Aetiology

There are numerous causes of taste disorders that, although common, remain misunderstood among the medical community. Most patients undergo numerous consultations, often with very little insight into their condition. This can have an adverse impact on a patient's quality of life. Furthermore, taste disorders, and especially distorted taste, can be an early symptom of significant or life-threatening illness (e.g., amyotrophic lateral sclerosis [ALS], multiple sclerosis, lung cancer, myasthenia gravis (with thymoma), or coronavirus disease 2019 [COVID-19]).[1][11][13]​​​​​​[34][35]​​[36]

Post-infectious

Upper respiratory tract infections: classically lead to olfactory dysfunction, but taste function may also be compromised. These taste disorders are often transient and reversible.

COVID-19: a potentially severe acute respiratory infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A systematic review reported a pooled prevalence of 44% for gustatory dysfunction among patients with COVID-19.[37] In one survey, 89% of patients with mild disease experienced complete resolution or improvement in altered sense of taste or smell after 4 weeks.[38] With the development of new variants of COVID-19, the prevalence of these symptoms is subject to change.[39] Most cases resolve relatively rapidly, however approximately 5% of patients may have prolonged symptoms.[13][25]​​

Chronic middle ear infections and inflammation: chronic or repeated episodes of middle ear infection or otitis media may affect afferent taste fibres of the chorda tympani.[40][41]

Iatrogenic/post-surgical

Altered gustatory function may arise secondary to surgical procedures. The afferent pathways involved depend on the nature of the procedure. Surgical taste disorders are among the most frequently encountered in specialised smell and taste disorder outpatient clinics. Procedures that may be complicated by post-surgical taste distortions or ageusia include:

  • Dental surgery: procedures involving the mandibular teeth associated with lingual nerve proximity are potentially threatening to taste pathways. Classically, prolonged lingual nerve damage following an inferior alveolar (dental) nerve anaesthetic delivery and wisdom tooth extraction are the interventions most likely to produce taste disorders.​​[41]

  • Tonsillectomy: approximately 35% of patients report transient taste disorders directly after tonsillectomy. Most resolve within 2 years of surgery, but rarely these taste disorders become permanent.[42][43][44] The underlying process is probably damage to the lingual branch of the glossopharyngeal nerve carrying taste fibres from the rear of the tongue.[41]

  • Middle ear surgery: this often alters taste function.[41]​ Damage is ipsilateral to the operated side and is often unnoticed by the patient. The underlying process is probably damage to the chorda tympani, which carries the taste fibres for the anterior two-thirds of the tongue. The chorda tympani runs unprotected through the middle ear cavity in close relation to the ossicles and the tympanum.

  • Upper airway endoscopy: any rigid endoscopy (e.g., micro-laryngoscopy) of the upper airways compresses the base of the tongue. This results in a transient taste disturbance in about 10% of patients.[45]

  • Maxillofacial and cranial surgery: direct cutting of the tongue, palate, or other oral structures (e.g., during cancer surgery or a velopharyngeal procedure) may be associated with taste dysfunction.[1]​ Surgery to the base of the skull (e.g., for a tumour or aneurysm) may also result in taste disturbance.​[41]​​

Most post-surgical taste disorders are unilateral after a unilateral surgical procedure. They are often transient. Typically, qualitative complaints (e.g., taste distortions, metallic taste) are noticed more often and are more likely to lead to a consultation than quantitative disorders (loss of taste). Taste disturbance is rarely noticed by patients following otological surgical procedures, even though an associated chorda tympani injury is well recognised.

Neurological

Taste disorders may accompany classic neurological diseases, such as stroke or peripheral lesions. Rarely, abnormal taste (especially distorted taste) can be an early symptom of a significant or life-threatening fatal condition, such as ALS, multiple sclerosis, or myasthenia gravis.[11][36]​​​[34] In such cases, taste disorders are often associated with other more bothersome symptoms such as dysphasia, dysphonia, or dysarthria. Well-recognised causes include:

  • Bell's palsy and Ramsay Hunt's syndrome: this is active infection of the geniculate ganglion of the facial nerve (VII) by varicella-zoster virus. Taste disorders are associated with facial nerve paralysis in approximately one third of cases.[46] Recovery often occurs following resolution of the paralysis.​

  • Stroke: every stroke lesion involving parts of the central taste pathway may produce taste disorders. The taste disorder is mostly unnoticed or less important in light of the other stroke sequelae. However, systematic investigation of stroke patients has revealed that up to 20% of patients have an altered taste function. The dysfunction may manifest as ipsilateral or bilateral ageusia.[22][47]​​

  • Head trauma: although olfactory function is more commonly altered after cranial trauma, taste dysfunction also occurs.[11]​ Few studies have investigated post-traumatic taste disorders, although it seems that the incidence increases with the severity of the injury.​​[46]

