Description

• The senses of smell and taste allow a full appreciation of the flavor and palatability of foods and also serve as a warning system against toxins, polluted air, smoke, and spoiled food.
• Physiologically, the chemical senses aid in normal digestion by triggering GI secretions. Smell/taste dysfunction may have a significant impact on quality of life.
• Loss of smell occurs more frequently than loss of taste, and patients frequently confuse the concepts of flavor loss (as a result of smell impairment) with taste loss (an impaired ability to sense sweet, sour, salty, or bitter).
• Smell depends on the functioning of CN I (olfactory nerve) and CN V (trigeminal nerve).
• Taste depends on the functioning of CNs VII, IX, and X. Because of these multiple pathways, total loss of taste (ageusia) is rare.
• Systems affected: nervous, upper respiratory

Epidemiology

Incidence
There are ~200,000 patient visits a year for smell and taste disturbances.

• Predominant sex: male > female. Men begin to lose their ability to smell earlier in life than women.
• Predominant age: Chemosensory loss is age dependent:
  • Age >80 years: 80% have major olfactory impairment; nearly 50% are anosmic.
  • Ages 65 to 80 years: 60% have major olfactory impairment; nearly 25% are anosmic.
  • Age <65 years: 1–2% have smell impairment.
• Estimated >2 million affected in the United States

Etiology and Pathophysiology

• Smell and/or taste disturbances:
  • COVID-19–common in mildly symptomatic patients (~65%) but even more common in patients needing hospitalization (85%) (1),(2),(3). While most recovery occurs within a few weeks, 7% of chemosensory disorders persist beyond 60 days (3). Emerging evidence suggests that recent chemosensory loss appears to be a good predictor of COVID-19 (4)[B].
  • Nutritional factors (e.g., malnutrition, vitamin deficiencies, liver disease, pernicious anemia)
  • Endocrine disorders (e.g., thyroid disease, diabetes mellitus, renal disease)
  • Head trauma
  • Migraine headache (e.g., gustatory aura, olfactory aura)
  • Sjögren syndrome
  • Toxic chemical exposure
  • Industrial agent exposure
  • Aging
  • Medications (see below)
  • Neurodegenerative diseases (e.g., multiple sclerosis, Alzheimer disease, cerebrovascular accident, Parkinson disease)
  • Infections (e.g., upper respiratory infection [URI], oral and perioral infections, candidiasis, coxsackievirus, AIDS, viral hepatitis, herpes simplex virus)
• Possible causes of smell disturbance:
  • Nasal and sinus disease (e.g., allergies, rhinitis, rhinorrhea, URI)
  • Cigarette smoking
  • Cocaine abuse (intranasal)
  • Hemodialysis
  • Radiation treatment of head and neck
  • Congenital conditions
  • Neoplasm (e.g., brain tumor, nasal polyps, intranasal tumor)
  • Systemic lupus erythematosus (SLE)
  • Bell palsy
  • Oral/perioral skin lesion
  • Damage to CN I/V
  • Possible association with psychosis and schizophrenia
• Possible causes of taste loss:
  • Oral appliances
  • Dental procedures
  • Intraoral abscess
  • Gingivitis
  • Damage to CN VI, IX, or X
  • Stroke (especially frontal lobe)
• Selected medications that reportedly alter smell and taste:
  • Antibiotics: amikacin, ampicillin, azithromycin, ciprofloxacin, clarithromycin, doxycycline, griseofulvin, metronidazole, ofloxacin, tetracycline, terbinafine, β-lactamase inhibitors
  • Anticonvulsants: carbamazepine, phenytoin
  • Antidepressants: amitriptyline, doxepin, imipramine, nortriptyline
  • Antihistamines and decongestants: zinc-based cold remedies (Zicam)
  • Antihypertensives and cardiac medications: acetazolamide, amiloride, captopril, diltiazem, hydrochlorothiazide, nifedipine, propranolol, spironolactone
  • Anti-inflammatory agents: auranofin, gold, penicillamine
  • Antimanic drugs: lithium
  • Antineoplastics: cisplatin, doxorubicin, methotrexate, vincristine
  • Antiparkinsonian agents: levodopa, carbidopa
  • Antiseptic: chlorhexidine
  • Antithyroid agents: methimazole, propylthiouracil
  • Lipid-lowering agents: statins

Genetics
May be related to underlying genetically associated diseases (Kallmann syndrome, Alzheimer disease, and other neurodegenerative disorders, migraine syndromes, rheumatologic conditions, endocrine disorders)

Risk Factors

• Age >65 years
• Poor nutritional status
• Smoking tobacco products

General Prevention

• Eat a well-balanced diet, with appropriate vitamins and minerals.
• Maintain good oral and nasal health, with routine visits to the dentist.
• Do not smoke tobacco products.
• Avoid noxious chemical exposures/unnecessary radiation.

Geriatric Considerations

• Elders are at particular risk of eating spoiled food or inadvertently being exposed to natural gas leaks owing to anosmia from aging.
• Anosmia also may be an early sign of degenerative disorders and has been shown to predict increased 5-year mortality (5)[B]. In a separate study, healthy elders (average age 76) with loss of smell had 46% increased mortality at 10 years, 30% increased mortality at 13 years. In an analysis of cause of mortality, only 30% of the increased mortality could be ascribed to neurodegenerative causes and only 6% to weight loss. The mechanisms of increased mortality for the remaining 64% were not obvious from the data (6)[B].

Pediatric Considerations

• Smell and taste disorders are uncommon in children in developed countries.
• In developing countries with poor nutrition (particularly zinc depletion), smell and taste disorders may occur.
• Delayed puberty in association with anosmia (± midline craniofacial abnormalities, deafness, or renal abnormalities) suggests the possibility of Kallmann syndrome (hypogonadotropic hypogonadism).

Pregnancy Considerations

• Pregnancy is an uncommon cause of smell and taste loss or disturbances.
• Many women report increased sensitivity to odors during pregnancy as well as an increased dislike for bitterness and a preference for salty substances.

Commonly Associated Conditions

URI, allergic rhinitis, dental abscesses