Worddefinition
At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas.
Disorders of gustation
- Updated 05.28.2024
- Released 10.17.2022
- Expires For CME 05.28.2027
Disorders of gustation
Introduction
Historical note and terminology
Galen (129-201 CE) correctly described the different functions of the three principal nerves supplying the tongue (lingual, glossopharyngeal, and hypoglossal nerves) and demonstrated their origin at the base of the brain (217).
Printed medical illustrations began in 1490 and, by the beginning of the 16th century, included representations of afferent connections from the special sensory organs to the brain (117). These were typically part of highly schematic diagrams of brain function representing the medieval cell doctrine, in which usually three “cells” or ventricles were assigned functions of sensory integration and imagination, cognition, and memory (118). Indeed, many early-16th-century woodcuts of the medieval cell doctrine show presumptive connections between the organs subserving the special senses, either with the most anterior cell or ventricle of the brain or with a specific portion of it--the sensus communis (ie, sensory commune, or common sense, a structure Aristotle had postulated is responsible for monitoring and integrating the panoply of sensations from which unified conscious experience arises) (117; 118; 119). A representation of the tongue and presumptive pathways to the brain are incorporated into many of these woodcuts (119; 119). These stereotyped 16th-century schematic images typically linked the tongue to an anterior cell or a portion of it. Such images linked the gustation via the tongue to "cells" of "ventricles" before the advent of more realistic images and schematics began to show the neural gustatory pathways beginning in the mid-sixteenth century.
Observational anatomy was largely lost from the time of Galen in the second century, and it became regimented and dogmatized with the scholasticism of the Middle Ages until a few anatomists began to seriously challenge Galen beginning in the 16th century. Most notably, Flemish anatomist Andreas Vesalius (1514-1564) provided much greater realism with the publication of his de Humani corporis fabrica (1543).
Vesalius depicted dissection of the face, isolating the tongue, as part of one of his elaborate depictions of the famous “musclemen.”
-
Vesalian "muscleman" woodcut, including dissection of the mouth and mandible (close-up detail)
The mandible has been split in the midline and separated. (Source: Vesalius' de Humani corporis fabrica [1543]. Courtesy of the U.S. National Library of Medicine, Bethesda, Maryland. Woodcut print edited by Dr. Douglas...
Vesalius also separately made a schematic of the cranial nerves, including innervation of the tongue, but the pathway for taste sensation had not been determined by this point.
Even after Vesalius championed a return to observational anatomy, the medieval cell doctrine and its associated representation of the olfactory pathways persisted well into the 19th century, even if it was gradually moved to the fringes of medical thought (119).
By the 18th century, anatomists were working to realistically represent anatomical structures, as epitomized by the copperplate engravings of Dutch Golden Age physician, anatomist, poet, and playwright Govert Bidloo or Govard Bidloo (1649-1713).
This continued into the 19th century, with amazingly realistic and detailed colored lithographs, including the outstanding examples of Irish-born English anatomist Richard Quain (1800-1887) and German anatomist Christian Wilhelm Braune (1831-1892) in his topographical anatomy atlas (1867-1872).
-
Dissection of the head and neck showing the lingual nerve, detail (cut away)
The lingual nerve passes through the cheek and underneath the anterior border of the mandible. The lingual nerve has been colored blue here for ease of visualization. (Source: Quain R. The anatomy of the arteries of the human b...
The structure of the tongue surface was first described in 1609 by Italian anatomist Giulio Cesare Casseri (1552-1616), and then in 1665 by Italian physician-anatomist Lorenzo Bellini (1643-1704), who published the observations made by Italian biologist, physician, and pioneering microscopist Marcello Malpighi (1628-1694) the previous year (100; 217). After the discovery of taste buds in fishes by German zoologist and comparative anatomist Franz von Leydig (1821-1908) in 1851, German anatomist and zoologist Franz Eilhard Schulze (1840-1921) suggested in 1863 that they were chemosensory structures. Nineteenth-century studies focused on cytologic features and nerve supply. By the late 19th century, physicians were readily examining tongues for clinical clues to their patients' illnesses, and this became the subject of numerous caricatures.
-
A physician examines a patient's tongue. Lithograph by Charles Vernier, 1853
"Le docteur. _ Ah! ... si vous avez mangé du raisin tout s'explique: vous avez alors la maladie des pommes de terre dont le remède est encore inconnu ... tranquillisez-vous!" [“The doctor. _ Ah! ... if you have eaten grapes, ev...
-
Sick man projects his tongue while a doctor takes his pulse. Colored lithograph
The patient holds green spectacles in his left hand. The woman holds a cup with a spoon. Lettering: Der patient (Das Vertrauen). A patient (Confidence). (Source: F. Silber, Berlin, Germany. Courtesy of the Wellcome Library, Lon...
