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Anatomy and Physiology of Eye Movements Kenneth W 2 Anatomy and Physiology of Eye Movements Kenneth W. Wright OCULAR POSITION Within the orbit, the eye is suspended by six extraocular muscles (four rectus muscles and two oblique muscles), suspensory liga- ments, and surrounding orbital fat (Fig. 2-1). A tug-of-war exists between the rectus and oblique muscles. The four rectus muscles insert anterior to the equator, and pull the eye posteri- orly, 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 signifi- cant 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 loosen- ing 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. 2-2). This dis- crepancy 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). 24 Whitnall's lig. Superior oblique m. Levator palpebrae Müller's m. Superior rectus m. Intraconal fat Lateral rectus m. Lockwood's lig. Inferior rectus m. Extraconal fat Inferior oblique m. FIGURE 2-1. Side view of extraocular muscles. Note that the rectus muscles pull the eye posteriorly while the oblique muscles pull the eye anteriorly. FIGURE 2-2. Diagram shows visual axis versus muscle/orbital axis. Note that the visual axes parallel the central sagittal plane, while the orbital axis of each eye diverges 23° from the visual axis. 25 26 handbook of pediatric strabismus and amblyopia 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. OCULAR MOVEMENTS Ductions The term ductions is used to describe monocular eye move- ments without regard for the movement of the fellow eye (Fig. 2-3). Ductions result from an extraocular muscle contraction A B E C F D G FIGURE 2-3A–G. Diagram of ductions, which are monocular eye movements. chapter 2: anatomy and physiology of eye movements 27 TABLE 2-1. Extraocular Muscles. Approximate Action muscle Anatomic Tendon Arc of from length insertion length contact primary Muscle (mm) Origin (mm) (mm) (mm) position Medial 40 Annulus 5.5 4 6 Adduction rectus of Zinn Lateral 40 Annulus 7.0 8 10 Abduction rectus of Zinn Superior 40 Annulus 8.0 6 6.5 Elevation rectus of Zinn Adduction Intorsion Inferior 40 Annulus 6.5 7 7 Depression rectus of Zinn Adduction Extorsion Superior 32 Orbit apex From 26 12 Intorsion oblique above temporal Depression annulus pole of Abduction of Zinn superior rectus to within 6.5 mm of optic nerve Inferior 37 Lacrimal Macular 1 15 Extorsion oblique fossa area Elevation Abduction that pulls the scleral insertion site toward the muscle’s origin while the opposing extraocular muscle simultaneously relaxes. The contracting muscle is referred to as the agonist and the relaxing muscle as the antagonist. An upward movement of an eye is referred to as supraduction or sursumduction, a down- ward movement is termed infraduction or dorsumduction, a nasal-ward movement is termed adduction, and a temporal movement is termed abduction. Torsional rotations (twisting movements) are known as cycloductions, with incycloduction (intorsion) referring to a nasal rotation of the 12 o’clock position of the cornea and excycloduction (extorsion) referring to a tem- poral rotation of the 12 o’clock position. Muscle Action Versus Field of Action The terms “muscle action” and the “field of action” are often confused. Muscle action refers to the effect of muscle contrac- tion on the rotation of the eye when the eye starts in primary position. Table 2-1 lists the muscle actions of each extraocular muscle. Horizontal rectus muscles have but one action: hori- zontal rotation of the eye. Vertical rectus and oblique muscles, 28 handbook of pediatric strabismus and amblyopia however, have three actions: vertical, horizontal, and torsional. The most robust action is termed the primary action, followed by the less obvious secondary and tertiary actions. It is impor- tant to remember the classic descriptions of primary, secondary and tertiary muscle actions as they relate to the eye when it is in primary position. In contrast, the field of action of a muscle is the position of gaze when an individual muscle is the primary mover of the eye. Granted, virtually all eye movements are the result of combined contraction and relaxation of multiple muscles, but there are eight positions of gaze where one muscle provides the dominant force (Fig. 2-4). For example, when one looks up, the brain recruits both the superior rectus and the inferior oblique muscles. Looking up and nasal, however, is the primary func- tion of the inferior oblique muscle, so this is the field of action of the inferior oblique muscle. A muscle’s function is best eval- uated by having the patient look into the field of action of the FIGURE 2-4. Diagram of the field of action of the extraocular muscles. Arrows point to the quadrant where the specified muscle is the major mover of the eye. SR, superior rectus; IR, inferior oblique; MR, medial rectus; SO, superior oblique; IO, inferior oblique; IR, inferior rectus; LR, lateral rectus. chapter 2: anatomy and physiology of eye movements 29 muscle. Thus, even though the secondary action of the inferior oblique muscle is abduction, evaluate inferior oblique function by having the patient look “up and nasal.” A patient with an inferior oblique palsy will show limitation of eye movement up and nasal. Note, for straight upgaze, the superior rectus muscle is the major elevator, and for straight down-gaze the inferior rectus is the major depressor, with the oblique muscles con- tributing little. Smooth Pursuit Versus Saccadic Eye Movements There are two basic forms of eye movements: smooth pursuit and saccadic. Smooth pursuit eye movements are generated in the occipital parietal temporal cortex, with the right cortex con- trolling movements to the right and the left cortex controlling movements to the left. In humans, smooth pursuit first occurs at 4 to 6 weeks of age. These are slow accurate eye movements requiring visual feedback from central foveal fixation. Smooth pursuit eye movements can follow visual targets moving at velocities up to 30° per second (30°/s). Clinically, accurate smooth pursuit indicates central fixation and in preverbal chil- dren is an indication of good vision. Saccadic movements are rapid eye movements with veloc- ities usually ranging from 200° to 700°/s, but saccades have been recorded up to 1000°/s. The peak velocity increases as the ampli- tude of the movement increases, and this relationship is termed the main sequence. Saccades are movements used to keep up with targets moving too fast for smooth pursuit and for quick refixation from one target to another. Saccadic eye movements develop before smooth pursuits, occurring as early as 1 week of age. Saccadic eye movements are generated in the frontal lobes and are under contralateral control; that is, right frontal lobe stimulation will result in a saccadic eye movement to the left. Saccadic movements can be voluntarily initiated, but they are not voluntarily controlled, and there is no significant visual feedback to adjust the amount of movement. It is thought that the amplitude of a saccadic movement is preprogrammed based on the degree of retinal eccentricity of the target; this is why saccadic movements are termed ballistic, analogous to the bal- listic trajectory of a cannon ball. The neuronal signal that initi- ates a saccade consists of a burst of high-frequency discharge or pulse to the agonist and inhibition of the antagonist. Because all neurons available are activated for eye movements greater than 30 handbook of pediatric strabismus and amblyopia 5°, the magnitude of a saccade is determined by the duration of the pulse. At the end of a saccade, tonic neuronal firing of the agonist and antagonist muscles occurs to hold the eye position referred to as the step. Vision during a saccadic eye movement is suspended or suppressed. Some have used the term saccadic omission for the process of cortical suppression.1 A tremendous force is required to produce a saccadic eye movement; therefore, the presence of saccadic eye movements indicates “good” muscle function. Only rectus muscles generate saccadic eye move- ments. When evaluating a patient with limited ductions, look for the presence of a normal saccadic eye movement into the field of limited ductions. If there is a brisk saccade in the direction of the limitation, this indicates good muscle function and suggests the limited movement is caused by restriction, not a muscle paresis. Optokinetic nystagmus (OKN) can be generated by a slowly rotating drum with stripes and used to evaluate smooth pursuit and saccadic eye movements.
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