Disorders of the Special Senses: Introduction
DISORDERS OF THE SPECIAL SENSES: INTRODUCTION
The four chapters in this section are concerned with the clinical aspects of the highly specialized functions of taste and smell, vision, hearing, and the sense of balance. These special senses and the cranial nerves that subserve them represent the most finely developed parts of the sensory nervous system. Dysfunctions of the eye and ear are, of course, the domain of the ophthalmologist and otorhinolaryngologist, but they also are of great interest to the neurologist. Some defects in the special sensory apparatus reflect the presence of systemic disease and others represent the initial or leading manifestation of neurologic disease. In keeping with the general scheme of this text, the disorders of the special senses and of ocular movement are discussed in a particular sequence: first, certain facts of anatomic and physiologic importance, followed by cardinal clinical manifestations of disease, and then the syndromes of which these manifestations are a part.
Disorders of Smell and Taste
The sensations of smell (olfaction) and taste (gustation) are suitably considered together. Physiologically, these modalities share the singular attribute of responding primarily to chemical stimuli; that is, the end organs that mediate olfaction and gustation are chemoreceptors. Also, taste and smell are interdependent clinically, as the appreciation of the flavor of food and drink depends to a large extent on its aroma, and an abnormality of one of these senses is frequently misinterpreted as an abnormality of the other. In comparison to sight and hearing, taste and smell play a less critical role in the life of the individual. However, chemical stimuli in communication between humans are probably very important for some functions that have not been fully explored. Pheromones (pherein, “to carry”; hormon, “exciting”), that is, odorants exuded from the body, as well as perfumes, play a part in sexual attraction; noxious body odors may repel. In certain vertebrates the olfactory system is remarkably well developed, rivaling the sensitivity of the visual system. Though humans were thought to be capable of discriminating as many as 10,000 different odorants based on work by Reed and others, recent experimental studies by Bushdid and colleagues have shown that this may be a vast underestimation.
Disorders of taste and smell can be persistently unpleasant, but only rarely is the loss of either of these modalities a serious handicap. Nevertheless, as all foods and inhalants pass through the mouth and nose, these two senses serve to detect noxious odors (e.g., smoke) and to avoid tainted food and potential poisons. The loss of these senses could then have serious consequences. Also, because a loss of taste and smell may signify a number of intracranial, neurodegenerative, and systemic disorders, they assume clinical importance.
Anatomic and Physiologic Considerations