Taste Disturbances

Background

Although they are relatively common, taste disorders can also be a sign that an acute or chronic medical condition is present. Taste disorders were identified as one of the first signs of COVID-19. Patients who experience a change in taste sensation most often seek help from their general or specialty dentist. These practitioners are well-positioned to use their knowledge of taste disorders such as dysgeusia in screening patients for causative conditions or organisms. Information on the normal physiology of taste sensation, including proposed new tastes; the etiology and pathophysiology of taste disorders and relationships between taste and COVID-19; and the clinical relevance of taste disturbances was gathered through a literature search.

Methods

The PubMed, Embase, Web of Science, and Google Scholar databases were searched for relevant literature regarding dysgeusia, ageusia, and other taste disorders. Papers dealing with the relationship of these disorders to local and systemic causes were included in the search.

Normal Taste Sensation

Anatomy and Physiology of Gustation

Taste signals from the oral cavity to the brainstem are believed to travel through the facial, glossopharyngeal, and vagus nerves, Some difficulty has been encountered in pinpointing a single cortical area as being responsible for the taste sensation.

The functional units of taste are the taste buds, which are located on the various tongue papillae. Taste bud receptors convert chemical energy from tastants into electrical energy for transmission. Taste buds can also be found on the palate, pharynx, epiglottis, and esophagus. In addition, remote receptors have been reported in tissues from the gastrointestinal tract, bladder, brain, respiratory tract, heart, buccal mucosa, sinuses, white blood cells, bone marrow, thyroid, keratinocytes, and testicles.

Five types of taste cells have been identified: type I or glialike; type II for bitter, sweet, and umami detection; type III for sour; type IV, which are pluripotent; and type V, which are marginal cells. The concept of taste mapping in which there were relative concentrations of specific taste sensations on the tongue has been largely replaced by a newer concept where all areas of the tongue represent the different tastes in nearly equal numbers (Figure 3 ). The cerebral cortex may have a similar topographic arrangement.

Figure 3.

Anatomic distribution of specific taste receptors. A, Older concept of concentration of specific taste receptors at specific sites of the tongue (taste mapping). B, Newer concept of clusters of blends of taste receptors localizing at specific anatomic areas of the tongue. Figure courtesy of Swetha Kannan and Dr. Sita Mahalakshmi Baddireddy.

(Courtesy of Thomas DC, Chablani D, Parekh S, et al: Dysgeusia: A review in the context of COVID-19. J Am Dent Assoc 153:251-264, 2022.)

Flavor and Taste Sensations

Flavor is the blend of taste, smell, and touch that acts to warn the individual about the safety and quality of the food being consumed. Multiple inputs from senses other than taste receptors contribute to flavor, including vision and hearing, accompanied by a particularly robust interaction between the gustatory and olfactory areas of the brain. Taste perception blends olfaction, gustation, and texture input.

The various taste sensations offer different benefits to humans, with both positive associations and negative ones, as follows:

• •Sweet sensation is believed to regulate food intake, glucose homeostasis, and energy balance. It may play a role in the search for newer drugs for diabetes or obesity.
• •Sour taste is believed to indicate acid.
• •Salt sensation is crucial in the normal development of the gustatory circuits in the central nervous system. Salt sensitivity in persons with normal blood pressure may predict a susceptibility to hypertension.
• •Umami occurs naturally in foods such as tomatoes, seafood, and egg yolks. The taste is mediated through the glossopharyngeal nerve and has positive and negative aspects. Positively, umami improves nutritional intake, enhances the choice of healthy foods, and improves food flavor. Negative effects tend to relate this taste to hepatotoxicity, asthma, migraine headache, and central nervous system damage. Testing for this taste has been proposed as a way to manage patients with dysgeusia.
• •Bitter taste receptors in the respiratory tract may play a crucial role in the body’s defense mechanisms against invading microorganisms. They may be able to detect certain bacteria and play a role in bronchodilation, offering a potential target for therapy. About 30 receptor genes code for bitter.
• •Fat taste (oleogustus) is a newer taste sensation that may be carried via the glossopharyngeal and chorda tympani nerves.
• •Calcium taste may help modulate calcium metabolism, intake, and homeostasis in mammals and has receptors on various genes.

Etiology and Pathophysiology of Taste Disorders

The range of taste disorders covers the spectrum from dysgeusia, or abnormal taste sensation, to ageusia, or loss of all taste sensation (Table 1 ). As mentioned, olfaction is a major component of taste, with most of the dysfunctions related to impaired olfaction rather than gustation. The mechanisms of these gustatory dysfunctions include abnormal transportation in which the tastant can’t reach the receptor, gustatory unit abnormality in which the peripheral sensory organs are at fault, and neuronal unit abnormality in which the peripheral or central nervous system has suffered damage. Gustatory units in the oral cavity can be exposed to inflammation or other pathologic conditions, or artificial restorative dental materials can affect gustation. In addition, sequence variations in the genes and their associated receptors can also affect taste. More than 50% of the drugs prescribed in the United States cause some disorder of the taste sensation.