Clinical manifestations
Presentation and course
• Patients with gustatory symptoms generally report diminution or absence of gustatory sensation (hypogeusia or ageusia, respectively) and forms of distorted gustation (parageusia). | |
• Except in unusual circumstances, hypergeusia is a subjective sensation of hyperacuteness of gustation. | |
• There is no evidence that pregnant women or migraineurs experience an objective increase in gustatory sensitivity. | |
• When patients report alterations in the quality of gustation in response to a tastant (ie, parageusias), the perceptions are almost universally unpleasant, a condition referred to as aliageusia (the perception of unpleasant taste from something that usually tastes pleasant). | |
• Aliageusias may involve the perception of fecal or rotten tastes (cacogeusia) or chemical or burned tastes (torqugeusia). | |
• Complaints of impaired “taste” are often symptoms of olfactory dysfunction because much of the flavor of a meal derives from olfactory stimulation. Indeed, the complex sensory experience of “flavor” during the consumption of foods and drinks cannot be constructed simply from combinations of the basic taste qualities (sweet, salty, sour, bitter, and umami or savory). | |
• Chemosensory deficit may be the first symptom--a "sentinel symptom"--in patients with COVID-19, but there is wide variation in the proportion of cases in which this is reported to occur. | |
• Presbygeusia (literally “elderly gustation” or “old-age gustation”) is the gradual loss of gustatory abilities that occurs in most people as they grow older. | |
• Clinically significant gustatory loss is common in the elderly but frequently unrecognized, in part, because deficits typically accumulate gradually over decades--indeed, self-reported gustatory impairment significantly underestimates prevalence rates obtained by gustatory testing. |
Patients use various expressions to describe disorders of taste or gustation (Table 1) (116).
Table 1. Terminology for Gustatory Function
Category |
Term | ||
Disturbance of perception (any) |
Dysgeusia | ||
Alteration of quantity |
Absent |
Ageusia | |
Decreased |
Hypogeusia | ||
Normal |
Normogeusia | ||
Increased |
Hypergeusia | ||
Alteration of quality |
Distorted (any) [general term] |
Parageusia | |
Specific distortions | |||
1 |
Any unpleasant distortion of taste |
Aliageusia | |
2 |
"Abhorrent, obnoxious," typically fecal or putrid |
Cacogeusia | |
3 |
"Twisted": chemical, metallic, bitter, salty, or burned |
Torqugeusia | |
4 |
Inappropriate chemosensory quality of consistent nature1 |
Heterogeusia | |
Hallucination2 |
Phantageusia | ||
|
Patients with gustatory symptoms generally report diminution or absence of gustatory sensation (hypogeusia or ageusia, respectively) and forms of distorted olfaction (parageusia). Except in unusual circumstances, hypergeusia is a subjective sensation of hyperacuteness of gustation. There is, for example, no evidence that pregnant women or migraineurs experience an objective increase in gustatory sensitivity.
When patients report alterations in the quality of gustation in response to a tastant (ie, parageusias), the perceptions are almost universally unpleasant, a condition referred to as aliageusia (the perception of unpleasant taste from something that usually tastes pleasant). Aliageusias, for example, may involve the perception of fecal or rotten tastes (cacogeusia) or of chemical or burned tastes (torqugeusia).
The terminology for alterations in the quality of a gustatory perception (eg, cacogeusia, torqugeusia) can also be employed with hallucinated odors (phantageusias); thus, one can speak of a "cacogeusic phantageusia" for the hallucination of a fecal taste or a "torqugeusic phantageusia" for the hallucination of a burned taste (82).
In addition, complaints of impaired “taste” are often symptoms of olfactory dysfunction because much of the flavor of a meal derives from olfactory stimulation. Indeed, the complex sensory experience of “flavor” during the consumption of foods and drinks cannot be constructed simply from combinations of the basic taste qualities (sweet, salty, sour, bitter, and umami or savory).
Dysgeusia in COVID-19. Disorders of the smell and taste are more common among individuals with COVID-19 than among individuals with influenza (28). Chemosensory clinical symptoms are present in at least half of patients with COVID-19 (05; 20; 145; 146; 168; 59). In one study of 922 participants, 20% reported ageusia, 5% reported hypogeusia, and 32% reported hypogeusia and dysgeusia (05).
The risk of COVID-19-associated smell or taste disturbance has progressively fallen with successive waves of infection with the Alpha, Delta, Omicron K, Omicron L, Omicron C, and Omicron B variants, based on their peak intervals (169); consequently, since the Omicron waves, smell and taste disturbances have been of less predictive value in the diagnosis of COVID-19 infection (169).
A chemosensory deficit may be the first symptom--a "sentinel symptom"--in patients with COVID-19, but there is wide variation in the proportion of cases in which this is reported to occur (39; 40; 41; 66; 99; 101; 160; 177; 38; 163). A meta-analysis of eight studies involving 11,054 patients with COVID-19 reported that olfactory and gustatory symptoms appear prior to general COVID-19 symptoms in 65% and 54% of the patients, respectively, based on European, United States, and Iranian data (180). However, a systematic review of 17 studies found the onset of dysosmia and dysgeusia occurred 4 to 5 days after other symptoms of the infection and that these chemosensory symptoms typically improved after 1 week, with more significant improvements in the first 2 weeks (182).
New-onset chemosensory dysfunction is also common in patients with symptomatic COVID-19 after complete vaccination. In one study of 153 such patients, approximately half reported associated rhinorrhea, which is a much higher proportion than in unvaccinated cases (209).
