeks found that the exercise was as effective as office-based orthoptic therapy.

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