The exact cause of a squint or strabismus is not fully understood. Six eye muscles controlling eye movement are attached to the outside of each eye. In each eye, two muscles move the eye right or 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 the eye muscles. Strabismus is especially common among children with disorders that affect the brain, such as cerebral palsy, Down syndrome, hydrocephalus and brain tumours. Cataracts and other causes of poor vision can also cause strabismus.