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 perception of the form and color of objects, the perception of spatial relationships and motion, and the visual control of movements. 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 both the organization of all sensory neuronal systems and the relation of perception to cognition. Indeed, we know more about vision than 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 its tissues are available for inspection through a transparent medium.
Impairment of visual function, expressed as defects in acuity and alterations of visual fields, obviously stands as the most important symptom of eye disease. A number of terms are commonly used to describe visual loss. Amaurosis is a general term that refers to partial or complete loss of sight. Amblyopia refers to any monocular deficit in vision that occurs in the presence of normal ocular structures. A major cause of amblyopia is the suppression by the brain of vision from one eye during early childhood caused by either strabismus, anisometropia (a significant difference in refractive error), or by media opacities. 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, 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. Impairment of vision may be unilateral or bilateral, sudden or gradual, episodic or enduring.
The common causes of failing eyesight vary with age. In infancy, congenital defects, retinopathy of prematurity, severe myopia, hypoplasia of the optic nerve, optic pits, and coloboma are the main causes. In childhood and adolescence, nearsightedness or myopia, and amblyopia as a result of strabismus are the usual causes (see Chap. 13), although a pigmentary retinopathy or a retinal, optic nerve, or suprasellar tumor may also begin at this age. In middle age, usually beginning in the fifth decade, a progressive loss of accommodation (presbyopia) is almost invariable (at this age, half or more of the amplitude of accommodative power is lost and must be replaced by plus lenses). Still later in life, cataracts, glaucoma, retinal vascular occlusion and detachments, macular degeneration, and tumor, unilateral or bilateral, are the most frequent causes of visual impairment.