Anatomy and Physiology of Eye Movements

Abstract

Within the orbit, the eye is suspended by six extraocular muscles (four rectus muscles and two oblique muscles), suspensory ligaments, and surrounding orbital fat (Fig. 8-1). A tug-of-war exists between the rectus and oblique muscles. The four rectus muscles insert anterior to the equator, and pull the eye posteriorly, while the two oblique muscles insert posterior to the equator providing anterior counterforces. Posterior orbital fat also pushes the eye forward. If rectus muscle tension increases, the eye will be pulled back causing enophthalmos and lid fissure narrowing. Simultaneous cocontraction of the horizontal rectus muscles in Duane’s syndrome, for example, can cause significant lid fissure narrowing and enophthalmos. In contrast, decreased rectus muscle tone causes proptosis and lid fissure widening. Conditions such as muscle palsies or a detached rectus muscle allow the eye to move forward and result in lid fissure widening. Rectus muscle tightening procedures such as resections tend to cause lid fissure narrowing whereas loosening procedures such as rectus recessions induce lid fissure widening. When the eye is looking straight ahead with the visual axis parallel to the sagittal plane of the head, the eye is in primary position. The vertical rectus muscles follow the orbits and diverge from the central sagittal plane of the head by 23°. Thus, the visual axis in primary position is 23° nasal to the muscle axis of the vertical rectus muscles (Fig. 8-2). This discrepancy between the vertical rectus muscle axis and the visual axis of the eye explains the secondary and tertiary functions of the vertical rectus muscles (see muscle functions, following). The term position of rest refers to the position of the eyes when all the extraocular muscles are relaxed or paralyzed. Normally, the position of rest is divergent (i.e., exotropic), with the visual axis in line with the orbital axis. The eyes of a patient under general anesthesia are usually deviated in a divergent position.