The importance of the visual system is reflected by the magnitude of its representation in the central nervous system. A large part of the cerebrum is committed to vision, including the perception of the form and color of objects, the perception of spatial relationships and motion, and the control of movements under visual guidance. The optic nerve, which is a tract of the central nervous system, contains more than a million fibers (compared to 50,000 in the auditory nerve). The visual system also has special significance in that study of this system has greatly advanced our knowledge of the organization of all sensory neuronal systems as well as the relationships between perception and conscious awareness. Indeed, we know more about neural underpinnings of vision than we do about any other sensory function. Furthermore, the eyes, because of their diverse composition of epithelial, vascular, neural, and pigmentary tissues, are virtually a medical microcosm, susceptible to many diseases, and their tissues are available for inspection through a transparent medium.
Impairment of visual function expressed as defects in acuity and alterations of visual fields constitute the most important symptom of eye disease. A number of terms are commonly used to describe the visual loss. Amaurosis is a general term that refers to partial or complete loss of sight. Amblyopia refers to any monocular deficit in vision in the presence of normal ocular structures. A major cause of amblyopia is the suppression by the brain of vision from one eye caused by either strabismus, anisometropia (a significant difference in refractive error), or media opacities during early childhood. Nyctalopia is a term for poor twilight or night vision and is associated with extreme myopia, cataracts, vitamin A deficiency, retinitis pigmentosa, and, often, color blindness. There are also a number of positive visual symptoms that are named based on their characteristics, including phosphenes, migrainous scintillations, visual illusions, and hallucinations. Irritation, redness, photophobia, pain, diplopia and strabismus, changes in pupillary size, and drooping or closure of the eyelids are other major ocular symptoms and signs.
APPROACH TO THE PROBLEM OF VISUAL LOSS
In the investigation of a disturbance of vision, one carefully inquires what the patient means when he describes the perceived change, for the disturbance in question may vary from near- or farsightedness to partial syncope, hemianopia, dizziness, or diplopia. Measurement of visual acuity is an essential part of the ocular examination. Inspection of the refractive media and the optic fundi, the testing of pupillary reflexes, color vision, and the plotting of visual fields complete this part of the examination. Examination of the eye movements is also essential, as discussed in Chap. 13.
The Snellen chart, which contains letters, numbers, or pictures arranged in rows of decreasing size, is used in the measurement of distance visual acuity (Fig. 12–1A). Each eye is tested separately ...