Available reports present conflicting data on whether COVID-19 severity is associated with either the degree of gustatory dysfunction or the prognosis for recovery of gustation, but multiple reports suggested that chemosensory symptoms were most common among those with milder presentations of COVID-19 (25; 43; 167; 184).
Psychophysical testing showed significantly reduced intensity perception and identification ability for both taste and smell functions in patients with COVID-19 (31), although some studies using quantitative testing found olfactory disturbances in nearly all subjects during the acute infection phase, whereas taste or chemesthetic deficits were low (65; 146). Curiously, psychophysical data produced substantially lower estimates of dysgeusia after COVID-19, perhaps because of misinterpretation of impaired retronasal olfaction as gustatory dysfunction (86). Smell and taste loss in COVID-19 are closely associated, although a minority of individuals can experience one or the other (32; 59). Most patients with taste dysfunction after COVID-19 did not have true taste dysfunction, but rather perceived altered flavor sensations associated with olfactory dysfunction (146). The taste of salt may be less severely affected than other taste qualities (32), whereas complete loss of bitter taste was the most prevalent specific ageusia (173).
Chemosensory impairment may be persistent (17; 20; 145; 146; 168), even 1, 2, or 3 years after mild COVID-19 (17; 20; 146; 191), although chemosensory recovery from the Omicron BA.1 subvariant was more favorable than that after the first wave of the pandemic (19). In a prospective observational study, measuring the prevalence of altered sense of smell or taste at follow-up and their variation from baseline, on 403 consecutively assessed adult patients who tested positive for SARS-CoV-2 RNA by polymerase chain reaction during March 2020, 66% reported an altered sense of smell or taste at baseline, whereas 14%, 7%, and 5% reported such alterations at 6 to 24 months, 2 years, and 3 years, respectively (20). Late improvement was possible: of the patients who still experienced smell or taste dysfunction 2 years after COVID-19, 28% and 38% recovered completely and partially, respectively, at the 3-year follow-up (21). Olfactory dysfunction is most likely to persist after 1 year, whereas objectively measured taste dysfunction has typically recovered by that time (191).
Gustatory hallucinations (phantageusias)
Gustatory hallucinations (phantageusias) can occur through irritative (eg, epilepsy, migraine) or "release" (eg, ageusia or hypogeusia) mechanisms (42).
Phantageusia and phantosmia are associated with decreased brain GABA levels (126). In a study of olfactory and gustatory hallucinations in 28 normal volunteers and 19 patients with persistent phantageusia or phantosmia, GABA levels in several CNS regions (ie, the cingulate, right and left insula, and left amygdala) were lower in patients before treatment than in normal volunteers and were the only biochemical changes found (126). After treatment that inhibited sensory distortions, CNS GABA levels increased to levels at or near normal, consistent with functional remission of these symptoms.
Epileptic olfactory and gustatory hallucinations. Olfactory hallucinations often occur with a lesion in the inferior and medial parts of the temporal lobe, whereas gustatory hallucinations can occur in temporal lobe disease with lesions in the insula and parietal operculum (109). Of course, epileptic discharges can also spread to these areas from areas that are proximate or intimately connected with the temporal lobe, operculum, and insula (96). For example, a 4-year-old boy had ictal fear as his primary epileptic manifestation, associated with complex visual and gustatory hallucinations, oral automatisms, and pain in the left leg; investigation showed that his seizures resulted from the rapid spread of epileptic discharges from his frontal lobes to the right anterior temporal region (96).
Of 718 patients investigated for intractable epilepsy with stereo-electroencephalography (SEEG), 30 (4%) manifested gustatory hallucinations as part of their seizures (77). Gustatory hallucinations, which occurred as one manifestation of parietal, temporal, or temporoparietal seizures, resulted from disorganization of the parietal or Rolandic operculum. Electrically induced seizures, which included a gustatory hallucination as an ictal event, occurred mainly with stimulation of the hippocampus and amygdala.
Selected cases with gustatory hallucinations indicate the range of lesions responsible:
Temporal lobe epilepsy following seronegative limbic encephalitis. A 25-year-old man with drug-resistant temporal lobe epilepsy following seronegative limbic encephalitis related to non-Hodgkin lymphoma experienced musicogenic right temporal seizures triggered by listening to rock music; the seizures were characterized by déjà vu, piloerection, and gustatory hallucinations (149). | |
Third-ventricular colloid cyst. A 46-year-old man experienced prolonged episodes of olfactory and gustatory hallucinations that abruptly resolved with stereotaxic aspiration of a third-ventricular colloid cyst (58). | |
Partial status epilepticus, resulting from glioblastoma multiforme involving the hippocampus and amygdala. In one patient, new-onset olfactory and gustatory hallucinations were a presenting feature of partial status epilepticus resulting from a previously unrecognized glioblastoma multiforme involving the hippocampus and amygdala (29). | |
Schizencephaly. A patient with schizencephaly and normal intelligence had frequent simple partial epileptic seizures with visual, auditory, and gustatory hallucinations (15). |
In contrast to individuals with "purely temporal" epilepsies, those with "temporal plus" epilepsies--characterized by seizures involving a complex epileptogenic network including the temporal lobe and neighboring structures (eg, orbito-frontal cortex, insula, frontal and parietal opercula, and temporo-parieto-occipital junction)--more frequently have gustatory hallucinations, rotatory vertigo, and auditory illusions at seizure onset (12). Indeed, the most frequently reported auras among individuals with "purely temporal" epilepsies are gustatory hallucinations, vestibular illusions, laryngeal and throat constriction, and atypical somatosensory symptoms (perioral and bilateral acral paresthesias, etc.) (08).
