|Year : 2014 | Volume
| Issue : 1 | Page : 69-76
Taste disorders: A review
Vijay Kumar Ambaldhage1, Jaishankar Homberhalli Puttabuddi2, Purnachandrarao Naik Nunsavath3, Yehoshuva R Tummuru4
1 Department of Oral Medicine and Radiology, PMNM Dental College and Hospital, Bagalkot, Karnataka, India
2 Department of Oral Medicine and Radiology, JSS Dental College and Hospital, Mysore, Karnataka, India
3 Department of Oral Medicine and Radiology, SIBAR Institute of Dental Sciences, Takkellapadu, Guntur, Andhra Pradesh, India
4 Department of Oral Medicine and Radiology, Institute of Dental Education and Advance Studies, Gwalior, Madhya Pradesh, India
|Date of Submission||18-Jun-2014|
|Date of Acceptance||25-Jul-2014|
|Date of Web Publication||26-Sep-2014|
Vijay Kumar Ambaldhage
PMNM Dental College and Hospital, Bagalkot, Karnataka
Source of Support: None, Conflict of Interest: None
| Abstract|| |
For maintenance of the health of an individual, taste sensation is very important. It is an important sensation that serves to assess the nutritious content of food, support oral intake, and prevent ingestion of potentially toxic substances. Disturbances in the perception of taste can lead to loss of appetite, causing malnutrition and thus distressing both the physical and psychological well-being of the patient. Oral physicians are often the first clinicians who hear complaints about alteration in taste from the patients. In spite of the effect of taste changes on health, literature on the diagnosis, pathogenesis, and precise treatment of taste disorders are less. Taste changes may lead patients to seek inappropriate dental treatments. Proper diagnosis of the etiology is the foremost step in the treatment of taste disorders. Thus, it is important that dental clinicians to be familiar with the various causes and proper management of taste changes. In this article, we have reviewed related articles focusing on taste disorders and their management, to provide a quick sketch for the clinicians. A detailed search was performed to identify the systematic reviews and research articles on taste disorders, using PUBMED and Cochrane. All the authors independently extracted data for analysis and review. Ultimately, 26 articles underwent a full text review. In conclusion, the research to date certainly offers us valid management strategies for taste disorders. Meanwhile, practical strategies with the highest success are needed for further intervention.
Keywords: Ageusia, gustatory disorders, hypogeusia, phantogeusia, taste bud
|How to cite this article:|
Ambaldhage VK, Puttabuddi JH, Nunsavath PN, Tummuru YR. Taste disorders: A review
. J Indian Acad Oral Med Radiol 2014;26:69-76
|How to cite this URL:|
Ambaldhage VK, Puttabuddi JH, Nunsavath PN, Tummuru YR. Taste disorders: A review
. J Indian Acad Oral Med Radiol [serial online] 2014 [cited 2019 May 22];26:69-76. Available from: http://www.jiaomr.in/text.asp?2014/26/1/69/141864
| Introduction|| |
Taste perception is an important factor in sustaining human life. The enjoyment of food is one of the greatest sources of pleasure in a human being's life. For food to be palatable, it is essential that one be sensitive to taste. Although commonly recognized as important for epicurean pleasure, taste perception is crucial for motivating food intake, to obtain energy and nutrients needed to maintain the body's functions. In addition, the senses of taste and smell provide warning messages and protection from food-borne and environmental toxins. 
