Disorders that affect the ocular muscles, ocular motor (III, IV, and VI) cranial nerves, or visual or ocular motor pathways in the brain produce a wide variety of neuro-ophthalmic disturbances. Because the anatomic pathways of the visual and ocular motor systems traverse major portions of the brainstem and cerebral hemispheres, neuro-ophthalmic symptoms and signs are often valuable in the anatomic localization and diagnosis of neurologic disease.
FUNCTIONAL ANATOMY OF THE VISUAL SYSTEM
Visual information enters the nervous system when light, refracted and focused by the lens, creates a visual image on the retina at the back of the eye (Figure 7-1). The action of the lens causes this image to be reversed in the horizontal and vertical planes. Thus, the superior portion of the visual image falls on the inferior retina and vice versa, and the temporal (lateral) and nasal (medial) fields are likewise reversed (Figure 7-2). The center of the visual field is focused at the fovea (central portion of the macula), where the retina’s perceptual sensitivity is greatest. Within the retina, photoreceptor cells (rods and cones) transduce incident light into neuronal impulses, which are transmitted by retinal neurons to the optic (II) nerve. At this and all other levels of the visual system, the topographic relations of the visual field are preserved.
Representation of the visual field at the level of the retina: the point of fixation is focused on the fovea, the physiologic blind spot on the optic disk, the temporal half of the visual field on the nasal side of the retina, and the nasal half of the visual field on the temporal side of the retina.
Representation of the visual field at the level of the optic nerve, chiasm, and tract. Quadrants of the visual field are designated ST (superior temporal), IT (inferior temporal), SN (superior nasal), and IN (inferior nasal).
PERIPHERAL VISUAL PATHWAYS
Each optic nerve contains fibers from one eye, but the nasal (medial) fibers, conveying information from the temporal (lateral) visual fields, cross in the optic chiasm (Figure 7-2). As a result, each optic tract contains fibers not from one eye, but from one half of the visual fields of both eyes. Because of this arrangement, prechiasmal lesions affect vision in the ipsilateral eye and retrochiasmal lesions produce defects in the contralateral half of the visual field of both eyes.
The optic tracts terminate in the lateral geniculate nuclei, where they synapse on neurons that project via the optic radiations to the primary visual or ...