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Because distinct flavors depend on aromas to stimulate the olfactory chemoreceptors, smell and taste are physiologically interdependent. Dysfunction of one often disturbs the other. Disorders of smell and taste are rarely incapacitating or life threatening, so they often do not receive close medical attention, although their effect on quality of life can be severe. Loss of olfaction and/or gustation in the elderly leads to reduced oral intake and can add to the debilitation of the patient. The inability to detect certain odors, such as gas or smoke, may be dangerous, and several systemic and intracranial disorders should be excluded before dismissing symptoms as harmless. Whether brain stem disease (involvement of the nucleus solitarius) can cause disorders of smell and taste is uncertain, because other neurologic manifestations usually take precedence.
Anosmia (loss of the sense of smell) is probably the most common abnormality (see below). Hyperosmia (increased sensitivity to odors) usually reflects a neurotic or histrionic personality but can occur intermittently with seizure disorders. Dysosmia (disagreeable or distorted sense of smell) may occur with infection of the nasal sinuses, partial damage to the olfactory bulbs, or psychologic depression. Some cases of dysosmia, accompanied by a disagreeable taste, result from poor dental hygiene. Uncinate epilepsy can produce brief, vivid, unpleasant olfactory hallucinations. Hyposmia (diminished sense of smell) and hypogeusia (diminished sense of taste) can follow acute influenza, usually temporarily.
Drying of the oral mucosa from heavy smoking, Sjögren's syndrome, radiation therapy of the head and neck, or desquamation of the tongue can impair taste, and various drugs (eg, those with anticholinergic properties and vincristine ) alter taste. In all instances, the gustatory receptors are diffusely involved. When limited to one side of the tongue (eg, in Bell's palsy), ageusia (loss of the sense of taste) is rarely noticed.
Rarely, idiopathic dysgeusia (distorted sense of taste), hypogeusia, and dysosmia respond to zinc supplementation.
Anosmia
Anosmia is
the complete loss of smell.
Anosmia occurs when intranasal swelling or other obstruction prevents odors from gaining access to the olfactory area; when the olfactory neuroepithelium is destroyed (as occurs in viral infections, atrophic rhinitis, or the chronic rhinitis of granulomatous diseases and tumors); or when the olfactory nerve fila, bulbs, tracts, or central connections are destroyed (eg, by head trauma, intracranial surgery, infections, or tumors). Head trauma is a major cause of anosmia in young adults; viral infections and Alzheimer's, in older adults. Prior URI, especially influenza infection, is implicated in 14 to 26% of all presenting cases of hyposmia or anosmia.
Most patients with anosmia have normal perception of salty, sweet, sour, and bitter substances but lack flavor discrimination, which is largely dependent on olfaction. Therefore, they often complain of losing the sense of taste (ageusia). If unilateral, anosmia is often unrecognized.
Diagnosis, if the cause is not apparent, requires thorough evaluation for intranasal and intracranial diseases and examination of the cranial nerves (see Approach to the Neurologic Patient: Cranial nerves) and of the upper respiratory tract (particularly the nose and nasopharynx). CT, with radiopaque dye, of the head to rule out tumors and unsuspected fractures of the floor of the anterior cranial fossa is obtained. A psychophysical assessment of odor and taste identification and threshold detection is performed as well.
Treatment of allergic or bacterial rhinitis and sinusitis or removal of nasal polyps and benign tumors may recover olfaction. Conditions causing destruction of the olfactory neuroepithelium or its central pathways do not lend themselves to effective treatment, although spontaneous recovery may follow regeneration of the olfactory neuroepithelium and its central pathways.
Last full review/revision November 2005
Content last modified November 2005
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