Ageusia (from negative prefix a- and Ancient Greek γεῦσις geûsis 'taste') is the loss of taste functions of the tongue, particularly the inability to detect sweetness, sourness, bitterness, saltiness, and umami (meaning 'savory taste'). It is sometimes confused with anosmia – a loss of the sense of smell. Because the tongue can only indicate texture and differentiate between sweet, sour, bitter, salty, and umami, most of what is perceived as the sense of taste is actually derived from smell. True ageusia is relatively rare compared to hypogeusia – a partial loss of taste – and dysgeusia – a distortion or alteration of taste.[1][2]
| Ageusia | |
|---|---|
| Pronunciation | |
| Specialty | Neurology |
Even though ageusia is considered relatively rare it can impact individuals of any age or demographic. Due to the COVID-19 pandemic, there has been an increase in reported cases of ageusia, making it more commonly diagnosed than before.[3]
Causes
Ageusia can stem from various underlying causes, including viral infections from SARS-CoV-2, sinusitis, the common cold, and influenza. Additionally, bacterial infections such as strep throat and salivary gland infections may also lead to this condition. [3]
Lifestyle factors like smoking, obesity, and poor nutrition can contribute to ageusia, as can systemic conditions like diabetes and high blood pressure. Certain autoimmune diseases like Sjogren's syndrome, neurological disorders such as Alzheimer's, Parkinson's, multiple sclerosis, and medication side effects may also be culprits. Nutritional deficiencies, particularly in zinc and vitamin B-12, as well as dry mouth conditions or damage to the taste buds, further heighten the risk of ageusia.[3]
The main causes of taste disorders are head trauma, infections of upper respiratory tract, exposure to toxic substances, iatrogenic causes, medicines, glossodynia (burning mouth syndrome (BMS))[2] and COVID-19.[4]
Head trauma can cause lesions in regions of the central nervous system which are involved in processing taste stimuli, including thalamus, brain stem, and temporal lobes; it can also cause damage to neurological pathways involved in transmission of taste stimuli.[citation needed]
Neurological damage
Tissue damage to the nerves that support the tongue can cause ageusia, especially damage to the chorda tympani nerve and the glossopharyngeal nerve. The chorda tympani nerve passes taste for the front two-thirds of the tongue and the glossopharyngeal nerve passes taste for the back third of the tongue. The lingual nerve (which is a branch of the trigeminal V3 nerve, but carries taste sensation back to the chorda tympani nerve to the geniculate ganglion of the facial nerve) can also be damaged during otologic surgery, causing a feeling of metal taste.[citation needed]
Problems with the endocrine system
Deficiency of vitamin B3 (niacin) and zinc can cause problems with the endocrine system, which may cause taste loss or alteration. Disorders of the endocrine system, such as Cushing's syndrome, hypothyroidism and diabetes mellitus, can cause similar problems. Ageusia can also be caused by medicinal side-effects from antirheumatic drugs such as penicillamine, antiproliferative drugs such as cisplatin, ACE inhibitors, and other drugs including azelastine, clarithromycin, terbinafine, and zopiclone.[citation needed]
COVID-19
Agnosia and anosmia in COVID-19 patients occur independently of nasal obstruction or other rhinitis symptoms, likely attributed to direct viral damage to olfactory and gustatory receptors[5]
Olfactory dysfunction has become a prominent symptom associated with a COVID-19 infection, as supported by growing evidence. Taste dysfunction has often been linked more closely with retronasal olfactory dysfunction rather than a direct impairment of taste itself. Research suggests that taste dysfunction may also present independently, rather than solely as part of an olfactory dysfunction trajectory. This unique tendency of SARS-CoV-2 to induce selective neurological impairment could contribute to the diverse presentation of taste and olfactory dysfunction observed.[6]
Research exploring the prevalence of taste disorders resulting from the COVID-19 pandemic suggests that a diverse range of individuals were affected. In European patients, the prevalence was reported at 55.2%, while in North American patients, it was 61.0%. Among Asian patients, the prevalence was lower at 27.1%, and similarly, in South American patients, it was 29.5%. Australian patients exhibited a prevalence of 25.0%. Furthermore, specific types of taste disorders were also noted, with ageusia observed in 28.0% of patients, hypogeusia in 33.5%, and dysgeusia in 41.3%.[6]
Comparisons among the different taste disorders show that dysgeusia, characterized by altered or distorted taste perception, exhibits a higher prevalence (41.3%) compared to ageusia (28.0%) and hypogeusia (33.5%) among patients with COVID-19 infection. The exact mechanisms underlying the differential presentation of taste disorders in these patients remain uncertain, but several potential explanations exist. All three types of taste disorder could arise from damage along the central taste pathway, involving areas such as the brainstem, thalamus, cranial nerves, or cerebral cortex. Previous evidence suggests that cerebral involvement in COVID-19 may occur during both early and late stages of the infection.
In April 2020, 88% of a series of over 400 COVID-19 disease patients in Europe were reported to report gustatory dysfunction (86% reported olfactory dysfunction).[7]
Other causes
Local damage and inflammation that interferes with the taste buds or local nervous system, such as that stemming from radiation therapy, glossitis, tobacco use, or the wearing of dentures, can also cause ageusia. Other known causes include loss of taste sensitivity from aging (causing a difficulty detecting salty or bitter taste), anxiety disorder, cancer, kidney failure and liver failure.
Diagnosis
Ageusia is diagnosed by an otolaryngologist, who can evaluate a patient's loss of taste among other things. To do this, a specialist will look into any other factors that could be causing ageusia, such as examining the head, nose, ears, and mouth. An otolaryngologist can also conduct a series of tests to assess the severity of ageusia, which includes identifying specific tastes that the patient can sense or recognize.[8][9]
References
External links
- MedTerms Online Medical Dictionary. "Ageusia" Archived 2013-03-21 at the Wayback Machine. Retrieved April 15, 2005.
- Family Practice Notebook. "Taste Sensation". Retrieved April 15, 2005.
- Massachusetts Eye and Ear Infirmary. "Taste Disorders". Retrieved May 26, 2010.