Eye Rotations, the Extraocular Muscles, and Strabismus Terminology

To understand how eye muscles move the eyeball, it is necessary to understand the geometry of the eye and the functions of the muscles. The eyeball rotates about three axes: horizontal, vertical, and torsional. These axes intersect at the center of the eyeball. Eye rotations are achieved by coordinated contraction and relaxation of six extraocular muscles—four rectus and two oblique—attached to each eye. The action of the muscles on the globe is determined by the point of rotation of the globe, as well as the origin and insertion of each muscle. Recent evidence suggests that the muscles also exert their effects on the globe via the extraocular muscle pulleys. Considering that we make at least 100,000 saccades alone each day, it is not surprising that many extraocular muscles are very resistant to fatigue. Extraocular muscles are also different from other skeletal muscles in many respects. For example, eye muscle fibers are richly innervated, and each motoneuron innervates only 10–20 muscle fibers, the smallest motor unit known in the body. Extraocular muscles also have more mitochondria and a higher metabolic rate than other skeletal muscles. Thus, extraocular muscles are one of the fastest contracting muscles. This property allows animals to shift gaze swiftly, so that they can avoid approaching predators or detect prey in the vicinity. The unique immunologic and physiologic properties of extraocular muscles may also explain why they are more susceptible to certain disease processes, such as Grave’s disease and chronic progressive external ophthalmoplegia, but more resistant to others such as Duchenne’s dystrophy, which mainly affects skeletal muscles in the rest of the body. The eyeball rotates about three axes: x-axis (naso-occipital or roll axis), y-axis (earthhorizontal or pitch axis), and z-axis (earth-vertical or yaw axis). Ductions refer to monocular movements of each eye. They include abduction, adduction, elevation (sursumduction), depression (deorsumduction), incycloduction or incyclotorsion, and excycloduction or excyclotorsion (see table on opposite page). Versions refer to binocular conjugate movements of both eyes, such that the visual axes of the eyes move in the same direction. They include dextroversion, levoversion, elevation (sursumversion), depression (deorsumversion), dextrocycloversion, and levocycloversion (see table).