The loss of taste sensation affects the quality of life. In spite of the discomfort and effect on the quality of life caused by a change in sensation of taste, very few treatments are available for the effective management of taste disorders. It could be due to the complexity of the taste system and fewer clinical studies on the subject. Dental practitioners are often the first clinicians to be presented with complaints about changes in taste.  Thus, it is important for the dental clinicians to be familiar with the etiology, diagnosis, and management of taste disorders.
| Method|| |
Our search for articles on taste disorders and management was carried out in April 2014, in the database of PUBMED and Cochrane. The language was restricted to English and the keywords used for the search were: Taste disorders, gustatory disorders, ageusia, hypogeusia, taste pathway, taste perception, gustatory testing, and ageusia management. The references of all the included studies were also searched. We included only the review and research articles. Two independent reviewers screened the titles and abstracts of all the articles for eligibility. When the reviewers felt that the abstract or title was potentially useful, the entire copies of the article were retrieved. Finally, 26 articles were retrieved for the final scrutiny.
| Taste Pathway|| |
The taste pathway is an intricate system comprised of various structures innervated by cranial nerves. The facial nerve (VII nerve) innervates the anterior two-thirds of the specialized mucosa covering the dorsal aspect of the tongue and soft palate through the chorda tympani. The glossopharyngeal nerve (IX nerve) innervates the posterior third of the tongue through its glossal branches, and the vagus nerve (X nerve) supplies sensation to the pharyngeal mucosa. ,
The tongue is covered with numerous papillae of four varieties. Filliform papillae are the most numerous and appear as short, rough structures covered with thick keratinized epithelium. They do not contain taste buds. The filiform and fungiform papillae are spread mainly at the tip and along the lateral borders of the tongue.The fungiform papillae are dispersed across the surface of the tongue. They are more box-like, with a connective tissue core and a thin covering of epithelium. Most of the fungiform papillae contain a single taste bud on the tip. The larger circumvallate papillae are located anterior and parallel to the sulcus terminalis, appearing dome-shaped, with a surrounding trough, called a crypt. The crypt is lined by the gustatory epithelium, which contains several taste buds. In humans there are 12-15 circumvallate papillae. Foliate papillae are present on the lateral sides of the tongue, appear like slits, and are supplied by the glossopharyngeal nerve. ,
Taste buds mediate the taste sensation. They consist of taste cells and nerve fibers within specialized epithelial structures. The taste molecules are detected by chemosensitive receptors present in the taste buds. Taste buds have the capacity to regenerate and have a half-life of 10 to 15 days. 
The taste buds contain three types of taste receptors: ,
Type I: Receptors detect the salt taste.
Type II: Receptors can be found in the fungiform and circumvallate papillae and detect sweet, bitter, and umami tastes.
Type III: Receptors are found in the circumvallate and foliate papillae and can detect sour taste.
Types of tastes
There are mainly five types of tastes, namely, salty, sweet, sour, bitter, and umami tastes. The sweet taste is perceived at the tip of the tongue, bitterness in the posterior part, whereas, salt and sour tastes are perceived in the lateral borders of the tongue. The umami is a fairly new term for a taste described as savory, which is created by the combination of glutamate with 5΄-ribonucleotides. The umami taste is abundant in foods including fish, crab, scallop, chicken, cheese, and black mushrooms. ,
Each taste cell in the taste buds expresses one of the five taste receptors that selectively interact with chemical tastants such as sodium chloride (Nacl), hydrogen chloride (Hcl), sucrose, quinine, and umami substances. Taste receptors contain two G protein-coupled receptor families, T1R and T2R. Gustducin is a trimetric G-protein complex that is involved in sweet, bitter, and umami taste transduction. ,
As the taste receptors are usually adapted to salivary environment, the taste of saliva is not normally recognized. However, the major ions, whose concentration fluctuates widely with changes in the flow rate of saliva, potentially stimulate the taste receptors. Transmission of the acid taste occurs by the blockage of the K + channels in the membrane of the taste receptors. A potential independent Na + channel is concerned with transduction of the salty taste, whereas, transmission of the sweet and bitter tastes is linked with specific membrane receptors, which are associated with second-messenger systems (cAMP and IP3). 
Stimulation of the taste receptors produces signals that travel through cranial nerves VII, IX, and X to the gustatory nucleus in the brain. The trigeminal nerve is associated with assigning sensations, such as, the temperature, texture, and 'hotness' of food. This is probably the reason why the complete loss of taste is rarer than loss of smell and indicates the importance of the olfactory system in the perception of taste and flavor. 
