Taste (gustation) is attributed to sensory information received from the oral cavity and oropharynx and is the perception accompanying oral intake. Taste comprises five basic taste qualities: sweet, bitter, salty, sour, and umami. Umami (pleasant, good, desirable taste), identified a decade ago, is what the Japanese call the perception for monosodium glutamate; its taste resembles that of chicken bouillon.  Additional taste functions have been identified, with fat “taste” that may be mediated by receptor transduction and nonspecific transport across the cell membrane     and spicy “sensation” (e.g., capsaicin, ginger) mediated by small nerve fibers. However, the common sense of the word "taste" often means "flavor," which in turn is a composite of several (nongustatory) chemosensory afferents including taste, spice, texture, and temperature (both mediated by the trigeminal nerve - i.e., cranial nerve V) and, importantly, olfaction (more precisely, retro-olfaction) perceived while eating.    As such, a patient's complaints about taste loss do not always reflect the underlying pathology. Taste testing is mandatory to exclude a primary olfactory or trigeminal nerve pathology that manifests clinically to the patient as a problem with taste.
According to testing with taste strips, 5.3% of the people considered as healthy have hypogeusia although very few have complete ageusia.  The evaluation of a patient presenting with taste dysfunction comprises the patient's history (including drug intake and nutritional elements), a detailed clinical exam (including oral and dental exam), and investigations to determine the underlying etiology.   
Taste-receptor cells are located in the taste buds, which are located on taste papillae, primarily within the oral cavity. The taste system is highly redundant, with bilateral distribution and bilateral transmission along multiple cranial nerves. There are 4 types of papillae: fungiform, foliate, circumvallate, and filiform. All, apart from filiform, have taste buds. Afferent gustatory fibers run from the taste buds through gustatory pathways. The anterior two-thirds of the tongue is supplied by the chorda tympani, a branch of the facial nerve (cranial nerve VII). The gustatory fibers from the chorda tympani enter the brainstem via the nervus intermedius nerve (cranial nerve VII bis). At its distal end, the gustatory fibers with the chorda tympani join the lingual nerve, a part of the mandibular branch of the trigeminal nerve (cranial nerve V). Surgical procedures involving the posterior mandible may damage the lingual nerve, causing a transient or permanent loss of taste. The posterior third of the tongue is innervated by the lingual branch of the glossopharyngeal nerve (cranial nerve IX) and the base of the tongue and epiglottis by the vagus nerve (cranial nerve X). From the cranial nerves, the fibers converge in the nucleus tractus solitarius (NTS) in the brainstem. Leaving the NTS, the taste fibers still run ipsilaterally and cross partially at midbrain level, projecting in both thalami and insulae. Gustation may be altered at several levels within these pathways. Taste-receptor neurons, like olfactory-receptor neurons, are capable of regeneration.
Although it may be generally anticipated that taste complaints are related to the taste of food, some patients may report taste change in the mouth when no food is present, and in some cases of taste complaint, food may taste normal and cover the abnormal taste reported when not eating. These may be reported as taste phantoms. It is important to distinguish the nature of the complaint.
In contrast to what was thought for almost a century, it is now recognized that humans have at least 5 basic taste qualities that can be distinguished by taste receptors.  In contrast to the previous theory that distinct areas on the tongue are responsible for decoding only one taste quality (e.g., sweet on the tip of the tongue), current understanding is that the entire tongue is able to perceive each of the taste qualities. Nevertheless, there are areas, such as the tip and the rear of the tongue, with higher global taste sensibilities. However, data suggest that there might be a gustotopy, similar to the cortical tonotopy related to hearing.  
Taste is an important stimulus for production of saliva and, vice versa, saliva is necessary for the perception of taste. Saliva is the natural oral solvent that dissolves food components to reach the taste receptors. Taste is also influenced by salivary composition, as a specific quality must be above normal saliva concentration in order to be perceived. Epidermal growth factor and nerve growth factor in saliva in taste function may play a role. Thus, both saliva quantity and quality can influence gustatory sensation.  Three pairs of major salivary glands and hundreds of minor glands distributed throughout the oral and pharyngeal mucosa produce this complex fluid, which contains minerals, enzymes, and immune globulin (IgA). The system is regulated by the autonomic sympathetic and parasympathetic pathways. Physiologic, pathologic, and iatrogenic processes may interfere with salivary output and thus induce dysgeusia.
Local causes of taste change can be due to local/regional conditions, including poor oral hygiene, oral/dental/sinus disease, oral infection (e.g., candidiasis), oropharyngeal pathosis, diet, and tobacco use.
Dysgeusia: the general terminology for any kind of taste disorder
Parageusia: qualitative taste impairment, which delineates a triggered taste distortion (e.g., bitter, metallic, or other taste perception occurs with eating/drinking)
Phantogeusia: qualitative taste impairment, which delineates a non-triggered, permanent or intermittent taste distortion, which includes several complaints, such as metallic taste or permanent bitter, sour, salty, or (even rarer) sweet taste
Hypogeusia: a quantitative taste disturbance producing reduced taste function
Ageusia: a quantitative taste disturbance producing absence of taste.
Any of these, except ageusia, can be lowered, enhanced, or unaffected by eating/drinking. These quantitative and qualitative disorders can occur together or alone (e.g., a patient with a bitter parageusia can have a normal or altered measured taste function).
Within the literature, "dysgeusia" is often used variably either as qualitative, quantitative, or general taste disorder, which can cause confusion.
Other classifications are based on the anatomic site of the lesion (e.g., peripheral nerve lesion, brainstem lesion, thalamic or fronto-orbital lesion).
The epidemiology of taste disorders is unclear. Most specialized smell and taste clinics report having 10 times more olfactory disorders than pure gustatory impairments;  however, this may reflect patterns of referral rather than true epidemiology. Olfactory impairment or loss occurs in 5% of the general population, so gustatory impairment may be present in about 0.5% of the population.  Furthermore, taste disorders may not be noticed by the patient or the physician (e.g., in stroke) and, if testing is not performed, may be unrecognized.  The lack of recognition and the physiologic particularities concerning taste, such as the capacity of touch to be interpreted as taste perception, suggest that taste impairments are underreported. Data suggest that around 5% of the general population have lowered taste function without necessarily being bothered by it or recognizing it.  Taste change is necessarily primarily based upon patient reports, which may underestimate the epidemiology of taste changes.
However, taste disorders are very common in head and neck cancer, with up to 75% of patients reporting taste loss. In some patients, full recovery does not occur and often umami taste is affected. Taste changes occur with regional radiation therapy, and with systemic chemotherapy and targeted therapies in patients having solid-tumor cancer or stem cell transplant therapy.  Approximately 60% of patients receiving systemic chemotherapy will report some dysgeusia, which has been associated with the type of cytotoxic agents and the presence of oral mucositis.  Alteration of sweet and salty perception in these patients may persist up to 3 years after cessation of cancer therapy. 
Weill Cornell Medical College
Division of Oncology
AB declares that he has no competing interests.
Director of Oral Health Services
Division of Head and Neck Surgery
Professor of Oral Medicine
Department of Surgery
City of Hope
JE declares that he has no competing interests.
Dr Andrei Barasch and Dr Joel Epstein would like to gratefully acknowledge Dr Stéphanie Collet, Dr Basile Nicolas Landis, and Dr Philippe Rombaux, previous contributors to this monograph. SC is an author of a reference cited in this monograph. BNL has been reimbursed for speaking on olfactory disorders at a neuroscience meeting in Switzerland and is an author of a number of references cited in this monograph. PR is an author of a number of references cited in this monograph.
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