Last updated: 02/17/2009

The extraocular muscles attach via tendons to the sclera (the white, opaque, outer protective covering of the eyeball) at different places just behind an imaginary equator circling the top, bottom, left, and right of the eye. The other end of each...

of these muscles attaches to a part of the orbit (the eye socket in the skull). These muscles enable the eyes to move up, down, to one side or the other, or any angle in between.

Normally, both eyes move together, receiving the same image on corresponding locations on both retinas. The brain fuses these matching images into one three-dimensional image. The exception is in strabismus, which is a disorder where one or both eyes deviate out of alignment, most often outwardly (exotropia) or toward the nose (esotropia). In this case, the brain receives two different images, and either suppresses one or allows the person to see double (diplopia). By weakening or strengthening the appropriate muscles to center the eyes, a person can correct this deviation. For example, if an eye turns upward, the muscle at the bottom of the eye could be strengthened.

The main purpose of eye muscle surgery is thus to restore straight eye alignment. The surgery is performed to align both eyes so that they gaze in the same direction and move together as a team; to improve appearance; and to promote the development of binocular vision in a young child. To achieve binocular vision, the eyes must align so that the location of the image on the retina of one eye corresponds to the location of the image on the retina of the other eye.

In addition to being used to correct strabismus, eye muscle surgery is also performed to treat such other eye disorders as nystagmus or such special types of congenital strabismus as Duane syndrome. Nystagmus is a condition in which one or both eyes move rapidly or oscillate; this condition can be improved by moving the eyes to the position of least oscillation. Duane syndrome is a disorder in which there is limited horizontal eye movement; it can sometimes be relieved by surgery that weakens an eye muscle.