Porth's Essentials of Pathophysiology, 4e

970

Nervous System

U N I T 1 0

Left visual field

Right visual field

Nasal

Temporal

Temporal

Whitened field no vision

Right eye

Left eye

Left

Right

Lesion 1

FIGURE 38-13. Diagram of optic pathways. The red lines indicate the right visual field and the blue lines the left visual field. Note the crossing of fibers from the medial half of each retina at the optic chiasm. Lesion 1 (right optic nerve) produces unilateral blindness. Lesion 2 (optic chiasm) may involve only those fibers that originate in the nasal half of each retina and cross to the opposite side in the optic chiasm; visual loss involves the temporal half of each field (bi-temporal hemianopia). Lesion 3 (right optic tract) interrupts fibers (and vision) originating on the same side of both eyes (homonymous), with loss of vision from half of each field (hemianopia).

1

Right optic nerve

Optic nerve

2

3

Lesion 2

Optic tract Lateral geniculate nucleus Optic radiation

Optic chiasm

Lesion 3

Right optic tract

The visual field refers to the area that is visible during fixation of vision in one direction 16,22,24 (see Fig. 38-13). As with a camera, the simple lens system of the eye inverts the image of the external world on each retina. In addition, the right and left sides of the visual field also are reversed. Most of the visual field is binocular, with both eyes focusing on one object and then fusing two images into one. The right binocular visual field (the nasal half of the right eye and the temporal half of the left eye) is seen by the left retinal halves of each eye. Likewise, the left binocular field is seen by the right retinal halves of each eye. This binocular field is subdivided into central and peripheral portions. Central portions of the retina provide high visual acuity and correspond to the field focused on the central fovea. The peripheral and surrounding portion provides the capacity to detect

objects, particularly moving objects. Beyond the visual field shared by both eyes, the left lateral periphery of the visual field is seen exclusively by the left nasal retina, and the right peripheral field by the right nasal retina. Disorders of the Optic Pathways Among the disorders that can interrupt the visual pathway are trauma, tumors, and vascular lesions. Trauma and tumors can produce direct injury or impinge on the optic pathways. Vascular insufficiency in any one of the arterial systems of the retina or visual pathways can seriously affect vision. For example, normal visual function depends on the adequacy of blood flow in the ophthalmic artery and its branches—the central retinal artery; the anterior and middle cerebral arteries, which supply the intracranial optic nerve, chiasm, and optic tracts; and the posterior cerebral artery, which supplies the LGN, optic radiation, and visual cortex. The adequacy of posterior cerebral artery function depends on that of the vertebral and basilar arteries that supply the brain stem. Visual Field Defects Visual field defects result from damage to the visual pathways or the visual cortex (see Fig. 38-13). The testing of the visual fields of each eye and of the two eyes together is useful in localizing lesions affecting the system. Perimetry or visual field testing, in which the visual field of each eye is measured and plotted in an arc, is used to identify defects and determine the location of lesions. Blindness in one eye is called anopia. If half of the visual field for one eye is lost, the defect is called hemianopia ; if a quarter of the field is lost, it is called quadrantanopia. Loss of the temporal or peripheral visual fields on both sides results in a narrow binocular field, commonly called tunnel vision. The loss of different

Frontal eye field (part of 8)

Somatosensory (3, 1, 2)

Somatosensory association (5, 7)

Visual (17)

(18) (19)

Second somatosensory

Visual association

Auditory (41) Auditory association (42, 22) FIGURE 38-14. Lateral view of the cortex illustrating the location of the visual, visual association, auditory, and auditory association areas.