The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017
Dysgeusia
The medical term dysgeusia is derived from the Greek root words dys (abnormal) and geusia (taste). Symptoms can range from a mild distortion of taste, dysgeusia, to a complete absence of taste, ageusia. Taste is a special sense conveyed from the taste buds to the brain over several pathways. There are five basic tastes: sweet, sour, salty, bitter, and umami (savory or delicious taste).
Anatomy and Physiology
The human tongue and mouth have areas of special epithelium containing taste receptors. Roughly 50—100 taste receptor cells are gathered inside onion-shaped structures called taste buds. Taste receptors are found on the tongue, palate, epiglottis, pharynx, larynx, and upper esophagus (Simon et al., 2006). The tongue is coated with rough tufts of epithelium that help to protect taste buds and aid in food handling. Taste is sensed when food dissolves in saliva and bathes taste buds. This stimulates the receptor cells through different channels, signaling pathways, and cranial nerves to the gustatory (taste) area of the brain where basic taste is sensed.
The receptor cells from the front of the tongue and palate connect to branches of the facial nerve (7th cranial nerve), and the back of the tongue is supplied by the glossopharyngeal nerve (9th cranial nerve). The vagus nerve, or the 10th cranial nerve, brings taste information from the throat, epiglottis, and esophagus and can mediate the reflex of swallowing or gagging (Simon et al., 2006). Different tastes are appreciated by differing chemical reactions within the receptor cells. Dysgeusia results from a disruption in this process.
History
Taste is important in distinguishing safe from harmful foods and can trigger an acceptance reflex, swallowing, or an avoidance reflex, gagging. In ancient days important people had personal tasters to try their food first to avoid being poisoned. It is reported that current world leaders may have food tasters as part of their staff for similar reasons. In recent years, scientists have debunked the “taste map” myth that there are very specific areas of the tongue dedicated to each taste.
Tests
In order to test taste, the tongue should be moist. The nerves that convey taste should be tested on each side of the front and back of the tongue with the tongue protruded. The taste can be applied with a moistened cotton-tip applicator. Using a taste questionnaire can be helpful as those who state easily tasting saltiness, bitterness, sourness, and sweetness almost certainly do not have dysgeusia (Malaty & Malaty, 2013).
The majority of those complaining of taste problems are found to have smell loss instead (Cowart, 2011). Because the sense of smell is strongly linked with the perception of taste, the olfactory (smell) system should be investigated by examining smell sensation in each nostril of the person with dysgeusia. Viewing the nasal system through a special scope can help diagnose some conditions. The physician may need computerized tomography scanning of the nose and sinuses for diagnosis.
Significance
The sense of taste helps us to avoid harmful or spoiled food. Loss of taste may lead to decreased appetite, weight loss, and failure to thrive. Normally, women have better taste perception than men, and taste lessens with aging.
Dysgeusia is almost always due to loss of smell sensation resulting from diseases of the nose or sinuses, head injury, or certain neurological diseases. Dysgeusia may be caused by poor mouth hygiene or inflammation, surgery of the middle ear or dental surgery, radiation therapy to head and neck, vitamin deficiencies (copper, zinc, B12, or niacin) or malnutrition, medications, head trauma, toxins, and chronic medical conditions (Malaty & Malaty, 2013).
Treatment
The treatment is to carefully examine and identify the cause of distorted taste and eliminate or treat it. Determine whether distorted taste is due to nasal or mouth disease and address the underlying problem. If applicable, limit exposures to smoke or chemicals that may affect olfaction. If dysgeusia is from a new medication, stopping medication should result in gradual return of taste. This is true for chemotherapy exposure as well since taste receptors renew themselves about every 10 days.
The prognosis of dysgeusia depends on the severity of the loss and other factors. It is reported that one-third to one-half of individuals report a returning sense of smell over time (Malaty & Malaty, 2013). If dysgeusia persists, the person should be referred to a specialist, specifically an ear, nose, and throat doctor.
Lauren C. Seeberger
See also: Ageusia; Bitter Sensation; Salty Sensation; Sour Sensation; Sweet Sensation; Taste Aversion; Taste System; Umami
Further Reading
Cowart, Beverly J. (2011). Taste dysfunction: A practical guide for oral medicine. Oral Diseases, 17, 2—6.
Malaty, John, & Irene A. C. Malaty. (2013). Smell and taste disorders in primary care. American Family Physician, 88(12), 852—859.
Simon, Sidney A., Ivan E. de Araujo, Ranier Gutierrez, & Miguel A. L. Nicolelis. (2006). The neural mechanisms of gustation: A distributed processing code. Nature Reviews Neuroscience, 7, 890—901.