What is squint (strabismus)?
Squint is a misalignment of the two eyes so that both the eyes are not looking in the same direction. This misalignment may be constant, being present throughout the day, or it may appear sometimes and the rest of the time the eyes may be straight.
It is a common condition among children. It may also occur in adults.
What causes squint?
The exact cause of squint is not really known. 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. This misalignment may be the same in all directions of gaze, or in some conditions the misalignment may be more in one direction of gaze, e.g., in squint due to nerve palsy.
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. Therefore it is important in all the cases of squint, especially in children, to have a thorough eye checkup to rule out any other cause of loss of vision.
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 can not 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.
What is the treatment for squint?
The aims of treatment of squint in order of importance are:
- Preserve or restore vision
- Straighten the eyes
- Restore binocular vision
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 parents are explained about the importance of this treatment, as their cooperation is very crucial for the success of this treatment.
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.