History And Definitions

The third, fourth, and sixth cranial nerves and the muscles they innervate are involved in moving the eyes to allow inspection of visual objects. The act of positioning the image of an object on the retinal fovea, which is the locus of highest visual acuity and the psychological center of the visual field, is referred to as fixation. This is accomplished by activating combinations of extraocular muscles to achieve a specific angle of globe rotation in order to place the fovea in direct line with an image. Fusion denotes the primitive reflex action, organized within the central nervous system (CNS), in which the visual axes of the two eyes are made to converge on a single object of regard. This allows the same image in visual space to be focused simultaneously on the foveas of both eyes, and the image is perceived as a single object. The visual axis is an imaginary line connecting the fovea of the retina with the object of regard. It defines the direction of gaze and corresponds to the Y axis on Figure 9-1 . Strabismus is present when there is deviation or misalignment of the visual axes.

There are nine cardinal positions of gaze, subdivided into primary, secondary, and tertiary positions. The primary position is straight ahead, and eccentric gaze develops when the two eyes deviate away from primary gaze. Secondary positions include straight up, straight down, straight in toward the nose, and straight out toward the temple. Tertiary positions are those away from the primary position

Figure 9-1 The axes of rotation of the eye. The Y axis corresponds to the line of sight when the eye is in the primary position looking straight a(Fram Miller, NR: Walsh andHoyt's Clinical Neuro-Ophthalmology, Vol 2, 4th ed. Baltimore, Williams & Wilkins, 1985.)

with combined simultaneous horizontal and vertical deviation. These tertiary positions include elevation in adduction, elevation in abduction, depression in adduction, and depression in abduction. Abduction refers to movement of the eye toward the temple in the horizontal plane about a vertical axis, and adduction is the movement of the eye toward the nose in the horizontal plane about a vertical axis (Z axis, Figure9-1). Rotational movements relate to the eye's turning about an axis in the coronal or frontal plane of the globe. The axis may be vertical (horizontal rotation) or horizontal (vertical rotation) or oblique (oblique rotation) at any angle. Torsional movements relate to an eye turning in the frontal plane about an anteroposterior axis through the center of rotation of the globe. The direction of torsion is specified by the movement of the 12 o'clock meridian of the limbus or corneal margin. Intorsion is defined as rotation of the point inward toward the nose or midline, and extorsion refers to rotation outward toward the temple. The rotation of both eyes in the same direction and at the same speed is known as conjugate gaze. This keeps the relative position of the visual axes constant (nearly parallel for distant objects, convergent for near objects). These conjugate movements of the two eyes are termed versions to distinguish them from vergence movements, in which the visual axes do not remain parallel but either converge or diverge.

Phoria denotes a latent eye deviation or deviation of the visual axes not present under conditions of binocular viewing. A phoria becomes manifest (becomes a tropia) when the image of the object of regard is made different in the two eyes such that the fusion mechanism cannot operate or operates inefficiently. Tropia is defined as deviation that is present with both eyes viewing, and hence is the condition that underlies diplopia. Diplopia, or double vision, is defined as seeing two separate images of the same object in visual space, with one of the images displaced from the other. Binocular diplopia results from the misalignment of the visual axes (the two eyes are looking in slightly different directions). This type is present only when the patient is viewing the object of regard simultaneously with both eyes and disappears when either eye is covered. Polyopia, or seeing three or more simultaneous images of a single object in visual space is categorically not a feature of binocular diplopia in humans. It is usually caused by an optical aberration in the refractive media of the eye, but a cerebral form also exists and is usually from lesions in the occipital-parietal lobes.

There are three systems of conjugate eye movement, including smooth pursuit, saccades, and the vestibulo-ocular reflex (VOR) that deviate the eyes in unison. Smooth pursuit eye movements stabilize on the fovea images of small objects moving in visual space. Saccades redirect the line of gaze to an object of interest, placing its image on the fovea. The VOR compensates for head movement, stabilizing gaze in space.

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