Dysgeusia

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.

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

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. It has been reported that 50% of chemotherapy patients have had either dysgeusia or another form of taste impairment. Examples of chemotherapy treatments that can lead to dysgeusia are cyclophosphamide, cisplatin, vismodegib, and etoposide. The exact mechanism of chemotherapy-induced dysgeusia is unknown.

Distortions in the taste buds may give rise to dysgeusia. In a study conducted by Masahide Yasuda and Hitoshi Tomita from Nihon University of Japan, it has been observed that patients with this taste disorder have fewer microvilli than normal. In addition, the nucleus and cytoplasm of the taste bud cells have been reduced. Based on their findings, dysgeusia results from loss of microvilli and the reduction of Type III intracellular vesicles, all of which could potentially interfere with the gustatory pathway. Radiation to head and neck also results in direct destruction of taste buds, apart from effects of altered salivary output.

Another primary cause of dysgeusia is zinc deficiency. While the exact role of zinc in dysgeusia is unknown, it has been cited that zinc is partly responsible for the repair and production of taste buds. Zinc somehow directly or indirectly interacts with carbonic anhydrase VI, influencing the concentration of gustin, which is linked to the production of taste buds. It has also been reported that patients treated with zinc experience an elevation in calcium concentration in the saliva. In order to work properly, taste buds rely on calcium receptors. 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".

Miraculin Found in miracle berries, sweetens nonsweet food and beverages.

Gymnema sylvestre Blocks the ability to taste sweetness.

There are also a wide variety of drugs that can trigger dysgeusia, including zopiclone, H₁-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.