Dysgeusia, also known as parageusia, is a distortion of the sense of taste. Dysgeusia is also often associated with ageusia, which is the complete lack of taste, and hypogeusia, which is a decrease in taste sensitivity. An alteration in taste or smell may be a secondary process in various disease states, or it may be the primary symptom. The distortion in the sense of taste is the only symptom, and diagnosis is usually complicated since the sense of taste is tied together with other sensory systems. Common causes of dysgeusia include chemotherapy, asthma treatment with albuterol, and zinc deficiency. Liver disease, hypothyroidism, and rarely, certain types of seizures can also lead to dysgeusia. Different drugs can also be responsible for altering taste and resulting in dysgeusia. Due to the variety of causes of dysgeusia, there are many possible treatments that are effective in alleviating or terminating the symptoms. These include artificial saliva, pilocarpine, zinc supplementation, alterations in drug therapy, and alpha lipoic acid.

Signs and symptoms

The alterations in the sense of taste, usually a metallic taste, and sometimes smell are the only symptoms.

Causes

Chemotherapy

A major cause of dysgeusia is chemotherapy for cancer. Chemotherapy often induces damage to the oral cavity, resulting in oral mucositis, oral infection, and salivary gland dysfunction. Oral mucositis consists of inflammation of the mouth, along with sores and ulcers in the tissues. Healthy individuals normally have a diverse range of microbial organisms residing in their oral cavities; however, chemotherapy can permit these typically non-pathogenic agents to cause serious infection, which may result in a decrease in saliva. In addition, patients who undergo radiation therapy also lose salivary tissues. Saliva is an important component of the taste mechanism. Saliva both interacts with and protects the taste receptors in the mouth. Saliva mediates sour and sweet tastes through bicarbonate ions and glutamate, respectively. The salt taste is induced when sodium chloride levels surpass the concentration in the saliva. and etoposide. Zinc "is an important cofactor for alkaline phosphatase, the most abundant enzyme in taste bud membranes; it is also a component of a parotid salivary protein important to the development and maintenance of normal taste buds". H<sub>1</sub>-antihistamines, such as azelastine and emedastine. Approximately 250 drugs affect taste, including Paxlovid, a drug used to treat COVID-19. Some describe so-called "Paxlovid mouth" as like a "mouthful of dirty pennies and rotten soymilk", according to the Wall Street Journal.

The sodium channels linked to taste receptors can be inhibited by amiloride, and the creation of new taste buds and saliva can be impeded by antiproliferative drugs. Drugs that act by blocking the renin–angiotensin–aldosterone system, for example by antagonizing the angiotensin II receptor (as eprosartan does), have been linked to dysgeusia. There are a few case reports claiming calcium channel blockers like amlodipine also cause dysgeusia by blocking calcium sensitive taste buds.

Pregnancy

Changes in hormone levels during pregnancy, such as estrogen, can affect the sense of taste. A study found that 93 percent of pregnant women reported some change in taste during pregnancy. In a case study, 22% of patients who were experiencing a bladder obstruction were also experiencing dysgeusia. Dysgeusia was eliminated in 100% of these patients once the obstruction was removed.

Dysgeusia often occurs for unknown reasons. A wide range of miscellaneous factors may contribute to this taste disorder, such as gastric reflux, lead poisoning, and diabetes mellitus. A minority of pine nuts can apparently cause taste disturbances, for reasons which are not entirely proven.

Certain pesticides can have damaging effects on the taste buds and nerves in the mouth. These pesticides include organochloride compounds and carbamate pesticides.

Normal function

The sense of taste is based on the detection of chemicals by specialized taste cells in the mouth. The mouth, throat, larynx, and esophagus all have taste buds, which are replaced every ten days. Each taste bud contains receptor cells. A single taste bud is innervated by several afferent nerves, while a single efferent fiber innervates several taste buds. Fungiform papillae are present on the anterior portion of the tongue while circumvallate papillae and foliate papillae are found on the posterior portion of the tongue. The salivary glands are responsible for keeping the taste buds moist with saliva.

A single taste bud is composed of four types of cells, and each taste bud has between 30 and 80 cells. Type I cells are thinly shaped, usually in the periphery of other cells. They also contain high amounts of chromatin. Type II cells have prominent nuclei and nucleoli with much less chromatin than Type I cells. Type III cells have multiple mitochondria and large vesicles. Type I, II, and III cells also contain synapses. Type IV cells are normally rooted at the posterior end of the taste bud. Every cell in the taste bud forms microvilli at the ends.