Table 1.

Taste Terminology

TASTE TERMINOLOGY	DESCRIPTION
Ageusia	Complete loss of taste sensation38,102
Aliageusia	Bad taste to normally considered "good" taste106
Dysgeusia	General term used for any abnormal taste sensation104
Gustation	The scientific term given for the process of taste sensation95
Hypergeusia	Increased sense of taste38
Hypogeusia	Diminished sense of taste38
Parageusia	Altered taste sensation in the presence of a tastant103
Phantogeusia	Taste sensation in the absence of a tastant103
Presbygeusia	An alteration in taste sensation considered as a result of aging106
Tastant	Any chemical that elicits taste sensation
Taste Agnosia	A term used when a person cannot recognize a taste sensation, independent of normalcy in intellect, sensory processing, and linguistic abilities107

(Courtesy of Thomas DC, Chablani D, Parekh S, et al: Dysgeusia: A review in the context of COVID-19. J Am Dent Assoc 153:251-264, 2022.)

Clinical Relevance

Conditions Related to Dysgeusia

Dentists are often the first clinicians that patients seek when they have taste disorders. Their most common symptoms are dysgeusia, phantogeusia, or a burning sensation. Dental-related factors that may cause taste alterations include local trauma, medications, infections, dental restorations, and salivary gland dysfunction. Radiation to the head and neck area, orofacial pain conditions (including burning mouth syndrome [BMS] and TMDs), Bell palsy, and Guillain-Barré syndrome have also been associated with dysgeusia.

Taste can also be altered by systemic conditions such as Sjögren syndrome (SS), chronic renal failure, end-stage liver disease, endocrine disorders, genetic disorders, neurologic disorders, and psychiatric conditions. Pregnant and postmenopausal women may experience alterations in taste function.

Among the autoimmune disorders related to dysgeusia are SS, autoimmune encephalitis, myasthenia gravis, systemic lupus erythematosus, multiple sclerosis, and Parkinson disease. With SS, the mechanisms that may produce dysgeusia include systemic inflammation, interaction with genetic pathways that modify gustation, an increased taste threshold, and small fiber neuropathy. Sweet taste is independent of salivation, so patients with SS tend to have minimal change in their ability to detect sweet tastants.

BMS patients can experience phantogeusia, which has a bitter, metallic, or burning quality. The bitter and metallic qualities may be related to cranial nerve IX being disinhibited in these patients after damage to or hypofunction of the chorda tympani nerve. BMS patients tend to have a higher taste threshold of all tastes except umami. “Supertasters” are individuals who are suspected to have increased fungiform papillae, which places them at higher risk for having BMS. Dysgeusia related to BMS may be related to psychological factors.

Upper respiratory infections, viral hepatitis, and oral cavity infections may increase the detection and identification of individual taste stimuli via taste buds, causing dysgeusia. Normal cell turnover of taste buds can be disrupted by inflammation. Other infections possibly associated with dysgeusia include HIV infections and their therapy, rhinosinusitis, hepatitis E, Helicobacter pylori, Lyme disease, leprosy, syphilis, and cytomegalovirus.

Often the change in taste sensation precedes the respiratory symptoms of COVID-19. Among the explanations of how COVID-19 affects gustation are damage to the central nervous system, abnormal zinc homeostasis, angiotensin-converting enzyme 2 receptor manifestation, and increased proinflammatory cytokines. The recovery of sensation in COVID-19 patients appears to be high, but little evidence is available to indicate if there is more permanent loss of taste sensation. When gustatory disturbances are identified with no concomitant symptoms of COVID-19, physicians should suspect the possibility of COVID-19 and suggest the patient self-isolate and be tested.

Diagnostic Testing

Taste-testing measures range from simple chairside testing to objective and measured research-level investigations of gustation. A clinician may use simple taste tests to evaluate a patient’s gustation, but this method is subjective and lacks sensitivity and specificity. Objective methods can be done using electrogustometry or the filter paper disk method, whole mouth method, taste strip method, or e-tongue method.

Effects of Smell and Taste on Nutrition

Smell and taste combine to modify the motivation to eat, the enjoyment of food, and the activation of the satiety centers in the brain. Taste disorders have reportedly affected body weight, but evidence linking taste disorders to poor nutrition is lacking; patients with dysgeusia have no substantial alteration in nutritional status. Patients with chronic kidney disease who have substantial taste disturbances, such as reduced taste acuity, abnormal salty taste, and metallic taste, may have difficulty complying with the recommended renal diet and can suffer nutritional deficits. Most researchers report significant reductions in the quality of life for patients with dysgeusia.

Clinical Significance.

Systemic disorders and infections can cause gustatory disorders, which can compromise patients’ quality of life, especially when the cause is COVID-19. Dental clinicians should routinely screen patients for taste disorders, especially during the COVID-19 pandemic. Early recognition of possible infection can lead to a diagnosis and permit prompt treatment. However, dentists must be aware that multiple factors other than COVID-19 can be causing the dysgeusia and ensure that patients undergo a comprehensive workup before making a diagnosis.