The physiology of olfaction is less well understood than that of the other special senses. In the past few years, however, discovery of the family of odor-receptor genes as well as inositol phosphate and cyclic nucleotide signaling pathways have led to a molecular basis of olfactory reception. Clinically, odorant molecules must traverse the nasal vault to reach the cribriform area and become soluble in the mucus overlying the exposed dendrites of receptor cells. Anatomic lack of access to the receptor cells of the first cranial nerve is the most common cause of olfactory dysfunction (hyposmia or anosmia). Polyps, septal deformities, and nasal tumors may all contribute to this inability of air to reach the area of the cribriform plate high in the nose where these receptors are located. Transient olfactory dysfunction often accompanies the common cold, nasal allergies, and perennial rhinitis. About 20% of impaired olfactory function is idiopathic, although it often follows a viral illness. Some have suggested administering large doses of vitamin A and zinc to such patients, although little evidence supports their use. Central nervous system neoplasms, especially those that involve the olfactory groove or temporal lobe, may affect olfaction. Head trauma accounts for less than 5% of cases of hyposmia. Absent, diminished, or distorted smell or taste has been reported in a wide variety of endocrine, nutritional, and nervous disorders. A great many medications have also been implicated.
Evaluation of olfactory dysfunction should include a thorough history of systemic illnesses and medication use as well as a physical examination focusing on the nose and nervous system. Most clinical offices are not set up to test olfaction, but such feats may at times be worthwhile if only to assess whether a patient possesses any sense of smell at all. Odor threshold should be tested in increasing concentrations. For example, use n-butyl alcohol (1-butanolol) in concentrations up to 4% in deionized water. Serial 3:1 dilutions in 12 steps produce an initial test of 46 ppm (v/v) and the maximum of 3055 ppm (at 4%). Odor identification can be tested using standardized choices (see references). In permanent hyposmia, counseling should be offered about seasoning foods with spices (eg, pepper) that stimulate the trigeminal as well as olfactory chemoreceptors and about safety issues such as the use of smoke alarms and electric rather than gas home appliances.
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