Smell and Taste Disorders
Because 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 the ability to enjoy food and drink and to appreciate pleasant aromas. They can also interfere with the ability to notice potentially harmful chemicals and gases and thus may have serious consequences. Occasionally, impairment of smell and taste is due to a serious disorder, such as a tumor.
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 integrates the information so that flavors can be recognized and appreciated. Some tastes--such as salty, bitter, sweet, and sour--can be recognized without the sense of smell. However, more complex flavors (raspberry, for example) require both taste and smell sensations to be recognized.
 See the figure How People Sense Flavors.
Generally, when people are in their 50s, the ability to smell and to taste starts to diminish gradually. In about 40% of older people, the ability to smell is significantly reduced.
A reduced ability to smell (hyposmia) and loss of smell (anosmia) are the most common disorders of smell and taste. Because distinguishing one flavor from another is based largely on smell, people often first notice that their ability to smell is reduced when their food seems tasteless.
The ability to 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, the ability to smell may be reduced because odors are prevented from reaching the smell receptors (specialized nerve cells in the mucous membrane lining the nose). Because the ability to smell affects taste, food often does not taste right to people with colds. Smell receptors can be temporarily damaged by the influenza (flu) virus. Some people cannot smell or taste for several days or even weeks after a bout of the flu, and rarely, loss of smell or taste becomes permanent.
Occasionally, serious infections of the nasal sinuses or radiation therapy for cancer causes a loss of smell or taste that lasts for months or even becomes permanent. These conditions can damage or destroy smell receptors.
The most common cause of permanent loss of smell is a head injury, as often occurs in a car accident. Permanent loss of smell results when fibers of the olfactory nerves--the pair of cranial nerves that connect smell receptors to the brain--are damaged or sheared at the roof of the nasal cavity. The roof of the nasal cavity is formed by a bone (cribriform plate) that separates the brain from the nasal cavity. Damage to the olfactory nerves can also result from fractures of the cribriform plate or tumors near this bone. Very few people are born without a sense of smell.
Oversensitivity to smell (hyperosmia) is much less common than loss of smell. Pregnant women commonly become oversensitive to smell. Hyperosmia can also be psychosomatic. Psychosomatic hyperosmia is more likely to develop in people who have a histrionic personality (conspicuous seeking of attention with dramatic behavior (see Section 7, Chapter 105)).
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 nerves. Other causes of dysosmia are poor dental hygiene and mouth infections. Dysosmia may be a symptom of depression. Seizures originating in the part of the brain where memories of smell are stored--the middle part of the temporal lobe--may produce brief, vivid, unpleasant smells (olfactory hallucinations). These smells are the part of the seizure called an aura, not a smell disorder.
A reduction in the ability to taste (hypogeusia) or loss of taste (ageusia) is usually caused by conditions that affect the tongue. Such conditions include a very dry mouth, heavy smoking (especially pipe smoking), radiation therapy to the head and neck, and side effects of drugs such as the chemotherapy drug vincristine and the antidepressant amitriptyline. In Bell's palsy, the sense of taste is often lost on the front two thirds of only one side of the tongue (the side affected by the palsy).
A distortion of taste (dysgeusia) may be caused by many of the same conditions that result in loss of taste, including depression and seizures. Burns to the tongue may temporarily destroy taste buds.
Diagnosis and Treatment
To test smell, doctors hold common fragrant substances (such as soap, a vanilla bean, coffee, and cloves) under the person's nose, one nostril at a time. The person is then asked to identify the smell. Taste can be tested using substances that are sweet (sugar), sour (lemon juice), salty (salt), and bitter (aspirin, quinine, or aloes). Doctors and dentists also check the mouth for infection and dryness. Rarely, computed tomography (CT) or magnetic resonance imaging (MRI) of the head is needed to identify structural abnormalities (such as a tumor, an abscess, or a fracture) near the cribriform plate.
Treatment depends on the cause of a smell or taste disorder. For example, sinus infections and irritation are treated with steam inhalation, nasal sprays, antibiotics, and sometimes surgery (see Section 19, Chapter 221). Tumors are surgically removed or treated with radiation, but such treatment usually does not restore the sense of smell. Doctors may recommend changing or discontinuing a drug, sucking on candy to keep the mouth moist, improving dental hygiene, or waiting several weeks to see if the cause of the problem (such as the flu) disappears. Zinc supplements, which can be purchased without a prescription, are claimed to speed recovery, especially from taste disorders due to a bout of the flu. However, the claim has not been substantiated.
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