Olfatory Nerve

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OLFATORY NERVE
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Cranial Nerve I: OLFATORY
DEFINITIONS Cranial nerve I (CN I) constitutes the primary receptor cells for the sense of smell. This nerve monitors the intake of airborne agents into the human respiratorysystem and largely determines, along with the sense of taste, the flavor and palatability of foods and beverages. In addition to purveying esthetic pleasures, CN I warns of spoiled foods, leaking natural gas, polluted air, and smoke, and mediates elements of basic communication (e.g., mother-infant interactions). Of particular interest to the neurologist is the fact that alterations in the ability tosmell are among the first, if not the first, signs of Alzheimer's disease and idiopathic Parkinson's disease.

Olfactory dysfunction can affect a patient's safety, nutritional status, and quality of life. Among 750 consecutive patients presenting to the University of Pennsylvania Smell and Taste Center with mainly olfactory problems, 68 percent experienced an altered quality of life, 46percent reported changes in appetite or body weight, and 56 percent noted adverse influences on daily living or psychological well-being.[1] Many anosmics report increased use of sugar and seasonings to compensate for their olfactory disorder, a situation detrimental to persons with diabetes and salt-sensitive hypertension. In experimental settings, older persons with diminished smell function ingestfoods that younger persons find distasteful, implying that such individuals are at considerable risk for food poisoning. It is well documented that a disproportionate number of elderly persons die

from accidental gas poisoning, which is due, in large part, to their decreased ability to smell. Most patients presenting with decreased "taste" function actually have decreased smell function. Theperception of decreased ability to "taste" during deglutition is usually due to a loss of flavor sensations derived from retronasal stimulation of the olfactory receptors rather than to a loss of taste bud-mediated sensations per se. Thus, such flavors as coffee, chocolate, vanilla, strawberry, pizza, licorice, steak sauce, root beer, and cola disappear when CN I is markedly damaged, leaving intactonly sweet, sour, salty, bitter, and umami (monosodium glutamate-like) sensations. Whole-mouth taste function

is much more resilient to pathological or traumarelated alterations than is olfactory function, in large part because the taste buds have redundant innervation from several cranial nerves (i.e., CN VII, IX, and X). The nomenclature for olfactory dysfunction is straightforward. Anosmiarefers to a loss of ability to smell, whereas hyposmia or microsmia refers to diminished ability to smell. Dysosmia is distorted or perverted smell perception, such as when a rose smells more like garbage than a rose (parosmia, cacosmia) or when a medicine-like smell is present in the absence of odor stimulation following head trauma (phantosmia). General or total anosmia implies inability to smellall odorants on both sides of the nose. Partial anosmia implies an inability to smell certain odorants. In some cases, partial anosmia is indicative of a decreased sensitivity to a broad spectrum of odorants (general hyposmia), with the decrement exceeding the absolute threshold

for only some odorants. Specific anosmia, the inability to smell one or a few odorants in the presence of an...