A squint, or strabismus, is a condition in which the eyes do not align properly. One eye turns inwards, upwards, downwards, or outwards, while the other one focuses at one spot.
It can happen all the time or intermittently.
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 because a disorder in the brain means that the eyes cannot correctly coordinate.
Strabismus also makes binocular vision impossible, so it is harder for the person to appreciate depth perception.
It is estimated to affect around 4 percent of the population in the United States.
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 is when the eye turns upwards
- Hypotropia is when the eye turns downwards
- Esotropia is when the eye turns inwards
- Exotropia is 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 most effective, strabismus can be treated 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 their doctor.
If a child has one eye closed, or turns their head when looking at you, 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 it develops in the first 6 months after birth.
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 other 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 illness or long-sightedness
- due to a lesion on a cranial nerve
If the eye cannot focus the light as it comes in through the lens, this is known as a refractive error.
Other problems that can lead to strabismus include:
- myopia, or short-sightedness
- hypermetropia, or long-sightedness
- astigmatism, where the cornea is not curved properly
A refractive error tends to make the affected eye turn inward, in an attempt to get 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.
Children and babies should have routine eye checks as they develop. The American Optometric Association recommends starting eye tests at 9 months, 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 ophthalmologist.
The ophthalmologist will probably use eye drops that dilate the pupils before the test is done.
The Hirschberg test, or Hirschberg corneal reflex test, is used to assess whether the patient has strabismus.
The ophthalmologist 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.
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.
Treatment options include:
- Glasses: If hypermetropia, or long-sightedness, is causing the squint, glasses can usually correct it.
- Eye patch: Worn over the good eye, a patch can get the other eye, the one with the squint, to work better.
- 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.
- Eye drops and eye exercises may help.
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.
A standard type of exercise for strabismus is home-based pencil pushups (HBPP).
To do HBPP, follow these steps:
- Hold a pencil at arm’s length, around midway between the eyes
- Look at the pencil while moving it toward the nose, and try to maintain a single image of it
- Keep moving the pencil toward the nose until you can no longer see it as a single image
- Hold the pencil at the closest point where a single image is possible
- If you cannot regain a single image, start again
A study of patients who did two sets of 20 “push-ups” each day for 12 weeks suggested that the exercise can be “an easy, cost-free, and effective therapy.”
Other home exercises for strabismus include “Swinging in the mirror.”