Although déjà vu is common and qualitatively similar whether it occurs as an epileptic aura or as a normal phenomenon, patients with epilepsy are more likely to report olfactory and gustatory hallucinations, prior fatigue and concentrated activity, associated derealisation, fear, and physical symptoms such as headaches or abdominal sensations (213).
Although epileptic presentations of phantageusia are typically "irritative" hallucinations in Cogan's dichotomy of hallucinations, some cases of postictal phantageusia may instead represent "release" hallucinations akin to a Todd paralysis (42). For example, a 32-year-old man with epileptic seizures due to a grade 1 astrocytoma of the right sylvian region experienced gustatory hallucinations as a postictal symptom (07).
Olfactory and gustatory hallucinations in Parkinson disease. Bannier and colleagues reported that olfactory hallucinations occurred in 10% of the 87 patients with Parkinson disease in their series (11). Olfactory hallucinations were rare, short-duration, unpleasant odors that were not frightening and were recognized by the patients as hallucinations (like their visual hallucinations typically are recognized as unreal). Compared with controls, patients with Parkinson disease and olfactory hallucinations had olfactory impairment of both detection and identification, but there was no difference in their olfactory abilities compared with patients with Parkinson disease but no olfactory hallucinations.
Solla and colleagues similarly reported that olfactory hallucinations occur frequently in patients with Parkinson disease, particularly in women, and are often concomitant with visual and auditory hallucinations without any association with olfactory impairment (198). In a sample of 273 patients, 141 with Parkinson disease and 132 healthy controls of similar age, olfactory hallucinations were found in 11% of those with Parkinson disease and none of the controls (198). Among patients with Parkinson disease and olfactory hallucinations, 3% also had gustatory hallucinations (198). Visual or auditory hallucinations and female gender were independent predictors of olfactory hallucinations (198).
Other studies have reported lower frequencies of olfactory and gustatory hallucinations in patients with Parkinson disease. Holroyd and colleagues studied 98 patients with Parkinson disease, among which there were 26 with visual hallucinations, only one of which (1% of the total) was reported to have concomitant gustatory hallucinations, whereas none apparently had auditory, tactile, or olfactory hallucinations (89). None of the patients, therefore, had any unimodal hallucinations other than visual.
Olfactory and gustatory hallucinations in migraine. Olfactory and gustatory hallucinations occur occasionally in migraineurs (09; 150). Among 46 new female referrals for migraine headache, 15% reported olfactory or gustatory hallucinations and distortions of body image as part of most migraine attacks (150).
Olfactory and gustatory hallucinations in psychotic disorders. Among patients with schizophrenic or schizoaffective disorder, auditory hallucinations are by far the most common, followed by visual hallucinations and then by tactile and olfactory or gustatory hallucinations (152).
There are subgroups with psychosis that experience a higher frequency of olfactory and gustatory hallucinations. A subgroup of schizophrenia, characterized by premorbid eating disorders (ie, eating disorders preceding fulfillment of diagnostic criteria for schizophrenia), is more common in women and is associated with a higher frequency of gustatory hallucinations (135). In a sample of 36 patients with first-episode psychosis reporting nonauditory hallucinations and 31 controls from the same clinical population without nonauditory hallucinations, dissociation remained significantly associated with nonauditory hallucinations after adjusting for childhood sexual abuse, other types of childhood adversity, and emotional distress (131).
Tactile, olfactory, and gustatory hallucinations are often thought to be rare in primary psychotic illness. In a descriptive study of a large cohort of psychotic patients with schizophrenia, schizoaffective disorder, or bipolar I disorder, olfactory and gustatory hallucinations were present in 15% and 4% of the total sample, respectively (Table 2) (127). Hallucinations in the various sensory modalities occurred in patients across these diagnoses, and no modality-specific pattern is characteristic of a particular type of psychotic disorder. Tactile, olfactory, and gustatory hallucinations were associated with somatic delusions, delusions of control, thought broadcasting, earlier age at onset, and a lifetime history of depressive episodes (127).
Table 2. Frequency of Hallucinations by Modality in Patients with Psychotic Disorders
Schizophrenia (n=133) |
Schizoaffective (n=101) |
Bipolar disorder (n=186) |
Total Sample (n=420) | |
Auditory |
76% |
68% |
27% |
52% |
Visual |
24% |
29% |
16% |
22% |
Tactile |
20% |
21% |
8% |
15% |
Olfactory |
17% |
23% |
8% |
15% |
Gustatory |
7% |
2% |
2% |
4% |
Total |
79% |
73% |
41% |
61% |
|
In another cross-sectional study of 766 patients with adult psychotic disorders across the diagnostic categories of schizophrenia (n = 227), schizoaffective disorder (n = 210), and bipolar I disorder (n = 329), 26% had visual hallucinations (36). Visual hallucinations were independently associated with hallucinations in other modalities, history of a suicide attempt, catatonic behavior, and specific types of delusions (ie, delusions of control and religious, erotomanic, and jealousy delusions) (36).
