strabismus (strəbĭzˈməs) [key], inability of the eyes to focus together because of an imbalance in the muscles that control eye movement; also called squint. It is a consequence of weakness or uneven development of one or more of the six small muscles that surround the eye. One or both eyes may be affected. Horizontal strabismus is caused when the eyes do not move together laterally; this condition is known as cross-eye if the eye turns inward or walleye if the eye turns outward. Vertical strabismus results when the eye rolls upward or downward in its socket. There is also torsional strabismus in which the eyes do not rotate together about their optical axes. Strabismus is usually present at birth and becomes apparent early in infancy, but it may also result from illness or injury. Because the condition results in perception of a double image, there is a tendency to use only one eye. It is important that treatment be started as soon as possible to prevent loss of sight in the unused eye. Corrective therapy includes exercise that strengthens eye muscles and prescription of corrective lenses. Sometimes a patch is placed alternately on each eye so that neither is allowed to become completely unused. If necessary, the eye muscles may be shortened or lengthened surgically.