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The Merck Manual of Medical Information--Home Edition
Section 6. Brain and Nerve Disorders
Chapter 72

Smell and Taste Disorders

click on thumbnail to see full-size imageSince disorders of smell and taste are rarely life-threatening, they may not receive close medical attention. Yet, these disorders can be frustrating because they can affect a person's ability to enjoy food, drink, and pleasant aromas. They can also interfere with a person's ability to notice potentially harmful chemicals and gases, which could have serious consequences. Now and then, a disorder that impairs smell and taste can be serious.

Smell and taste are closely linked. The taste buds of the tongue identify taste; the nerves in the nose identify smell. Both sensations are communicated to the brain, which combines the information to recognize and appreciate flavors. While some tastes--such as salty, bitter, sweet, and sour--can be recognized without the sense of smell, more complex flavors (raspberry, for example) require both taste and smell sensations to be recognized.

A loss of or reduction in the ability to smell (anosmia) is the most common disorder of smell and taste. Because distinguishing one flavor from another is based in large part on smell, people often first notice a reduced sense of smell if their food seems tasteless.

The sense of smell can be affected by changes in the nose, in the nerves leading from the nose to the brain, or in the brain. For example, if nasal passages are stuffed up from a common cold, smell may be decreased simply because odors are prevented from reaching the smell receptors. Because the ability to smell affects taste, food often doesn't taste right to people with colds. Cells that sense smell can be temporarily damaged by the influenza (flu) virus; some people can't smell or taste for several days or even weeks after a bout of the flu.

Occasionally, the loss of smell or taste lasts for months or even becomes permanent. Cells that sense smell can be damaged or destroyed by serious infections of the nasal sinuses or by radiation therapy for cancer. However, the most common cause of permanent loss of smell is head trauma, as often occurs in a car accident. Fibers of the olfactory nerve--the nerve that contains smell receptors--are sheared at the cribriform plate--the bone at the skull base that separates the intracranial space from the nasal cavity. Rarely, a person is born without a sense of smell.

Oversensitivity to smell (hyperosmia) is much less common than anosmia. A distorted sense of smell that makes innocuous odors smell disagreeable (dysosmia) may result from infections in the sinuses or partial damage to the olfactory nerve. Dysosmia also may be caused by poor dental hygiene, which can result in mouth infections that smell bad and are sensed by the nose. Sometimes people with depression develop dysosmia. Some people who have seizures originating in the part of the brain where smell is perceived (the olfactory center) experience brief, vivid, unpleasant smell sensations (olfactory hallucinations). (see page 347 in Chapter 73, Seizure Disorders) These disagreeable smells are part of the seizure, not a misinterpretation of an odor.

A diminished or lost sense of taste (ageusia) is usually caused by conditions that affect the tongue. Examples are very dry mouth, heavy smoking (especially pipe smoking), radiation treatment to the head and neck, and side effects of drugs such as vincristine (a cancer drug) or amitriptyline (an antidepressant).

A distortion of taste (dysgeusia) may be caused by many of the same factors that result in loss of taste. Burns to the tongue may temporarily destroy taste buds, and Bell's palsy (a one-sided paralysis of the face caused by malfunction of the facial nerve) (see page 341 in Chapter 71, Cranial Nerve Disorders) may dull the sense of taste on one side of the tongue. Dysgeusia may also be a symptom of depression.

Diagnosis

Doctors can test smell using fragrant oils, soaps, and foods--coffee or cloves, for example. Taste can be tested using substances that are sweet (sugar), sour (lemon juice), salty (salt), and bitter (aspirin, quinine, aloes). A doctor or dentist also checks the mouth for infection or dryness (too little saliva). Rarely, a computed tomography (CT) or magnetic resonance imaging (MRI) scan of the brain is needed.

Treatment

Depending on the cause of the taste disorder, a doctor may recommend changing or stopping a suspected drug, keeping the mouth wet by sucking on candy, or just waiting several weeks to see if the problem disappears. Zinc supplements, which can be purchased without a prescription, are claimed to speed recovery, especially from taste disorders that follow a bout of the flu. The effect, however, has not been scientifically confirmed.

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