Hypogeusia is a medical term for a diminished sense of taste, reduced ability to taste or reduced sensitivity to taste 1. Hypogeusia and dysgeusia (unpleasant perception of a tastant), can have a substantial, negative impact on general health and quality of life 2. For example, taste disorders are known to contribute to or exacerbate anorexia, malnutrition, and depression. In other clinical situations, such as cancer and AIDS, ensuing taste abnormalities are associated with poor patient outcome 3. The incidence of taste disorders is the same among men and women, but that women with milder symptoms are more likely to seek treatment 4. In spite of the clinical significance of taste dysfunction, its underlying mechanisms remain largely unknown. At present, there is no specific treatment for severe generalized taste loss 5.
The sense organ specialized for taste (gustation) consists of 10,000 taste buds approximately. These taste buds are ovoid bodies and measure about 50 to 70 micrometers 6. In humans, the taste buds are located in various anatomic locations, such as in the mucosa of the epiglottis, palate, pharynx, and in the walls of the papillae of the tongue 7. At the apical portion of each taste bud are receptors that get exposed to the oral cavity. Within each taste bud lie four morphologically distinct types of cells (Basal cells, type 1 dark cells, type 2 light cells and type 3 intermediate cells) 6. Basal cells are likely to be immature taste cells that do not extend processes into the taste pore. The latter three types of cells are sensory neurons, and they are responsible for responding to taste stimuli or tastants. Taste sensation can be evoked by a diverse group of chemicals ranging from simple ions (H+, Na+) to inorganic molecules to carbohydrates and complex proteins 8.
The pathophysiology of hypogeusia depends on the factor leading to its cause. In a person undergoing chemotherapy and radiotherapy treatment, anatomic changes in taste bud cells and sometimes death of taste bud cells can occur due to the higher rate of cell turnover. Also, as saliva is essential for transporting stimulants to the taste cells, damage to the major salivary glands during the treatment of cancer can lead to a loss of taste. The presence of infection or inflammation in the nearby areas may trigger apoptosis, which can lead to a reduction in the number of taste bud cells. They may also disrupt the ability to detect taste stimulants by chemosensitive hairs 9.
There are a variety of conditions that can lead to hypogeusia, such as damage to the nerve of taste sensation (lingual and glossopharyngeal nerve) in the anterior and posterior portion, dietary deficiencies, systemic conditions such as hypothyroidism, and diabetes mellitus, pernicious anemia, Sjogren syndrome, and Crohn disease 10. Cranial nerve lesions affecting gustatory function include neuritis due to herpes zoster, dissection of the cervical arteries, space-occupying processes in the cerebellopontine angle (meningioma or neurinoma), and the neoplastic lesions affecting the skull base. It can also result from iatrogenic lesions (following laryngoscopic manipulations), neuralgia, and polyneuropathies (due to conditions such as diphtheria, porphyria, lupus, or amyloidosis).
Injury to the chorda tympani nerve during an ear surgery, laryngoscopy, or any dental surgical treatment can lead to changes in the taste perception. The presence of any infection or trauma can also lead to injury of this nerve carrying the sensory taste fibers from the tongue. Any damage to the lingual branch of the ninth cranial nerve during tonsillectomy may also result in taste loss 11. The taste bud cells become altered in certain systemic disorders which can secondarily lead to loss of taste through neuropathy or changes in the environment of the oral cavity. The disorders include autoimmune diseases (Sjogren syndrome), hypertension, diabetes mellitus, renal disorder, liver disorder, and hyperthyroidism 12.
The normal aging process can also lead to a decline in the taste perception, but complete taste loss is rare. A reduction in taste sensation among elderly patients is common because of age-related regression in taste cells, reduced production of saliva, and an individual’s inability to chew food completely correlates with tooth loss 13. Also, geriatric patients are affected by hypogeusia due to polypharmacy, dietary deficiency, and as a secondary complication of some oral and systemic disease.
The gustatory nucleus is a paired nucleus located in the medulla; these are also referred to as solitary nuclei. They receive impulses from the taste buds present on the tongue through VII, IX and X cranial nerves. After receiving these impulses, the nuclei send profuse projections to various regions of the brain such as the amygdala, pons, lateral hypothalamus, ventral posterior thalamic nucleus as well as to the primary and secondary gustatory cortical regions.
Patients with cancer in any head and neck region receiving radiotherapy can present with hypogeusia as radiation therapy can injure the taste buds, transmitting nerves, and affect the salivary flow by damaging the salivary glands, resulting in gustatory dysfunction.
