What the Numbers Mean: An Epidemiological Perspective on Taste and Smell

Causes of Taste and Smell Loss

Smell and taste loss are relatively common problems that have a tremendous impact on a person’s quality of life. While the sensory systems of smell (olfaction) and taste (gustation) are often grouped together clinically, each has its own unique features and potential clinical problems. The brain processes taste and smell differently, so damage to either sense can cause different types of problems, depending on the degree of dysfunction.

Fortunately, a complete loss of smell, taste, or both, is rare, and individuals can often compensate for partial loss. Also, taste and smell are the only sensory systems that have the capacity to regenerate after damage, so some recovery of function is usually possible.

Major determinants of taste and smell loss include:

• Sex and age
  • Age-related smell loss is more prominent in men.
  • Taste is more resistant to age-related loss.
• Environment
  • Airborne toxic odors can permanently damage the sense of smell.
• Trauma
  • Frontal head injury can permanently damage the sense of smell.
• Disease or illness
  • Chronic rhinitis and allergies can adversely affect the sense of smell and reduce the appreciation of food; the effect may be short-term or permanent.
  • Infections of the middle ear can partially damage the sense of taste.
• Prescription medications
  • Certain prescription drugs can affect taste quality perception.
• Cancer treatment
  • Chemotherapy and radiation treatment for head and neck cancer can damage the sense of taste.

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Assessing and Differentiating Taste and Smell Loss

Mild taste and smell loss often goes unnoticed. Some smell loss may accompany seasonal nasal congestion or a head cold. This type of smell loss usually goes unreported and unmeasured, because people generally consider it to be a temporary and mild annoyance.

More serious smell loss can be caused by nasal obstruction that requires corrective surgery or by chronic viral infections with swelling that require special medications. This type of loss could be dangerous because an individual may fail to detect smoke or spoiled food. It is sufficiently distressing that the patient may seek help.

Otolaryngologists—sometimes called otorhinolaryngologists—are physicians who specialize in diseases of the ear, nose, and throat, including problems affecting taste and smell. An otolaryngologist will conduct a physical examination of the nose and mouth and perform customized tests to assess the degree of any smell and/or taste loss.

As part of the exam, the physician will take a personal history and ask the patient to complete a questionnaire to fully describe the problem ("self-reporting"). It is very important that the physician diagnose whether the loss is due to a problem with the sense of smell, taste, or both. In many cases, patients claim a loss of taste because food has become less enjoyable or seems bland, but testing shows that the problem actually involves the sense of smell.

This example points to a very important interaction between taste and smell: our appreciation of food flavor is a complicated mix of taste (sweet, sour, salty, bitter), smell (aromas), texture (smoothness), and temperature (hot and cold) sensations felt through chewing or drinking. Food aromas generated by chewing activate the sense of smell by way of a special channel in the roof of the throat. If this channel is blocked due to nasal inflammation, for example, aromas can not reach the person’s sense of smell, and a major contributor to the enjoyment of flavor is lost.

Unlike hearing or vision tests, the tests to measure taste and smell loss are not based upon a standard, internationally-accepted procedure. There are several different tests for taste and smell, and the choice of test may be due to convenience, ease-of-use, the nature of the loss, and cost.

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Tests for the Loss of Smell

In the clinical setting in the United States, where time, cost, and insurance are critical factors, smell loss is usually assessed with a "scratch-and-sniff" test. This test consists of a booklet with 10–40 individual sheets that have microencapsulated beads impregnated with specific odors. The patient scratches the beads with a pencil to release the odor and then identifies the odor from a list of five choices. This type of test measures one’s ability to recognize a particular odor, but does not measure one’s sensitivity or threshold to certain smells. Once the test is completed, the number of errors is recorded and then compared against a standard number of responses by normal individuals. This type of test is relatively quick (under 30 minutes), novel, and fun. The test assumes a normal level of cognitive function, so it is not suitable for individuals with dementia and other types of neurodegenerative disease.

A more exacting test of smell function can be conducted in the academic research environment or in clinical trials. In these cases, patients are recruited to participate in a series of tests in which time and convenience are secondary to a more thorough examination. These examinations can include measures of threshold sensitivity and odor identification. The test equipment is often customized and not suitable for a physician’s office. Its use requires special training.

