What is squint (strabismus)?

Squint is a misalignment of the two eyes so that both the eyes are not looking in the same direction.
Squinting of eyes may occur occasionally or may be present throughout the day. It may appear in only one eye or may alternate between the two eyes.
It is a common condition among children but may also occur in adults.

Types of Squint
It could be :
Convergent squint also known as esotropia, where one eye is turned in towards the nose. This is the most common form of squint seen usually in children.

Divergent squint also known as exotropia, where one eye turns out away from the centre of the face.
Vertical deviations known as hypo or hypertropias are also seen but are not common.

What causes squint?

Squinting eye could be because of one of the following reasons
  • Refractive (focusing) abnormality
  • Eye muscle imbalance
  • Injury
  • Due to other diseases or illness or associated with some syndrome.
The movement of each eye is controlled by six muscles. Each of these muscle acts along with its counterpart in the other eye to keep both the eyes aligned properly. A loss of coordination between the muscles of the two eyes leads to misalignment.

Sometimes a refractive error hypermetropia (long sight) may lead to inward deviation of the eye. Poor vision in an eye because of some other eye disease like cataract, etc. may also cause the eye to deviate.

Squint can become obvious after chronic illness and it can also run in families.
What is binocular vision?

Under normal circumstances, when both the eyes have good vision and they are aligned properly, they focus on the same object. Each of the eyes sends picture of the same object, viewed from a slightly different angle. These two images reach the brain, where they are fused to form a single three-dimensional picture with depth perception. This is known as binocular single vision.

What are the problems with squint?
When the eyes are not aligned properly, each of the eyes is focusing on a different object and sends signal to the brain. These two different images reaching the brain lead to confusion and may have either of the two effects: A child would ignore the image coming from the deviated eye, and thus sees only one image. But in the process, he loses the depth perception. This suppression of the image from the deviating eye results in poor development of vision in this eye, which is known as amblyopia. An adult cannot ignore the image from either eye, and therefore has double vision. This can be very annoying and may interfere with work.

What are the symptoms of squint?
In a child, the parents may notice the deviation of eyes. It is important to remember that the eyes of a newborn are rarely aligned at birth. Most establish alignment at 3-4 weeks of age. Therefore squint in any child who is more than one month old must be taken seriously and should be evaluated by an ophthalmologist.

Adults may notice double vision, or misalignment of the eyes.

How is squint diagnosed?
The squint is diagnosed by the ophthalmologist. He or she would do a few special tests to confirm the squint, to try and find out the cause and to quantify the amount of deviation. In some cases there may be a false appearance of squint due to broad nasal bridge in a child. An ophthalmologist will be able to differentiate between a true squint and false squint.

The aims of treatment of squint in order of importance are:
  • Preserve or restore vision
  • Straighten the eyes
  • Restore binocular vision

The different treatment modalities are four Os:
  • Optics (Spectacles)
  • Occlusion (Patching)
  • Orthoptics (Eye Exercises)
  • Operation (Surgical Correction)

First of all, the eyes are checked to see if they have any refractive error that may be responsible for squint. If there is any significant refractive error present, it is treated first. In some cases (accommodative squint) a correction of refractive error is all that may be required to treat squint.

Next the eyes are checked for presence of amblyopia. It is important to treat the amblyopia before the surgery for squint.

The squint is treated by surgery of either one or both the eyes. The surgery involves weakening or strengthening of the relevant muscles to restore the balance and to get a good coordination. In some cases with double vision, prisms may be added in the glasses to ease the symptoms.

When should the squint be treated?
In a child, the treatment of squint and any associated amblyopia should be started as soon as possible. Generally speaking, the younger the age at which amblyopia is treated; the better is the chance of recovery of vision. Remember that the child would never grow out of squint. A delay in treatment may decrease the chances of getting a good alignment and the vision.