Centrally, there are two pathways for taste perception - one ascends to the hypothalamus and the other to the thalamus and then to the gustatory center of the cortex. A disturbance along these pathways can result in taste perception changes. 
Role of saliva
Taste sensitivity is affected by the interaction between the taste substance and saliva. Saliva helps in the perception of taste by the following mechanisms: 
- Solubilization of the tastants in the saliva.
- Possible chemical interactions with the various components of saliva.
- Diffusion and dilution of the tastants in the saliva.
Hence, salivary secretion plays a key role in taste, including the transport of taste substances and the protection of taste receptors. Saliva protects the taste receptor from damage by dryness, infection, and from disuse atrophy. This property of saliva may be related to taste disorders. 
Role of zinc
Zinc is a vital constituent in both the repair and maintenance of taste buds. Zinc helps in the synthesis of the protein gustin, which is linked to the building of taste buds. Decrease in the salivary gustin/carbonic anhydrase VI causes taste and smell disorders, which can be treated with zinc. It is a cofactor for alkaline phosphatase, an important enzyme in the taste bud membrane. Thus, taste disorders can be treated using zinc. ,
| Taste Versus Flavor|| |
Taste is the perception of salty, sweet, sour or bitter by the tongue; whereas, Flavor is the combination of taste, smell, and the trigeminal sensation. Flavor is a complete sensory experience arising from the ingested stimulant molecules and involves taste, smell, texture, and temperature. 
The loss of taste results particularly from a change in a person's perception to any of the five taste modalities. Damage to the olfactory system or the trigeminal nerve causes a loss of perception of flavor, with or without changes in taste perception. It is thought that 75% of the flavor sensation is produced by odorants. In case of loss of flavor without measurable loss in taste, the cause is usually a loss of olfaction through any upper respiratory tract infection, aging, neurodegenerative disease, trauma to the head or nasal and paranasal sinus disease. Thus, patients with taste loss complaints may require olfactory testing. ,
When one eats food, aromas are released that enter the nose through a retronasal passage connecting the roof of the mouth with the nose. Nerve endings in the olfactory bulb in the nose send these smell stimuli to the brain. It is the aroma, when combined with the stimuli of taste, temperature, and texture that give the food a 'flavor'. It is the integration of these stimuli by the brain that distinguishes between food substances. ,,
| Classification|| |
Taste disorders are classified based on two principles: type and site of the lesion.
On the basis of the type of lesion, taste disorders are grouped as: ,,
- Hypergeusia - increased sensitivity to taste
- Hypogeusia - decreased sensitivity to taste
- Dysgeusia - taste confusion
- Ageusia - complete loss of taste
According to Fikentscher 1987, taste disorders are classified based on the site of the lesion, for example, dysguesia is due to: 
- Epithelial Disorders ¯ defect in the detection of taste due to disorders of the mucosa and taste buds
- Neural Disorders ¯ defect in the transmission and perception of the taste stimulus due to neural disorders
- Central Disorders ¯ central lesions, such as, brain tumor, surgeries, head trauma, Alzheimer's disease, and so on.
Taste disorders can also be classified into three categories on the basis of the state of impairment.
I category - External damage to the gustatory papillae and taste buds: Caused by dry mouth (xerostomia, hyposalivation), tongue coating, atrophic glossitis, iatrogenic causes (e.g., dental treatment or exposure to radiation), burns, exposure to toxic substances and other external sources of damage.
II category - Internal damage to the gustatory papillae and taste buds: Caused by zinc deficiency, aging, excessive medication intake, vitamin deficiency, systemic disease (e.g., bulimia, anorexia, hypothyroidism, Cushing's syndrome, diabetes mellitus, liver disease, and others), infections of the upper respiratory tract, and exanthema dysgeusia.