Zinc deficiency is also responsible for abnormalities in taste perception in an otherwise healthy person and cases of drug-induced taste disorders 14. It can also result from some local injury and inflammation in the surrounding structure. The damage can be a result of burns, lacerations, surgery, and local anesthesia). Taste function can also be affected by local antiplaque medicaments that get excreted into saliva, some infections (dentoalveolar, periodontal, and soft tissue infections), vesiculobullous conditions, complete and partial removable prostheses, metallic dental restorations, and dysfunction of the salivary gland.
Certain drugs including antibiotics (ampicillin, macrolides, metronidazole, quinolones, tetracycline), antineoplastic agents, neurologic medications (anti parkinsonism, central nervous system stimulants, migraine medications) cardiovascular drugs (antihypertensives, diuretics, statins, antiarrhythmics), antipsychotics, tranquilizers, tricyclic antidepressants, thyroid medications, antihistamines, bronchodilators, antifungals, and antivirals have also been reported to cause hypogeusia as a side effect 15.
Figure 1. Map of taste buds on human tongue
Majority of the taste dysfunctions are caused by impairment of smell (olfactory) rather than taste perception 6. The most common causes of olfactory (smell) dysfunction include allergic rhinitis, chronic rhinosinusitis and upper respiratory infection 16. Any condition that results in a compromised environment for the mediators of chemosensation (e.g. tongue, saliva, oral mucosa, neural pathways, and neurotransmitters) results in altered taste perception at any age. The mechanistic cause of taste dysfunction can be of three types:
- Transport, sensory or neuronal problem (Table 1) 17.
- Drug-induced taste disorder was found to be the most common etiology among the patients visiting a taste clinic in Japan (Table 2) 4.
- The drug interactions and side-effects index lists over 200 drugs with chemosensory dysfunction. A partial list of these medications affecting taste is shown in Table 3 18.
Oral sources of altered taste function are common and can be evaluated by a dentist. Trauma (burns, lacerations, surgery, and local anesthesia), local antiplaque medications and drugs excreted into saliva, infections (dentoalveolar, periodontal, soft tissue), vesiculobullous conditions, removable prostheses, metallic dental restorations, and salivary dysfunction can also directly or indirectly affect taste function 17. Gastroesophageal reflux disease (GERD) can produce apparent phantogeusia (perception of taste that occurs in the absence of a tastant), which may be intermittent or persistent and are often described as sour 19.
Systemic conditions such as hypothyroidism, diabetes mellitus, pernicious anemia, Sjogren syndrome and Crohn’s disease are also known to cause taste dysfunction 20. Radiation therapy can injure the taste buds, transmitting nerves, and salivary glands, resulting in gustatory dysfunction 20. Zinc deficiency is responsible for taste perception abnormalities in otherwise healthy persons and in drug-induced taste disorders 21. Finally, aging or factors associated with aging may render individuals more vulnerable to gustatory dysfunction 19.
Occasionally, patients complain that the sweet orange juice turns sour and bitter, if consumed immediately after they brush their teeth. This effect on sweet taste is transient and can be explained by the action of the detergent sodium dodecyl sulfate, a common ingredient in toothpastes 8.
Moreover, aging or factors associated with aging may also render individuals more vulnerable to dysfunction of the gustatory system 13.
Table 1. Mechanistic cause of taste dysfunction17 ]
Table 2. Causes of taste dysfunction in 2,278 Japanese patients4 ]
Table 3. Drugs causing taste dysfunction18 ]
Hypogeusia is a medical term for a diminished sense of taste, reduced ability to taste or reduced sensitivity to taste 1. Taste dysfunction is often associated with nausea, decreased appetite, and dry mouth, especially in patient undergoing chemotherapy 22.
Hypogeusia can create serious health issues. hypogeusia, if induced by drugs, can lead to worsening of various geriatric problems such as anorexia, cachexia, and incontinence. It can be a risk factor for cardiovascular disease, diabetes mellitus, cerebrovascular stroke, and other illnesses that require a specific diet.
When the taste sensation is affected, the person may change his/her eating habits and may have a dietary deficiency. Some people may eat less resulting in loss of weight, while others may eat in higher quantity resulting in weight gain. In severe cases, hypogeusia can lead to depression.
There are many causes of hypogeusia and the diagnosis is often not made by a single clinician, it is essential to have an interprofessional team to manage patients with hypogeusia.
A clinician evaluating a patient who has gustatory dysfunction must understand that “taste” complaints are usually symptoms of an olfactory dysfunction 6. The distinction between true taste loss (bitter, sweet, salty, sour, or umami) and olfactory loss, the inability to perceive complex flavors of food, will help clarify the patient’s diagnosis 23. Qualitative gustatory dysfunction is more frequent than the quantitative dysfunction 24.