  • Neurodegenerative disease (e.g., Parkinson's disease, Alzheimer's dementia, ALS, multiple sclerosis): most neurodegenerative diseases are associated with olfactory disorders, and case reports suggest that these diseases may also affect taste.​​[46][11]​​ There are cases reporting dysgeusia to be one of the first symptoms of Parkinson's disease and ALS.[34][48]​ Taste disorders in multiple sclerosis are clearly related to the site of the plaques (e.g., brainstem or insula). Many patients with dementia are unaware of their taste deficit.​[46]​​

  • Epilepsy: gustatory (and olfactory) auras (dysgeusia) may manifest as a result of seizure activity.​[46]

  • Myasthenia gravis: dysgeusia as a first symptom of myasthenia gravis due to thymoma has been reported.[36]​​

  • Guillain-Barre syndrome: there have been a few reports of dysgeusia in these patients.[46]

Metabolic or systemic

Very little is known about the influence of systemic and metabolic diseases on taste function. Systemic conditions (including some documented as isolated case reports) associated with taste disturbance include:

  • Inflammatory-mediated salivary gland disease, such as Sjogren syndrome (primary or secondary).[1]​​​​

  • Renal insufficiency: several studies have shown that taste function is influenced quantitatively and qualitatively by renal insufficiency and dialysis.[49][50]​​[51]​​ However, often taste loss is unnoticed. 

  • Hepatic insufficiency: hepatitis and liver failure may cause taste disorders.​[52]​​

  • Diabetes mellitus: taste disorders have been reported in diabetes.[1][11]

  • Thyroid dysfunction: thyroid disease and thyroid treatments (radioactive iodine therapy, anti-thyroid medication) have been associated with taste disorders.​[11][53]

  • Burning mouth syndrome: this condition is a combination of bilateral lingual paraesthesia and oral pain, sometimes with dysgeusia or other taste dysfunction. The pathophysiology of this condition is unclear, but neuropathic changes within the oral mucosa may be responsible.[1]​ Its prevalence is highest in peri- and post-menopausal women, and may be more common in 'super-tasters' (people with enhanced abilities to detect taste).

  • Ageing is also associated with diminution of taste.[24][23]

Toxin- or drug-induced

Transient disappearance of taste function is a major adverse effect of oncological radiotherapy.[27]​ 

Radiotherapy to the head and neck may directly affect taste receptors and produce irreversible changes to salivary gland function, with the resulting xerostomia significantly affecting taste.

Systemic chemotherapy may cause taste change that is usually reversed on cessation of treatment and may be related to direct toxicity and from secretion of medication in saliva.[28][30]​​​​​[32]​​​​​ However, dysgeusia in these patients may persist indefinitely.[33][54]​​ Taste changes in patients with cancer have been shown to have an impact on both the patients' weight and quality of life.[55]

Almost any drug or medicine has the potential to cause taste disorders.[56]​ Consequently, there are countless case reports on diverse drugs causing taste disorders. Drug- and medicine-induced taste disorders seem to be reversible after drug discontinuation.[11]​ Commonly implicated medicines include ACE inhibitors, antibiotics (e.g., metronidazole), and antifungals (particularly terbinafine).[1]

Neoplastic

Tumours of the pontocerebellar angle: typically, these tumours affect audition and balance. However, facial nerve function and taste function may be disturbed if the tumour impinges on the nervus intermedius part of cranial nerve VII.​[46]

Paraneoplastic: there have been several reports of sweet dysgeusia as an initial symptom of small cell lung cancer and thymoma; in some cases, the taste disturbance has been associated with hyponatraemia.[1][36][57]

Nutritional deficiencies

Little is known about the basis of taste disorders due to nutritional deficiencies. Nevertheless, some deficiencies have been associated with taste dysfunction. These include:

  • Iron deficiency: chronic iron deficiency may be accompanied by burning mouth syndrome and abnormal taste symptoms.[58]

  • Vitamin B12 deficiency: less commonly, abnormal taste may accompany glossitis.

  • Zinc deficiency: can lead to quantitative or qualitative taste disorders such as taste reduction, taste loss, or dysgeusia.[59] Zinc supplements may be used to improve taste acuity in patients with zinc deficiency/idiopathic taste disorders, however the evidence to support its use is conflicting.[1][59][60]​​ A trial with zinc supplementation may be beneficial, and should be considered in patients with a history of gradual-onset taste loss if there are no clinical features to suggest another cause.

Periodontal disease

Periodontal disease such as draining fistulas, dental abscess, necrotising gingivitis, cellulitis and oromucosal infection by Candida species may cause altered taste.[61] Similarly, localised dysgeusia may be the result of local allergic reactions to dental restorative materials.[13] A thorough oral examination is always recommended.

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