III category ¯ Disturbance of the taste sensation neural pathway : It could be the result of peripheral or central nerve damage, such as, taste bud degeneration occurring after chorda tympani nerve injury or head trauma.
Depending on the results of the gustatory testing, the taste disorders can be classified as follows:
- Total Ageusia - complete lack of taste sensation to all types of tastes
- Partial Ageusia - complete lack of taste sensation to few tastes
- Total hypogeusia - decreased sensation to all types of tastes
- Partial hypogeusia - decreased sensation to few tastes
- Hypergeusia - increased sensitivity to taste
- Dysgeusia - taste confusion or presence of a strange, distorted taste sensation
| Etiological Factors|| |
The various causes of gustatory disorders can be tabulated as shown in [Table 1].
| Pathophysiology|| |
In addition to various etiological factors, normal aging can also cause taste changes. In particular, lowered taste sensitivity is common among elderly patients, possibly because of age-related changes in the taste buds, decreased salivary production, and an inability to chew the food properly. Some of the causes and their possible mechanism of dysguesia are as mentioned in [Table 2].
| Clinical Evaluation and Diagnosis|| |
There are four basic components in the clinical evaluation of a patient with gustatory disorder: Detailed history, physical examination, psychophysical evaluation, and medical imaging.
The history plays a very important role in the etiological diagnosis. Often associated events with the onset of the gustatory complaint give the most important clue as to the cause of the disorder. A detailed history of changes in medication and dental procedures (e.g., extraction of teeth, surgeries, types of toothpaste, and oral rinses used) should also be taken into consideration. ,
The physical examination plays an important role in the etiological diagnosis. Abnormalities seen in the oral and nasal cavities may provide key etiological information and focus attention on a local factor in the nose or oral cavity, to explain the development of the gustatory disorder. In cases of phantom taste complaints, it is essential to rule out oral health problems that may contribute to these. A thorough oral examination should be performed, including assessment of the possible abnormalities in the microbial flora of the oral cavity. ,,
The role of imaging is in many ways an extension of the physical examination and in the case of inflammatory processes in the oral cavity, nose, and paranasal sinuses may provide anatomical and etiological diagnostic information. Examinations using imaging techniques to rule out or prove the presence of damage to central nervous structures and in particular to the brain stem, thalamus, and pons may be necessary. ,
A psychophysical evaluation is essential to corroborate the patient's complaints, determine the efficacy of treatment, and measure the degree of permanent impairment. The practitioner should also be sensitive to the patient's psychological state. Depression may be the result of a taste problem or contribute to a taste complaint. In either case, referral for psychological counseling should be considered, although not as a first step. ,
| Gustatory Testing/Taste Testing|| |
Taste and smell function can be readily tested at the chair side. The testing generally falls into two categories: Taste testing and smell testing. As the olfactory diseases are partially or fully responsible for the taste disorders, when a patient presents with a taste complaint, it is important to investigate whether the change in taste function is due to smell abnormalities. The various taste testing methods are as follows: ,
- Self-reporting questionnaire method
- Visual analog scale (traditional method)
- Clinical bitterness masking test for phantogeusia
Whole-mouth test/sip and spit method
A whole-mouth taste test is used to assess the patient's ability to detect, identify, and evaluate the intensity of different concentrations of sweet, sour, salty, and bitter taste solutions. Flavor can be enhanced by monosodium glutamate, a phenomenon referred to as umami.
In this method, a solution of a predetermined concentration of a sweet, salty, bitter, or sour substance is gargled and then spit out. The patient is asked to identify the taste and the concentration can be varied to determine threshold sensitivity. The disadvantage of this method is, regional damage (e.g. on the front or tip of the tongue) would be masked by stimulation of the remaining taste cells in the oral cavity. 
Spatial taste test (tongue mapping)
A spatial test is used to assess different areas of the mouth, as localized areas of impairment can be undetected. A cotton swab dipped in a special taste solution is placed in different areas of the mouth. The throat is evaluated by having a patient swallow part of each taste solution. The individual is requested to assess taste quality and intensity. 