In individuals with schizophrenia, referential or control delusions promote the generation or maintenance of olfactory hallucinations, which are associated with and frequently co-occur with hallucinations in other modalities, particularly somatic, tactile, and gustatory hallucinations (115). Olfactory hallucinations are also associated with social anxiety and depression in schizophrenics, with self-smells being particularly associated with self-deprecation (115).
Among 86 outpatients with DSM-IV delusional disorder, those with comorbid Axis I disorders (47%) had a fairly specific collection of additional features, which included more common olfactory and gustatory hallucinations, personality psychopathology, somatic delusions, and suicide risk (46); they also had more severe psychopathology, particularly in terms of emotional dysregulation. In contrast, those with delusional disorder without psychiatric comorbidity (ie, "pure" delusional disorder; 53%) had worse overall neurocognitive performance, mainly affecting working memory (46).
Among 43,339 patients with schizophreniform or affective disorder diagnoses, 0.54% had tactile hallucinations, 0.24% had olfactory hallucinations, and 0.06% had gustatory hallucinations (13). Tactile, olfactory, and gustatory hallucinations were associated with schizophreniform diagnosis, worse disorder severity, male gender, and black ethnicity (13). In fully adjusted multivariate statistical models, tactile and olfactory hallucinations were independently associated with prior mention of use of non-benzodiazepine hypnotics (“Z drugs,” such as zolpidem or zaleplon) (13).
Patients have been reported who were mistakenly thought to suffer from depression but actually had unnoticed drug-induced dysosmia and dysgeusia (125). Among individuals that superficially appear depressed, it is essential to distinguish (1) those with dysosmia or dysgeusia with reactive depressive manifestations and (2) those with primary neuropsychiatric illness with olfactory or gustatory hallucinations (125).
Prognosis and complications
Disorders of the chemosensory senses, smell and taste, are usually less disabling than disorders of the other special senses (vision and hearing). Nevertheless, chemosensory impairment adversely affects quality of life, and self-reported changes in flavor perception is a predictive factor for involuntary weight loss (158; 44). Taste and smell alterations are common in pediatric cancer patients due to chemotherapy (71; 210) and are associated with impaired nutritional status 6 months after chemotherapy (71).
Chemosensory dysfunction in COVID-19. COVID-19-associated chemosensory loss has a substantial negative impact on health-related quality of life beyond mere inconvenience (40; 55). Indeed, altered taste and smell with Covid-19 may cause severe disruption of daily living and quality of life that impacts psychological well-being, physical health, and interpersonal relationships (26; 40; 55). Affected individuals variously reported reduced desire and ability to eat and prepare food; weight gain, weight loss, and nutritional insufficiency; reduced emotional well-being; and impaired intimacy and social bonding (26; 68). "Reduced enjoyment of food" was the most common complaint (87%) in one study (40).
Among healthcare workers who contract COVID-19, olfactory and gustatory loss was associated with emotional distress, anxiety, and depression (52). Moreover, the psychological impact tends to persist even after recovery from the disease, adding to the risk of work-related distress (52).
The loss of smell and taste improves at a high rate after disease onset in most series (16; 66; 94; 137; 170; 25; 154; 178; 200; 205; 32; 38). However, a substantial proportion (approximately 4% to 5%) of patients with COVID-19 might develop long-lasting change in their sense of smell or taste (204). Rapid recovery of olfaction is observed in two thirds of COVID-19-infected people, but the remainder experience a slower pattern of recovery (108). By 1 month, almost all patients have a significant improvement in olfactory threshold and odor discrimination but not odor identification (94; 154). The recovery of gustation typically occurs before the recovery of olfaction (74). The mean duration of anosmia is 7 days (103), but many of these cases progress from anosmia to hyposmia and parosmia.
Nevertheless, some series report that between one third and one half of patients have persistent qualitative changes in olfaction (parosmia or phantosmia), typically accompanied by qualitative disorders of gustation (parageusia and phantageusia) (148; 57; 67; 90; 190; 134). Some experience long-term deficits with no self-reported improvement at 6 months (57; 67; 90). Persistent parosmia is common even in those who report at least some recovery of olfactory function (90).
Persistent loss of smell or taste was not associated with persistent SARS-CoV-2 infection (16).
Age younger than 40 years and presence of nasal congestion at the time of COVID-19 infection were associated with improved rates of smell recovery, whereas difficulty breathing at the time of COVID-19 infection and prior head trauma were associated with worsened rates of recovery (41).