When examining patients with taste dysfunction, subjective assessment of the chief complaint along with the objective evaluation of the head, neck, and oral region and review of the patient’s medical condition, dental condition, current, and past medications, and social history is necessary.
- Detailed history: To identify the etiology of the disease, the events associated with the onset of the gustatory complaint are of utmost importance. A complete history, including systemic illness, current medication, and dental procedures, also merits consideration 25. Any change in the medication also requires evaluation.
- Physical examination: To identify the local factors leading to the development of the taste disorder, the oral cavity needs an examination for local factors 25.
Apart from this, there are various tests available for assessment of taste sensation, such as electro/chemo-gustometry, spatial analyses, psychophysical evaluation and medical Imaging.
- Psychophysical evaluation: This is essential to identify the patient’s complaints and in measuring the degree of permanent taste loss. The clinician must also be sensitive to the psychological state of the patient. Depression can result from a taste problem or contribute to a taste complaint 25.
- Medical Imaging: This is done to obtain anatomical and etiological diagnostic information. Imaging techniques help in ruling out or confirm the presence of any damage to the structures of the central nervous system, particularly to the brain stem, thalamus, or pons.
The electrogustometry has its basis the principle of applying weak electrical currents to the different taste buds in the oral cavity, whereas the chemogustometry uses specific taste solutions to examine the taste sensitivity. Based on these tests, the patient’s ability to identify and evaluate the intensity of different types of tastes, such as sweet, salty, sour, and bitter taste gets assessed. Since the localized areas of impairment can be undetected, in the spatial test, different regions of the mucosa of the oral cavity get evaluated. In this test, a cotton swab is dipped in a particular taste solution and then applied to different areas of the oral mucosa. To assess the efficiency of the taste buds present on the throat, the patient is asked to swallow part of each taste solution. The patient is then asked to assess the quality and intensity of the taste.
Some other tests include the use of stimuli in the form of a filter paper saturated with tastant/taste strips. The patient is then asked to identify the taste. The strips have an advantage over the taste solution of having a long shelf life.
The clinician can also evaluate gustatory dysfunction by applying a topical anesthetic (unflavored 2% lidocaine) on the dorsal surface of the tongue. The anesthetic is applied on one side, first starting from the anterior two-third and progressing toward the posterior one-third, followed by application to the contralateral side in the same manner. If the chief complaint gets eliminated, the source of taste disorder is considered to be local. But if it persists, then other factors such as a systemic condition or some lesion in the central nervous system may be suspected.
Hypogeusia treatment involves finding and treating the underlying cause of hypogeusia. Some taste disorders do not require any treatment as they resolve spontaneously 8. There is no particular therapeutic regime for a taste disorder like hypogeusia. If it is chemotherapy-induced, it is potentially reversible by the cessation of the use of offending medication. However, discontinuation of drugs to treat the taste disorder is not always possible in patients, particularly with life-threatening conditions such as cancer, diabetes mellitus, and uncontrolled infections. Supplements are an option, such as zinc gluconate, particularly in patients undergoing radiotherapy/chemotherapy in the dosage of 140mg/day or alpha-lipoic acid in the dosage of 600 mg/day for few months may restore taste 21.
In cases of dysgeusia (unpleasant perception of a tastant) and burning mouth disorder, tricyclic antidepressants and clonazepam can be helpful 20. In case of severe dysgeusia, topical anesthetics such as lidocaine gel may help 24. Following trauma or surgery affecting the nerve supply to the taste buds, no specific therapy is available, only time will tell whether the condition will improve 20. The condition may either improve gradually on its own or may remain the same. In patients with xerostomia, an artificial saliva is a therapeutic option 26.
An orofacial cause can usually be traced, treated and followed by the dental surgeon 17.
In the absence of specific treatment for a diagnosed taste dysfunction, the most important aspect of treatment is teaching the patient to cope with the disorder 8. Some of the common self-care strategies used by patients with taste dysfunction include eating smaller and more frequent meals, using more condiments, using more fats and sauces, eating blander foods, adding something sweet to meats, sucking on hard candy, eating more boiled foods, using more salt, oral care before eating, eating cold foods, avoiding beef, and spicy foods 22.
Many patients become concerned about the seriousness of their disorder and also develop depression. The thought of an untreatable, but not life-threatening disease, as most chemosensory disorders are, is often more acceptable for a patient than an undiagnosed one. The clinician should be sensitive to the patient’s psychological state. Depression may be the result of a taste disorder or contribute to a taste complaint. In either case, referral for psychological counseling may be considered, although not as a first step 19. Furthermore, if a cause cannot be established, patients should be referred to a multidisciplinary taste and smell center 17.
The success rate of the treatment of hypogeusia depends upon the etiology. Many patients develop depression as they remain concerned about the seriousness of their disorder.References
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