Filter-paper disk method
In this test, the filter paper or a dissolvable strip is coated with a known concentration of a sweet, salty, bitter or sour material. The filter paper or strip is placed on a particular area of the tongue or palate and patient is asked to identify the taste. The concentration can be changed to determine the threshold sensitivity. The objective is to activate major regions of taste cells, to check whether the individual has partial taste deficit or complete damage. 
In electrogustometry, weak electrical currents are delivered to the various taste bud fields. It is used to measure the taste threshold by passing a transient controlled anodal current through the tongue mucosa. When current passes through the tongue a distinctive metallic taste is felt. ,
Edible taste strips
The edible taste strips are prepared from methylcellulose solutions that are dried to form thin films. The maximum amount of a tastant that could be included in a 2.54 × 2.54 cm taste strip is 5% for each class of tastant (sweet, sour, salty, bitter, and umami). 
Flavor discrimination test
A flavor discrimination test is often used to evaluate the combination of both sensations of taste and smell. Four different solutions are used, each with a different degree of sweetness. The patient is requested to taste the solutions in random order. ,
Taste testing will be incomplete without olfactory testing. Smell testing can be accomplished by asking patients to identify common smells (e.g., chocolate, coffee, mothballs) after exposure (one nostril at a time). A deficit in smell function, either bilaterally or unilaterally, may be related to an obstruction in the sinuses or to an insidious neurological process. ,,
| Management of Taste Disorders|| |
Taste as well as smell impairments are stressful for patients, particularly if a diagnosis is not established and treatments are not effective. Practitioners must counsel patients with coping strategies and behavioral modification techniques. Moreover, patients need to be advised on nutrition and appetite issues to prevent nutritional deficiencies. Special attention has to be paid to possible underlying illnesses and medicines taken by the patient. ,
Specific conditions related to taste dysfunction, such as, hyposalivation, poor oral hygiene, use of tobacco products and/or alcohol, have relatively simple solutions. Discontinuing the etiological habit, chewing sugarless gum or candy for taste and salivary stimulation, or prescribing sialogogue can be used for individuals with residual salivary gland function. Chewing gum or candy may also cover the unpleasant taste and provide symptomatic relief.
In patients with gastric reflux, acid pump inhibitors, such as, omeprazole, pantaprazole, and lansoprazole are effective. In cases of exposure to toxins or drugs, elimination of the offending agent often improves the problem. If trauma is the cause, no specific therapy is available, but the condition may improve in time with regeneration of the nerve cells. 
Taste loss and xerostomia in patients being treated with radiotherapy can be prevented by protecting healthy oral tissues from the effects of irradiation. One method is by minimizing the radiation field by shielding or avoiding irradiation of the tongue and salivary glands. Avoiding extraneous irradiation is often possible by recent advances in radiation treatment techniques, such as 3-D conformal radiation therapy and intensity-modulated radiotherapy (IMRT). , Another preventive method is the use of a cytoprotective agent. Amifostine, a thiol compound, protects normal organs from the oxidative damages induced by chemotherapy or radiotherapy. However, Amifostine itself can produce side effects, such as vomiting, hypotension, flushing, metallic taste, and dry mouth. 
Necrotic oral lesions such as Acute Necrotizing Ulcerative Gingivitis, Acute Necrotizing Ulcerative Periodontitis, chronic ulcers, and malignant ulcers may present with smell and taste changes, which may be secondary to gram-negative bacterial overgrowth. In such cases, use of topical antiseptics (e.g., chlorhexidine gluconate) or systemic antimicrobials (e.g., metronidazole) may be considered, along with specific treatment for the lesions. 
