Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

22HANDBOOK OF PEDIATRIC STRABISMUS AND AMBLYOPIA

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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 movements 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.

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 downward 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 temporal 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 contraction 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: horizontal rotation of the eye. Vertical rectus and oblique muscles,

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 contributing 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 controlling 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 children is an indication of good vision.

Saccadic movements are rapid eye movements with velocities usually ranging from 200° to 700°/s, but saccades have been recorded up to 1000°/s. The peak velocity increases as the amplitude 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 ballistic trajectory of a cannon ball. The neuronal signal that initiates 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

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 movements. 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. As the drum rotates toward the patient’s right, there is a smooth pursuit eye movement to the right to follow the stripe. As the end of the stripe passes, there is a fast saccadic movement to the left to refixate on the next stripe. At target velocities less than 30°/s, smooth pursuit keeps pace with the target. At velocities between 30° and 100°/s, smooth pursuit movements progressively lag behind the target. At velocities greater than 100°/s, OKN is not evoked. OKN can be used to evaluate saccadic and smooth pursuit eye movements. Look at the fast phase of OKN to evaluate saccadic movements and the slow phase to evaluate smooth pursuit.

ANATOMY OF THE EYE MUSCLES

Rectus Muscles

The four rectus muscles originate at the orbital apex at the annulus of Zinn and course anteriorly to insert on the anterior aspect of the globe. The “straight” course of the rectus muscles gives rise to the term rectus. The rectus muscle insertions form a progressive spiral termed the spiral of Tillaux around the corneal limbus. The medial rectus muscle is the closest to the limbus (5.5 mm), then the inferior rectus (at 6.5 mm), the lateral rectus (at 7.0 mm), and the superior rectus is the furthest from the limbus (8.0 mm). The muscle–scleral insertion line has

FIGURE 2-5. Diagram of distance of the rectus muscle insertions from the limbus (in millimeters, mm). Note that the medial rectus muscle inserts closest to the limbus and the distances increase, going counterclockwise from the medial rectus toward the superior rectus, which inserts furthest from the limbus.

a horseshoe configuration with the rounded apex pointing toward the cornea (Fig. 2-5). One can remember this as the horseshoes are always galloping toward the cornea. The scleral thickness behind the rectus insertions is the thinnest of the eye, being only 0.3 mm thick. Hooking a rectus muscle requires passing the hook several millimeters behind the central muscle insertion to clear the posterior aspect of the horseshoe insertion. The widths of the insertions are all approximately 10 mm, and the distance between insertions or intermuscle spacing is only 6 to 8 mm. Because of the proximity of the rectus muscle insertions, it is easier than you might think to hook the wrong muscle during strabismus surgery. An important number to remember is the rectus muscle length, which is 40 mm for all rectus muscles and is also the length of the orbit. Rectus muscles are innervated from the intraconal side of the muscle belly at the junction of the anterior two-thirds and posterior one-third of the muscle.