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- Ageusia is the absence of the sense of taste.
- Other taste disorders include hypogeusia (decreased ability to taste), dysgeusia (disordered ability to taste), and phantogeusia (perception of a phantom taste).
- Taste disorders can be total (affecting all tastes), partial (affecting several tastes), or specific (affecting only one or a select few tastes).
- Loss of taste or smell may provide insight to other underlying neurologic or metabolic conditions.
- Smell and taste are important for human safety, nutrition, and quality of life.
- Patients with taste deficits have been noted to eat spoiled foods, decrease their daily oral intake, and report less satisfaction in life. Associated loss of smell may cause patients to ignore dangerous household scents such as smoke or carbon monoxide.
The number of newly diagnosed patients with loss of taste is unknown:
- There are a wide array of causes.
- There is currently no internationally accepted standard for measuring the impairment of the chemosenses.
- The physiology of taste and human variance in taste is not well understood.
- Most likely underestimated
- One study demonstrated that hypogeusia is present in approximately 5% of the population, whereas complete ageusia is very rare and may occur in 1 to 2 individuals per 1,000.
Etiology and Pathophysiology
- Taste receptor cells are found on the surface of the tongue:
- These specialized receptors detect ≥1 of the 5 basic tastes: sweet, umami, bitter, sour, and salty.
- Umami is described as savory and thought to be created by the neurotransmitter combination of glutamate with 5′-ribonucleotides.
- The sense of taste is chemically initiated when food and saliva interact with taste receptor cells and then afferent signals from the taste buds project to the nucleus of the solitary tract in the medulla, in turn causing a series of relays to the thalamus and the postcentral somatosensory cerebral cortex.
- The anterior two-thirds of the dorsal tongue contains filiform papillae and fungiform papillae, mainly at the lateral margins. Larger circumvallate papillae are located anterior and parallel to the sulcus terminalis (which separates the anterior and posterior tongue). Foliate papillae are also found on the lateral borders of the tongue.
- The taste buds contain three types of receptors: Type I receptors detect salt taste; type II receptors detect sweet, bitter, and umami tastes; and type III receptors detect sour taste.
- Innervation of the tongue for taste are the following:
- The anterior two-thirds and lateral margins of the tongue (CN V—trigeminal nerve): chorda tympani division of the facial nerve (CN VII)
- The posterior one-third of the tongue: the glossopharyngeal nerve (CN IX)
- Soft palate: the palatine branch of the greater superficial petrosal division of CN VII
- Oropharynx: the vagus nerve (CN X)
- Oral mucosa: free nerve ending of the trigeminal nerve signaling sensations of touch, pain, and temperature
- Ageusia or hypogeusia may actually reflect a dysfunction in smell or olfactory function:
- Flavor consists of a full chemosensory experience involving taste buds, olfaction, temperature, and texture.
- Complete loss of taste is rare, due to redundancy of the taste system via myriad innervation.
- Olfactory dysfunction is the most common cause of taste disturbance.
- Sinusitis, allergic rhinitis, nasal polyposis, craniofacial injuries, and other obstructive nasal pathologies can block odorant particles from reaching the olfactory nerve (CN I).
- Middle ear disease (i.e., otitis media) or recent otologic surgery may also lead to reduced taste due to irritation of the chorda tympani.
- Nonolfactory causes can be divided into three categories:
- Local: radiation therapy, oral infections/gingivitis, dentures, dental procedures, dry mouth, taste bud damage (scarring, trauma, invasive cancers)
- Systemic: cancer, renal failure, hepatic failure, nutritional deficiency (vitamin B3, zinc), Cushing syndrome, hypothyroidism, diabetes mellitus, infection (viral), drugs (see next section: antirheumatic and antiproliferative, corticosteroids and chemotherapeutic agents)
- Neurologic: Bell palsy (chorda tympani inflammation), familial dysautonomia, multiple sclerosis, viruses that invade the nerves involved with taste, epilepsy, stroke; neurodegenerative disease (i.e., Parkinson disease)
- Many medications are known to alter taste by changing the composition or flow rate of saliva or by affecting the taste receptor function. Medications include but not limited to the following:
- Acarbose, acetazolamide, amitriptyline, angiotensin II receptor antagonists, aspirin, atorvastatin, captopril, carboplatin, cetirizine, cisplatin, clidinium, clomipramine, clopidogrel, cocaine, corticosteroids, cyclophosphamide, diazoxide, dicyclomine, doxorubicin, enalapril, etidronate, fluoxetine, fluvoxamine, indomethacin, isotretinoin, levodopa, lithium, methimazole, methotrexate, metronidazole, penicillamine, pentamidine, phenytoin, propantheline, propylthiouracil, rifabutin, ritonavir, rivastigmine, selegiline, spironolactone, sulfadoxine, terbinafine, topiramate, and venlafaxine
Preference for tastes varies, likely due to a combination of exposures, learned taste, gender, maturity, and genetic variation.
- Increased age
- Infection (especially involving the nose and sinuses, throat, or lower airway)
- Trauma or surgery to the head or face
- Poor oral hygiene
- Smoking or alcohol use
- Environmental exposures (e.g., vapors, gases, adhesives, mold, fungus)
- Metabolic or neurologic disorders
- Chemotherapy or radiation exposure
- No specific recommendations at this time for the prevention of ageusia
- However, avoiding medications with known side effects of taste alteration may help.
Commonly Associated Conditions
Adrenocortical insufficiency, amyloidosis, cystic fibrosis, diabetes mellitus, gastroesophageal reflux, herpes zoster oticus, hypertension, kidney disease, meningitis/encephalitis, multiple sclerosis, Sjögren syndrome, stroke, vitamin B or zinc deficiency
Cancer patients who undergo systemic chemotherapy or radiation to the head and neck can commonly present with smell and/or taste dysfunction after treatment.
Common neurodegenerative diseases such as Alzheimer or Parkinson can present with decreased or loss of taste as an early sign of developing disease.