The relationship between zinc and taste perception has been well-established. Zinc deficiency in humans and animals impairs sensitivity to taste and odors, altering perception. Clinical evidence suggests that exogenous zinc sulfate administration successfully improves taste and odor disorders.  Few studies have reported an effective treatment for patients with idiopathic or persistent taste disturbance. In a recent study, zinc gluconate (50 mg, thrice daily) had a positive effect on taste disorders in a zinc-deficient population and also in patients with idiopathic taste loss, regardless of the serum zinc level.  Zinc supplementation is believed to aid in treating taste disorders by promoting the proliferation of normal taste bud cells, even in patients without zinc deficiency. ,
Zinc supplementation has shown variable results in the studies of radiotherapy (RT) in head and neck cancer patients. Zinc sulfate (45 mg t.i.d.) was compared with placebo during RT in 18 patients and improvement in taste was seen in those on zinc, based on whole-mouth threshold taste testing.  Another trial compared zinc sulfate (45 mg/day) versus placebo in 169 patients during RT, and found fewer patients in the zinc group reporting taste change (73 vs. 84%). 
Alpha-lipoic acid, an important coenzyme and antioxidant in many cellular pathways within the body, has also been suggested for treating idiopathic dysgeusia. Femiano and colleagues found that 91% of the patients with idiopathic dysgeusia, who took α-lipoic acid (200 mg every eight hours) showed some improvement and 46% experienced total resolution. These authors suggested that α-lipoic acid may mitigate or reverse the neuropathic changes related to idiopathic dysguesia. ,
The recent research suggests that lipid peroxidation in the oral mucosal epithelium may lead to the production of carbonyls that impart a metallic taste sensation in the oral cavity. Ferrous iron reacts with the skin and produces a series of aldehydes and ketones causing a metallic odor. An analogous mechanism may produce a metallic taste in the mouth. Thus, antioxidants may prevent or reduce lipid peroxidation in food and body systems and may also play a role in the prevention of the metallic taste. Vitamins C and E are natural antioxidants and are commonly found in human blood and act as antioxidants by reacting with reactive oxygen species (ROS) and free radicals. However, decreases in the antioxidant levels from cancer or its therapy may result in increased irreversible lipid peroxidation in the blood and body tissues leading to a metallic taste. 
Tricyclic antidepressants and Clonazepam have been used in the management of neuropathic oral conditions, including burning mouth syndrome. Clonazepam causes brain stem serotonergic descending inhibition and is, therefore, a valuable aid in these problems. ,
The published recommendations for easing taste and odor problems include:
- Avoiding the use of metallic restorative materials such as silver amalgam to reduce the risk of metallic taste
- Reducing the consumption of foods that taste metallic or bitter in taste such as red meat, coffee, and tea
- Increasing the consumption of high-protein, mildly flavored foods such as chicken, fish, dairy products, and eggs
- Adding seasonings and spices to enhance flavors if hypogeusia or hyposmia are experienced
- Serving foods at cold temperature to reduce unpleasant flavors and odors
- Practicing good oral hygiene including frequent brushing of teeth and use of mouthwash
- Using sialogogues such as sugar-free gums or sour-tasting drops
- Using saliva substitutes and lubricating solutions containing mucin and carboxy-methylcellulose.
In case of hypogeusia, due to trauma to nerves during maxillofacial surgeries, especially extraction of mandibular molars, surgical procedures to repair nerve damage may be done.  In a review by Ziccardi and Steinberg, they found that trigeminal nerve microsurgery was an option for treatment of patients with nerve injury. However, the timing is critical in determining whether the procedure is warranted. 
Finally, supplementation of foods and beverages with taste (e.g., herbs, spices), smell, temperature, and textural stimulants (e.g., crunchy, smooth, fizzy) improves palatability and flavor as well as the desirability of eating.
| Conclusion|| |
Taste and smell disturbances affect the quality of life of a large proportion of patients. The availability of testing for the integrity of taste and smell remains limited. Patients are often subjected to unnecessary and sometimes irreversible and damaging treatment of teeth and other oral mucosal tissues. Thus, proper oral examination and identification of the local factors can prevent unnecessary treatment by dentists.
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[Table 1], [Table 2]