Presbygeusia. Presbygeusia (literally “elderly taste” or “old-age taste”) is the gradual loss of taste that occurs in most people as they grow older. The elderly have higher detection and recognition thresholds for taste than younger individuals, and taste sensitivity is further compromised by medications and comorbid medical problems (185; 202). Healthy elderly people have higher thresholds for each taste category (sweet, sour, salty, bitter, and umami or savory), particularly for bitter substances, with the average threshold for taste rising fourfold in the elderly (62; 185; 202). The changes in taste sensitivity make food seem relatively tasteless and contribute to difficulties in complying with dietary regimens (such as a low-salt diet for the management of hypertension) (185).
Chemosensory deficits reduce the pleasure obtained from eating and represent risk factors for nutritional deficiencies and nonadherence to dietary regimens (53; 185). Lower olfactory perception in elderly women is associated with lower interest in food-related activities (such as cooking and eating a varied diet), a lower preference for foods with either a predominantly sour or bitter taste (such as citrus fruits) or pungency (such as horseradish), a higher intake of sweets, and a lower intake of low-fat milk products (53). Nutritional adequacy should be assessed in elderly women with a self-reported or measured difficulty in perceiving odors or flavor (53). Flavor-enhanced food positively affects food intake and can increase the enjoyment of food and improve the quality of life for elderly patients with chemosensory deficits (185). Flavor enhancements with simulated flavors can amplify odor intensity, improve the enjoyment of eating, and facilitate adequate nutrition in the elderly with hyposmia (but not anosmia) (185). Simulated flavors are mixtures of odoriferous substances that are either extracted from natural substances (as with concentrated orange juice or vanilla) or synthesized de novo (as with vanillin in artificial vanilla).
Burning mouth syndrome. Burning mouth syndrome is an uncommon painful intraoral disorder affecting mostly postmenopausal women (202). The pain is described as an uncomfortable, constant burning sensation in the mouth, affecting particularly the anterior tongue, palate, and lips. This may be associated with paresthesias or a numb sensation in the mouth or on the tip of the tongue, as well as a sensation of dry mouth and increased thirst without associated mucosal lesions. Approximately two thirds of affected individuals report dysgeusia or persistent abnormal taste sensations (which might be termed “palingeusia”) (202). Most have alterations in sensations of saltiness, and some have alterations in sweet, sour, and bitter tastes (202). The persistent taste sensations are most often described as bitter or metallic (torqugeusia) (202). Risk factors include older age, female gender, menopausal status, status as a “supertaster” (with a high-density of lingual papillae), upper respiratory infection, previous dental procedures, medications, traumatic life events, and stress (23). The etiology of burning mouth syndrome appears to be multifactorial; various conditions have been associated with it, including menopause, endocrine disorders (such as diabetes and hypothyroidism), nutritional disorders (such as deficiencies of iron, zinc, thiamine, riboflavin, pyridoxine, folate, and cobalamin), xerostomia (such as from Sjögren syndrome or other causes), medications (especially anticonvulsants and angiotensin-converting enzyme inhibitors), gastroesophageal reflux, postnasal drip, oral candidiasis, tongue plaque, removable dentures, halitosis, and mouth irritation (such as from overbrushing of the tongue, mouthwash, or acidic drinks) (202; 179). Many cases, however, are idiopathic. Burning mouth syndrome can be disabling and may cause various complications, including anorexia, weight loss, insomnia, irritability, depression, anxiety, and impaired socialization. The treatment depends on the etiology (if it can be identified). For primary (idiopathic) burning mouth syndrome, saliva-replacement products, oral rinses, clonazepam (applied topically or taken orally), gabapentin, selective serotonin reuptake inhibitor antidepressants, alpha-lipoic acid, B vitamins, and capsaicin have all been employed, with variable but limited success (although high-quality trials are lacking) (144; 222).
Psoriatic dysgeusia and burning mouth syndrome. Oral psoriasis is an overlooked cause of dysgeusia. Manifestations can include patches of red skin with yellow or white edges, sores, peeling skin on the gums, pustules, pain or burning (especially when eating spicy foods), fissured tongue, geographic tongue (red patches on the tongue resembling islands on a map), and swollen or infected gums.
-
Geographic tongue associated with oral psoriasis, and also diabetes and seborrheic dermatitis
A geographic tongue is often asymptomatic but may be associated with a burning sensation, especially with eating hot, acidic, and spicy foods. Photograph by Dimitrios Malamos on November 23, 2015. (Creative Commons Attribution ...
Clinical vignette. A 66-year-old patient presented to an oral medicine clinician with a 10-week history of “burning tongue, red at the tip, white bumps in the rear,” dysgeusia, and difficulty speaking (60). His physician had previously placed him on a nystatin topical rinse for 1 week without resolution. He had replaced his usual toothpaste with one that was supposedly free of the surfactant sodium lauryl sulfate. Eating spicy foods was not problematic. Pain was negligible on waking up but increased as the day progressed, although it did not interfere with his sleep. The patient was taking apixaban for atrial fibrillation, atorvastatin for cholesterol, lansoprazole for gastric reflux, fexofenadine and azelastine for seasonal allergies, and a vitamin D supplement. He had no known drug allergies. On examination, the anterior dorsal tongue was erythematous, but there was no lymphadenopathy or other abnormalities. He was referred to an allergist for food hypersensitivity, and the evaluation studies were negative. His symptoms seemed to improve after he eliminated paprika from his diet but then recurred.
