What is the role of gustatory dysfunction in the etiology of taste and smell disorders, and how is COVID-19 associated with anosmia and dysgeusia?

Updated: Jul 07, 2020
  • Author: Eric H Holbrook, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

Much of what is perceived as a taste defect is truly a primary defect in olfaction resulting in an alteration of flavor. The components that comprise the sensation of flavor include the food's smell, taste, texture, and temperature. Each of these sensory modalities is stimulated independently to produce a distinct flavor when food enters the mouth.

Taste may be enhanced by tongue movements, which increase the distribution of the substance over a greater number of taste buds. Adaptation in taste perception exerts a greater influence than in other sensory modalities.

Other than smell dysfunction, the most frequent causes of taste dysfunction are prior URTI, head injury, and idiopathic causes, but many other causes can be responsible.

Lesions at any site from the mucosa, taste buds, unmyelinated nerves, or cranial nerves to the brain stem may impair gustation.

Oral cavity and mucosal disorders including oral infections, inflammation, and radiation-induced mucositis can impair taste sensation. The site of injury with radiotherapy is probably the microvilli of the taste buds, not the taste buds themselves, since taste buds are thought to be radioresistant.

Poor oral hygiene is a leading cause of hypogeusia and cacogeusia. Viral, bacterial, fungal, and parasitic infections may lead to taste disturbances because of secondary taste bud involvement.

Normal aging produces taste loss due to changes in taste cell membranes involving altered function of ion channels and receptors rather than taste bud loss. [3, 36]

More than 200 medications have been associated with taste disorders. [37] Clinicians need to be aware of this, especially with regard to patients taking numerous drugs.

Malignancies of the head and neck, as well as of other sites, are associated with decreased appetite and inability to appreciate flavors.

Use of dentures or other palatal prostheses may impair sour and bitter perception, and tongue brushing has been shown to decrease taste acuity.

Surgical manipulation may alter taste permanently or temporarily. Resection of the tongue and/or portions of the oral cavity, most commonly for reasons of malignancy, decreases the number of taste buds. Radiation and chemotherapy damage taste receptors and decrease salivary flow, altering taste perception. In otologic surgery, stretching or transection of the chorda tympani nerve may result in temporary dysgeusia. Bilateral injury still may not result in permanent taste dysfunction, because of the alternate innervation through the otic ganglion to the geniculate ganglion via the greater superficial petrosal nerve.

Gastric bypass surgery can also have adverse olfactory and gustatory effects. In a study by Graham et al of 103 patients who underwent Roux-en-Y gastric bypass, sensory changes in taste and smell were reported by 73% and 42% of these individuals, respectively, [38]  although patients seem to have less olfactory loss if the bypass is done laparoscopically. [39]

Nutritional deficiencies are involved in taste aberrations. Decreased zinc, copper, and nickel levels can correlate with taste alterations. Nutritional deficiencies may be caused by anorexia, malabsorption, and/or increased urinary losses.

Endocrine disorders also are involved in taste and olfactory disorders. Diabetes mellitus, hypogonadism, Sjögren syndrome, and pseudohypoparathyroidism may decrease taste sensation, while hypothyroidism and adrenal cortical insufficiency may increase taste sensitivity. Hormonal fluctuations in menstruation and pregnancy also influence taste.

AIDS patients often complain of alterations in taste, and detection thresholds of glutamic acid and hydrochloride are higher in patients suffering from AIDS. [40]

Heredity is involved in some aspects of gustation. The ability to taste phenylthiourea (bitter) and other compounds with an –N-C= group is an autosomal dominant trait. Studies have shown that phenylthiourea tasters detect saccharin, potassium chloride (KCl), and caffeine as more bitter. Type I familial dysautonomia (ie, Riley-Day syndrome) causes severe hypogeusia or ageusia because of the absence of taste bud development.

Direct nerve or CNS damage, as in multiple sclerosis, facial paralysis, and thalamic or uncal lesions, can decrease taste perception.

Many other diseases can affect gustation (eg, lichen planus, aglycogeusia, Sjögren syndrome, renal failure with uremia and dialysis, erythema multiforme, geographic tongue, cirrhosis).

COVID-19

According to the American Academy of Otolaryngology - Head and Neck Surgery (AAO-HNS), anecdotal evidence indicates that anosmia and dysgeusia are symptoms of coronavirus disease 2019 (COVID-19). The AAO-HNS recommends that in patients in whom other respiratory diseases, such as allergic rhinitis, acute rhinosinusitis, and chronic rhinosinusitis, are not present, the occurrence of anosmia or hyposmia, as well as dysgeusia, should raise suspicion for COVID-19 infection. [41, 42, 43]  The Centers for Disease Control and Prevention (CDC) has added "new loss of taste or smell" to its list of symptoms that may arise 2-14 days after exposure to the COVID-19 virus (ie, severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]). [44]  Loss of taste or smell has also been added by the World Health Organization (WHO) to its list of less common COVID-19 symptoms. [45]

A study by Speth et al of 103 patients with COVID-19 found the prevalence of olfactory dysfunction to be 61.2%, with the condition occurring on median infection day 3. A strong correlation was reported between the severity of olfactory dysfunction and the severity of loss of taste. In addition, patients with olfactory dysfunction tended to have more severe shortness of breath. The investigators also found that olfactory dysfunction was less common in older age and more prevalent in females. [46]

Another study, a literature review by Aziz et al, indicated through pooled analysis that taste sensation is altered in almost 50% of patients with COVID-19, although it was suggested that, due to underreporting, the prevalence may be even higher. [47]

A study by Boscolo-Rizzo et al of adult patients with mild COVID-19 reported that within 4 weeks of taste or smell alteration, this symptom partially or completely resolved in 89% of them. [48, 49]


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