Diagnosis

In general, gustatory disorders are challenging to diagnose and evaluate. Because gustatory functions are tied to the sense of smell, the somatosensory system, and the perception of pain (such as in tasting spicy foods), it is difficult to examine sensations mediated through an individual system. In addition, gustatory dysfunction is rare when compared to olfactory disorders.

Diagnosis of dysgeusia begins with the patient being questioned about salivation, swallowing, chewing, oral pain, previous ear infections (possibly indicated by hearing or balance problems), oral hygiene, and stomach problems. The initial history assessment also considers the possibility of accompanying diseases such as diabetes mellitus, hypothyroidism, or cancer. In this test, three drops of liquid are presented to the subject. One of the drops is of the taste stimulus, and the other two drops are pure water. Direct scaling cannot be used to determine if a taste stimulus is being perceived at abnormal levels. In this case, magnitude matching is used, in which a patient is asked to rate the intensities of taste stimuli and stimuli of another sensory system, such as the loudness of a tone, on a similar scale. Although electrogustometry is widely used, there seems to be a poor correlation between electrically and chemically induced sensations.

Diagnostic tools

Certain diagnostic tools can also be used to help determine the extent of dysgeusia. Electrophysiological tests and simple reflex tests may be applied to identify abnormalities in the nerve-to-brainstem pathways. For example, the blink reflex may be used to evaluate the integrity of the trigeminal nerve–pontine brainstem–facial nerve pathway, which may play a role in gustatory function.

Structural imaging is routinely used to investigate lesions in the taste pathway. Magnetic resonance imaging allows direct visualization of the cranial nerves. Furthermore, it provides significant information about the type and cause of a lesion. Cell cultures may also be used.

In addition, the analysis of saliva should be performed, as it constitutes the environment of taste receptors, including transport of tastes to the receptor and protection of the taste receptor. Typical clinical investigations involve sialometry and sialochemistry.

Treatments

Artificial saliva and pilocarpine

Because medications have been linked to approximately 22% to 28% of all cases of dysgeusia, researching a treatment for this particular cause has been important. Xerostomia, or a decrease in saliva flow, can be a side effect of many drugs, which, in turn, can lead to the development of taste disturbances such as dysgeusia. Pilocarpine is a cholinergic drug, meaning it has the same effects as the neurotransmitter acetylcholine. Acetylcholine has the function of stimulating the salivary glands to actively produce saliva. The increase in saliva flow is effective in improving the movement of tastants to the taste buds. The patients prescribed the zinc reported experiencing improved taste function and less severe symptoms compared to the control group, suggesting that zinc may be a beneficial treatment. There is not a sufficient amount of evidence to determine whether or not zinc supplementation is able to treat dysgeusia when low zinc concentrations are not detected in the blood.

Zinc infusion in chemotherapy

It has been reported that approximately 68% of cancer patients undergoing chemotherapy experience disturbances in sensory perception such as dysgeusia. In a pilot study involving twelve lung cancer patients, chemotherapy drugs were infused with zinc in order to test its potential as a treatment. The results indicated that, after two weeks, no taste disturbances were reported by the patients who received the zinc-supplemented treatment while most of the patients in the control group who did not receive the zinc reported taste alterations.

Altering drug therapy

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The effects of drug-related dysgeusia can often be reversed by stopping the patient's regimen of the taste altering medication. In one case, a forty-eight-year-old woman who had hypertension was being treated with valsartan. Due to this drug's inability to treat her condition, she began taking a regimen of eprosartan, an angiotensin II receptor antagonist. After eight years of taking the drug, he developed a loss of taste sensation and numbness in his tongue. It can also be administered in capsules or can be found in foods such as red meat, organ meats, and yeast. Animal research has also uncovered the ability of ALA to improve nerve conduction velocity. This is cause for evaluation and management of their dysgeusia. In patients undergoing chemotherapy, taste distortions can often be severe, and make compliance with cancer treatment difficult. Symptoms including paranoia, amnesia, cerebellar malfunction, and lethargy can also manifest when undergoing histidine treatment. Due to the large number of persons affected by taste disorders, basic and clinical research are both receiving support at different institutions and chemosensory research centers across the United States.

See also

  • Anosmia
  • Parosmia

References

  • Dysgeusia at NIH