(Source: Ferris WJ, Mikula S, Brown R, Farquharson A. Oral psoriasis of the tongue: a case report. Cureus 2019;11[12]:e6318. Creative Commons Attribution [CC BY 3.0] License, https://creativecommons.org/licenses/by/3.0.)
On biopsy, periodic acid-Schiff (PAS) staining showed candidiasis, and H&E staining at medium magnification demonstrated elongated rete pegs (ie, the epithelial extensions that project into the underlying connective tissue in both skin and mucous membranes, also known as rete processes or rete ridges).
(H&E, medium magnification) (Source: Ferris WJ, Mikula S, Brown R, Farquharson A. Oral psoriasis of the tongue: a case report. Cureus 2019;11[12]:e6318. Creative Commons Attribution [CC BY 3.0] License, https://creativecomm...
The papillary connective tissue was noted to show lymphocytic inflammation and dilated blood vessels approximating the epithelial margins. Lower magnification showed marked collections of neutrophils seen in the parakeratin, consistent with Munro microabscesses (ie, a cardinal sign of psoriasis where they are seen in the hyperkeratotic and parakeratotic areas of the stratum corneum). He was treated with of oral fluconazole 100 mg daily for 2 weeks, but his condition further deteriorated. A fungal DNA identification culture and sensitivity assay was negative for candida, aerobic, anaerobic, and acid-fast bacilli.
(Source: Ferris WJ, Mikula S, Brown R, Farquharson A. Oral psoriasis of the tongue: a case report. Cureus 2019;11[12]:e6318. Creative Commons Attribution [CC BY 3.0] License, https://creativecommons.org/licenses/by/3.0.)
At this point, a consultant determined that the patient previously had been treated for cutaneous psoriasis that had begun in his teens with dry, red, blotchy lesions of the legs, trunk, and face. Calcipotriene ointment (a vitamin D analogue) for the trunk and leg lesions and desonide (a topical steroid) for the facial lesions had been sufficient to control the condition. The patient’s lesions subsequently resolved with a topical dexamethasone elixir rinse supporting a diagnosis of oral psoriasis. The steroid rinse was discontinued as a challenge, and in 2 weeks, the erythema and sensitivity returned. On resuming the topical steroid regimen, the lesions again resolved. A biological therapeutic was considered but deferred as the more conservative topical was effective.
-
Psoriatic dysgeusia and burning mouth syndrome: condition resolved with steroid treatment
(Source: Ferris WJ, Mikula S, Brown R, Farquharson A. Oral psoriasis of the tongue: a case report. Cureus 2019;11[12]:e6318. Creative Commons Attribution [CC BY 3.0] License, https://creativecommons.org/licenses/by/3.0.)
-
Psoriatic dysgeusia and burning mouth syndrome: condition returned after discontinuing steroid
(Source: Ferris WJ, Mikula S, Brown R, Farquharson A. Oral psoriasis of the tongue: a case report. Cureus 2019;11[12]:e6318. Creative Commons Attribution [CC BY 3.0] License, https://creativecommons.org/licenses/by/3.0.)
Cerebrovascular disease. Taste disorders are common, although underrecognized, in acute stroke, with hypogeusia occurring in approximately 30% of cases (79). Stroke-related hypogeusia is typically unilateral with brainstem strokes (113) but is strictly unilateral in only a minority of patients with cerebral infarction (79). Risk factors for stroke-associated gustatory loss include male gender, greater stroke-related functional impairment, dysphagia, and anterior circulation location, especially involving the peri-opercular frontal lobe (79). Interruption of the gustatory pathway in the brainstem usually occurs with lateral pontine stroke (113). Thalamic hypogeusia may be associated with a cheiro-oral syndrome (181). Stroke-related damage to the left insula causes an ipsilateral deficit in taste intensity but a bilateral deficit in taste recognition, suggesting that the left insula is dominant for taste recognition (165; 139). Stroke-associated dysgeusia and hypogeusia often persist, whereas other deficits improve and can contribute to unwanted weight loss poststroke (139; 61).
Alpha-synucleinopathies. Although less common than olfactory defects, impaired taste appreciation is also present in about a quarter of patients with clinically defined Parkinson disease, independent of age, disease severity, or olfactory deficits (189). Given the sparing of the first- and second-order taste neurons in Parkinson disease, a disorder of taste may indicate involvement of primary or secondary gustatory cortical areas, although confounding by drug effects (such as anticholinergic drugs) and changes in salivary constitution are possible (189).
Substance abuse. Taste detection and taste recognition thresholds of individuals with substance use disorder are impaired, and the threshold scores for sour, salty, sweet, and umami taste recognition are significantly lower compared with non-users (207).
Myasthenia gravis. Among 1688 patients with myasthenia gravis enrolled in a disease registry, 6% had taste disorders (208). Taste disorders were significantly more common in women, and those with severe symptoms, refractory myasthenia gravis, or thymoma-associated myasthenia gravis, and were less common with ocular myasthenia gravis (208). Taste disorders often occurred after the onset of myasthenia gravis and often responded to myasthenia gravis treatments (208).
