• Smell may be lost temporarily when a person smokes or has a cold or seasonal allergy.
• Smell may be lost permanently after a head injury.
• People may lose their sense of taste if they have a condition that causes a very dry mouth.
• Doctors can test smell by using common fragrant substances.
• Taste is tested by using sweet, salty, sour, and bitter substances.
• Infections are treated with antibiotics, and blockages are removed, but sometimes the ability to smell is not restored.

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, and 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 (such as raspberry) require both taste and smell sensations to be recognized.

How People Sense Flavors To distinguish most flavors, the brain needs information about both smell and taste. These sensations are communicated to the brain from the nose and mouth. Several areas of the brain integrate the information, enabling people to recognize and appreciate flavors. A small area on the mucous membrane that lines the nose (the olfactory epithelium) contains specialized nerve cells called smell receptors. These receptors have hairlike projections (cilia) that detect odors. Airborne molecules entering the nasal passage stimulate the cilia, triggering a nerve impulse in nearby nerve fibers. The fibers extend upward through the bone that forms the roof of the nasal cavity (cribriform plate) and connect to enlargements of nerve cells (olfactory bulbs). These bulbs form the cranial nerves of smell (olfactory nerves). The impulse travels through the olfactory bulbs, along the olfactory nerves, to the brain. The brain interprets the impulse as a distinct odor. Also, the area of the brain where memories of odors are stored—the smell and taste center in the middle part of the temporal lobe—is stimulated. The memories enable a person to distinguish and identify many different odors experienced over a lifetime. Thousands of tiny taste buds cover most of the tongue's surface. A taste bud contains several types of taste receptors with cilia. Each type detects one of the five basic tastes: sweet, salty, sour, bitter, or savory (also called umami, the taste of monosodium glutamate). These tastes can be detected all over the tongue, but certain areas are more sensitive for each taste. Sweetness is most easily identified by the tip of the tongue, whereas saltiness is best appreciated at the front sides of the tongue. Sourness is best perceived along the sides of the tongue, and bitter sensations are readily detected in the back one third of the tongue. Food placed in the mouth stimulates the cilia, triggering a nerve impulse in nearby nerve fibers, which are connected to the cranial nerves of taste (the facial and glossopharyngeal nerves). The impulse travels along these cranial nerves to the brain, which interprets the combination of impulses from the different types of taste receptors as a distinct taste. Sensory information about the food's smell, taste, texture, and temperature is processed by the brain to produce a distinct flavor when food enters the mouth and is chewed.

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.

Causes

Smell: 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. Polyps, tumors, other infections in the nose, seasonal allergies (allergic rhinitis), and smoking (tobacco) may interfere with the ability to smell. 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.

Did You Know? Occasionally, smell and taste disorders are due to a serious disorder, such as a tumor. Because the ability to smell and taste decreases with aging, older people may eat less and become undernourished.

A common cause of permanent loss of smell is a head injury, as may occur 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 from infections (such as abscesses) or tumors near this bone.

Alzheimer's disease and some other degenerative brain disorders can also damage the olfactory nerves, commonly causing of loss of smell. A very few people are born without a sense of smell.

Spotlight on Aging
After age 50, the ability to smell and to taste gradually begins to decrease. The membranes lining the nose become thinner and drier, and the nerves involved in smell deteriorate. Older people can still detect strong smells, but detecting subtle smells is more difficult. As people age, the number of taste buds also decreases, and those that are left become less sensitive. These changes tend to reduce the ability to taste sweet and salt more than the ability to taste sour and bitter. Thus, many foods start to taste bitter. Because smell and taste are diminished as people age, many foods taste bland. The mouth tends to be dry more often, further reducing the ability to taste and smell. Also, many older people have a disorder or take drugs that contribute to dry mouth. Because of these changes, older people may eat less. Then, they may not get the nutrition they need, and if they already have a disorder, their condition may worsen.

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 (characterized by conspicuous seeking of attention with dramatic behavior---see Personality Disorders: Histrionic (Hysterical) Personality).

Some disorders can distort the sense of smell, making innocuous odors smell disagreeable (a condition called dysosmia). These disorders include the following:

• Infections in the sinuses
• Partial damage to the olfactory nerves
• Poor dental hygiene
• Mouth infections
• Depression
• Viral hepatitis, which may cause dysosmia that results in nausea triggered by otherwise inoffensive odors

Seizures originating in the part of the brain where memories of smell are stored—the middle part of the temporal lobe—may produce a brief, false sensation of vivid, unpleasant smells (olfactory hallucinations). These smells are part of the intense feeling that a seizure is about to begin (called an aura) and do not indicate a smell disorder. Brain infections due to herpesviruses (herpes encephalitis) may also cause olfactory hallucinations.

Taste: A reduction in the ability to taste (hypogeusia) or loss of taste (ageusia) usually results from conditions that affect the tongue, usually by causing a very dry mouth. Such conditions include Sjögren's syndrome, heavy smoking (especially pipe smoking), radiation therapy to the head and neck, dehydration, and use of drugs (including antihistamines and the antidepressant amitriptyline). Nutritional deficiencies, such as decreased zinc, copper, and nickel levels, can alter both taste and smell.

In Bell's palsy, the sense of taste is often impaired on the front two thirds of one side of the tongue (the side affected by the palsy). But this loss may not be noticed because taste is normal or increased in the rest of the tongue. Burns to the tongue may temporarily destroy taste buds. Neurologic disorders, including depression and seizures, may impair taste.

A distortion of taste (dysgeusia) may be caused by inflammation of the gums (gingivitis) and by many of the same conditions that result in loss of taste or smell, including depression and seizures. Taste may be distorted by some drugs, such as the following:

• Antibiotics
• Anticonvulsants
• Antidepressants
• Certain chemotherapy drugs
• Diuretics
• Drugs used to treat arthritis
• Thyroid drugs

Diagnosis

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. Smell can also be tested more formally using standardized commercial smell test kits. Taste can be tested using substances that are sweet (sugar), sour (lemon juice), salty (salt), and bitter (aspirinSome Trade Names
BAYER
, quinineSome Trade Names
QUALAQUIN
, or aloes).

Doctors and dentists check the mouth and nasal passages for abnormalities, including infection and dryness. If the cause is not apparent, computed tomography (CT) or magnetic resonance imaging (MRI) of the head is needed to look for structural abnormalities (such as a tumor, an abscess, or a fracture) near the cribriform plate.

Treatment

Treatment depends on the cause of a smell or taste disorder. For example, sinus infections and irritation may be treated with steam inhalation, nasal sprays, antibiotics, and sometimes surgery (see Nose, Sinus, and Taste Disorders: Treatment). Nutritional deficiencies need to be corrected. Tumors are surgically removed or treated with radiation, but such treatment usually does not restore the sense of smell. Polyps in the nose are removed, sometimes restoring the ability to smell. People who smoke tobacco should stop. Other recommendations may include the following:

• Changing or stopping a drug
• Sucking on candy to keep the mouth moist
• Improving dental hygiene
• Waiting several weeks to see if the cause of the problem (such as the flu) disappears

Rarely, zinc supplements, which can be purchased without a prescription, are effective, especially for distortion of smell or for reduction or distortion of taste when no cause has been identified.

Last full review/revision July 2008 by Michael Jacewicz