Table of Contents
What is crossed eyes, exactly?
Strabismus, or squint, is a disorder in which the eyes are misaligned and point in different directions. Only one eye can look directly at the object of current interest while the other eye turns inward, outward, up or down. The misalignment may be continual or it may come and go; the turned eye may straighten at times, and the straight eye may turn. The result of any of these variations may be diplopia, or double vision.
The two main types of squint are esotropia, in which one or both eyes turn inward; and exotropia, in which one or both eyes turn outward.
What causes strabismus?
The condition is neurological in nature. That is, the problem lies in the brain, which controls the muscles that govern the movements of the eye. Accordingly, squint is common among children with neurological disorders such as cerebral palsy, Down’s syndrome, hydrocephalus and brain tumours. Occasionally, too, strabismus may be a symptom of a cataract. By far the most common cause of squint in children, however, is hypermetropia.
How does hypermetropia cause squint?
Children have very strong powers of accommodation, and accommodation can cancel hypermetropia and allow clear vision. If a hypermetropic child tries to see a distant object clearly, he or she will accommodate strongly and this will cause the eyes to converge. But convergence will, in turn, cause double vision. This is unpleasant and distracting and the brain will suppress the image from one eye so that diplopia is avoided. The effect of such suppression in childhood can be serious and will lead to defective vision in the squinting eye. If this is not treated early, the loss of vision may be permanent.
Are there any other risk factors?
Sometimes it runs in families, but many people with strabismus have no relatives with the problem.
How common is strabismus in children?
Some 4 to 5 per cent of all children have crossed eyes. It occurs equally in boys and girls.
I thought only children had crossed eyes, but now I’ve seen them in adults, too. Is this the same condition?
Yes. Most adults with crossed eyes have had them since childhood. However, sometimes adults develop strabismus because of certain medical problems such as diabetes, thyroid disease, myasthenia gravis, brain tumours, stroke or other neurological disorders.
At what age does strabismus usually begin?
Studies of newborn babies have found that strabismus is not usually present at birth but develops within the first month of life, as vision develops. Some babies, however, squint from birth because the muscles that turn the eyes outward are not working properly. In such a case, even if the child’s head is held so that it cannot turn, the eyes will not move out beyond the central position in the attempt to see an attractive object such as a toy.
Most babies look cross-eyed at least some of the time. Are they?
Not really. But it is true that infants’ eyes drift in and out of focus during the first few months of life. Their eyes sometimes cross as they begin to focus on close objects. This is normal because,’ as we have seen, the eyes typically turn inward as a person focuses on a near object.
The broad bridge of the nose and extra eyelid skin characteristic of babies add to the impression of strabismus. The confusion is so common there is even a name for it – pseudostrabismus, the appearance of crossed or misaligned eyes in very young children. Children with widely set eyes and excess skin around the bridges of their noses often appear to have exotropia, in which the eyes turn outward. But usually what they have is pseudoexotropia. As a child’s face begins to take shape and his eyes focus better, the condition disappears.
There is a condition known as epicanthus in which the inner ends of the eyelids turn down to cover the inner comers of the eyes. This condition causes a strong, but false, impression of a convergent squint (esotropia).
How can I tell if my child has a true squint?
If, from birth, the eyes cannot turn out fully, or if regular crossing persists after the age of six months, you should immediately have your child seen by an ophthalmologist. Other tell-tale signs to look out for are:
- your baby seems to have trouble seeing
- her eyes seem to drift
- she rubs them a lot
- or she tilts her head at an awkward angle.
You can make a preliminary diagnosis yourself by shining a torch into your child’s eyes. When he is looking at the light, a reflection can be seen on the front surfaces of the corneas. If the eyes are aligned properly, the light reflection will be in the same location in each eye – right in the centre of the pupil. If the child has true strabismus, the reflection will appear in a different location in each eye. In fact, sometimes parents first notice the abnormal light reflection in flash photos of their babies.
Don’t most kids outgrow crossed eyes?
No. As we have seen, what happens is that their brains learn to ignore the image of the misaligned eye and see only the image of the straight, better-seeing eye. However, the appearance of crossed eyes remains but, much more importantly, other problems already hinted at occur. These are discussed later in this chapter.
Double vision is likely to be troublesome to people who develop strabismus as adults, because the brain is already trained to receive images from both eyes and can’t ignore the image from the turned eye.