Biological basis
Etiology and pathogenesis
• Gustation, like disorders of the other special senses, can be conveniently divided into conductive, sensorineural, and central disorders, where (1) conductive disorders involve transmission of the sensory stimuli to the sensory receptors (usually but not always by impeding transmission); (2) sensorineural disorders involve dysfunction of the sensory receptors or conduction of signals from the sensory receptors to the brain; and (3) central disorders involve dysfunction of processing sensory information within the CNS, particularly within the brainstem and cerebrum. | |
• Age-related gustatory loss (presbygeusia) is common in the elderly and results from normal aging, certain diseases (especially Parkinson disease and dementia with Lewy bodies), medications, surgical interventions, and prior environmental exposures. | |
• The elderly have higher gustatory thresholds, perceive suprathreshold tastes less intensely, and are less able to discriminate tastants or recognize and identify common tastants. | |
• COVID-19-related chemosensory dysfunction results primarily from a loss of function of olfactory sensory neurons and taste buds, mainly caused by infection, inflammation, and subsequent dysfunction of supporting non-neuronal cells in the mucosa. |
Gustatory anatomy
The tongue and taste buds. Modern anatomical illustrations show the details of lingual anatomy clearly with labeling.
The tongue was very well illustrated at the beginning of the 19th century with clear anatomic drawings showing the angle of vallate papillae near the back of the tongue by German physician-anatomist Samuel Thomas von Sömmerring (1755-1830) (199). Still, even though some papillae were noticed and drawn, their function was not understood for about 60 years.
Taste buds in humans were first independently described in 1868 by Swedish physician Otto Christian Lovén (1835-1904) and German anatomist Gustav Schwalbe (1844-1916) (133; 187; 188).
They are bulb-shaped structures composed of about 50 to 120 bipolar cells. The average person has about 10,000 taste buds that are replaced approximately every 2 weeks.
Lingual papillae are small structures on the upper surface of the tongue that give it a rough texture. The four types of papillae on the human tongue have different structures and are accordingly classified as circumvallate (or vallate), fungiform, filiform, and foliate.
The root of the tongue, the "lingual tonsil," is free of papillae and instead has mucous glands and lymphatic tissue. The large vallate papillae are positioned in a V-shaped array at the back of the tongue. The sulcus terminans is located posterior to the vallate papillae and serves to separate the tongue from the lingual root. From the vallate papillae posteriorly to the tip of the tongue, the tongue is covered in filiform and fungiform papillae.
All lingual papillae except the filiform papillae are associated with taste buds.
Foliate papillae (without taste buds) were first reported in humans by German-born Dutch anatomist Bernhard Siegfried Albinus (originally Weiss; 1697-1770) in 1754 (04): they are located bilaterally along the posterolateral margins of the tongue surface and consist of parallel rows of ridges (folia) and valleys, which lie adjacent to the lower molar teeth (87).
Fungiform papillae have taste buds at the apex, whereas vallate and foliate papillae have taste buds arranged in arrays along the epithelial walls of the papillary trenches.
There are also extra-lingual taste buds embedded in the surface mucosa without papillae (eg, on the soft palate, oropharynx, epiglottis, and the upper esophagus).
-
Circumvallate papillae: isolated taste bud, supporting or protecting cell, and taste cell
Legend: (A) isolated taste bud, from whose upper free end project the ends of the taste cells; (B) supporting or protecting cell; (C) taste cell. (Source: Hollis EW. A Text-Book in General Physiology and Anatomy. New York, Cinc...
-
Vertical section of foliate papillae of the rabbit, passing across the folia (Ranvier)
The foliate papillae (papillae foliata) encompass numerous projections arranged in several transverse folds on the lateral lingual margins just anterior to the palatoglossal fold. (Source: Plate 1021 in: Gray H. Anatomy of the ...
-
Transverse vertical section through papilla foliata of a 4-month-old infant
(x18) "The papillae foliatae, or gustatory lamellae of Turner, of the human tongue, offer an excellent area in which to study the taste-bulbs. They consist of five to eight irregular folds or ridges, with rounded or flattened c...
Sensorineural pathways. Three cranial nerves carry taste signals to the brainstem: the facial nerve (via the chorda tympani from the anterior two thirds of the tongue and via the greater petrosal nerve from the soft palate), the glossopharyngeal nerve (from the posterior one third of the tongue), and the vagus nerve (from a small area of the epiglottis).
The glossopharyngeal nerve mediates taste in the back third of the tongue. (Source: Gray H, Spitzka EA. Anatomy: Descriptive and Applied. Philadelphia and New York: Lea & Febiger, 1913. The original figure has been...
The chorda tympani nerve arises from the mastoid segment of the facial nerve, carrying afferent special sensation from the anterior two thirds of the tongue via the lingual nerve (a sensory nerve that arises from the mandibular division of the trigeminal nerve) as well as efferent parasympathetic secretomotor innervation to the submandibular and sublingual glands.
The chorda tympani nerve exits the cranial cavity through the internal acoustic meatus along with the facial nerve.