Six eye muscles, controlling eye movement, are attached to the outside of each eye. In each eye, one muscle moves in the eye to the right, and one muscle moves the eye to the left. The other four muscles move it up or down and at an angle.
To line up and focus both eyes on a single target, all of the muscles in each eye must be balanced and working together. In order for the eyes to move together, the muscles in both eyes must be coordinated. The brain controls these muscles.
With normal vision, both eyes aim at the same spot. The brain then combines the two pictures into a single, three-dimensional image. This three-dimensional image gives us depth perception.
When one eye is out of alignment, two different pictures are sent to the brain. In a young child, the brain learns to ignore the image of the misaligned eye and sees only the image from the straight or better-seeing eye. The child then loses depth perception.
Adults who develop strabismus often have double vision because their brains have already learned to receive images from both eyes and cannot ignore the image from the turned eye. A child generally does not see double.
Strabismus is especially common among children with disorders that may affect the brain, such as:
- Cerebral palsy;
- Down syndrome;
- Brain tumors;
A cataract or eye injury that affects vision can also cause strabismus. The vast majority of children with strabismus, however, have none of these problems. Many do have a family history of strabismus.
Good vision develops during childhood when both eyes have normal alignment. Strabismus may cause reduced vision, or amblyopia, in the misaligned eye.
The brain will pay attention to the image of the straight eye and ignore the image of the crossed eye. If the same eye is consistently ignored during early childhood, this misaligned eye may fail to develop good vision, or may even lose vision. Strabismic amblyopia occurs in approximately half of the children who have strabismus.
Amblyopia can be treated by patching or blurring the stronger eye to strengthen and improve vision in the weaker eye. If amblyopia is detected in the first few years of life, treatment is usually successful. If treatment is delayed, amblyopia may become permanent. As a rule, the earlier amblyopia is treated, the better the result for vision.