An "olfactometer" is an odor delivery system that allows the technician to pass precise concentrations of a known odor to the nose of the patient. The patient indicates the lowest concentration that is detectable, and names the odor. In this way, the patient’s smell response profile can be determined and compared against a normal standard.

An accurate assessment of smell loss includes:

• Physical examination of the ears, nose, and throat.
• Personal history (for example, exposure to toxic chemicals or trauma).
• Questionnaire (self-report).
• Validated test supervised by a health care professional.
• Discussion of treatment options, such as surgery, antibiotics, or steroids.

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Tests for the Loss of Taste

The sense of taste regulates food intake and choice and is essential for life. Consequently, the brain organization of the sense of taste is complicated and redundant. It is very difficult to damage the taste system completely without a threat to life. Specialized taste cells are located throughout the tongue, roof of the mouth, and throat, so it is very difficult to test the entire system.

Two types of taste tests are used. The first test uses "whole mouth stimulation" or the "sip and spit" method. In this method, a solution of a known concentration of a sweet, salty, bitter, or sour substance is gargled and sloshed in the mouth and then discarded. The patient is asked to identify the taste substance, and the concentration can be varied to determine threshold sensitivity. It is an easy test to administer but assumes severe taste loss. Regional damage (for example, on the front or tip of the tongue) would be masked by stimulation of the remaining taste cells elsewhere in the mouth.

The second test uses a more localized application of taste substances. In this test, filter paper or a dissolvable strip is impregnated with a known concentration of a sweet, salty, bitter, or sour substance, and the filter paper or strip is placed on a specific part of the tongue or palate. The patient is asked to identify the taste substance. The concentration can be varied to determine threshold sensitivity. This test is also easy to administer. The goal is to activate major regions of taste cells to determine whether the individual has partial taste deficit or damage.

An accurate assessment of taste loss includes:

• Physical examination of the ears, nose, and throat.
• Dental examination and assessment of oral hygiene.
• Personal history (for example, trauma to mouth, cancer, or medications).
• Questionnaire (self-report).
• Validated test supervised by a health care professional.
• Discussion of treatment options, such as proper oral hygiene or different medications.

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Implications of Taste and Smell Loss

Smell and taste contribute greatly to our food choices, diet, nutrition, and overall health status. Studies have demonstrated that the ability to recognize some tastes is apparent at birth—for example, the pleasure response to sweetness and the disgust at bitterness. A response to salt develops during the first year of life. There is considerable genetic and cultural variability in our food preferences, and these have significant associations with a variety of health risk factors such as obesity, high blood pressure, and cardiovascular disease.

A genetic trait in humans (although it may be modified by external or environmental causes) is the ability or inability to taste the bitter quality of two related substances: phenylthiocarbamide (PTC) or 6-n-propylthiouracil (PROP). Tasters of PROP and especially super-tasters (those who rate PROP as being extremely bitter) have been shown to have a more intense appreciation for other tastes, such as other bitter substances, sweet, salt, and acidic tastes. Super-tasters, tasters, and non-tasters of PROP may be described as living in different sensory worlds, with super-tasters experiencing a neon world and non-tasters experiencing a pastel world. Tasters and non-tasters may also have differing health risk profiles, including increased or decreased susceptibility to certain diseases, and preference or tolerance for tea, coffee, alcohol, or cigarettes. This is a relatively new and growing field of investigation.

A complete loss of the ability to smell (anosmia) or taste (ageusia), although rare, may seem insignificant compared to blindness and deafness. Sufferers of these conditions, however, describe their lives as being nearly unbearable since they are deprived of the many pleasant tastes and odors of daily living. The consequences of the more common conditions of a reduced or diminished ability to smell (hyposmia) or taste (hypogeusia) go well beyond the inconvenience of a bland meal. Impairment of smell or taste could result in eating spoiled or contaminated food or not being aware, through smell, of a gas leak. Some patients complain not of smell or taste loss but, instead, of distortions such as persistent unpleasant or painful sensations described as bad, foul, rancid, acidic, metallic, or burning. These problems may result in diagnosis of a condition known as dysgeusia (altered or abnormal tastes) or burning mouth syndrome. Such patient complaints increase with age and lead to major quality of life issues.