Description

• Physiologically, the senses of smell and taste aid in normal digestion by triggering GI secretions.
• 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.

Prevalence

• Predominant sex: male > female. Men begin to lose their ability to smell earlier in life than women.
• Predominant age:
  • 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.
• Prior to the COVID-19 pandemic, an estimated >2 million affected in the United States.
• Commonly occurs with SARS-CoV-2, the virus responsible for COVID-19

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).Unexplained chemosensory loss is a good predictor of COVID-19 (4),(5)[B].
  • Nutritional factors (e.g., malnutrition, vitamin deficiencies, liver disease, pernicious anemia)
  • Endocrine disorders (e.g., thyroid disease, diabetes mellitus, renal disease)
  • Migraine headache (e.g., gustatory aura, olfactory aura)
  • Sjögren syndrome
  • Toxic exposures
  • Neurodegenerative diseases (e.g., multiple sclerosis, Alzheimer disease, cerebrovascular accident, Parkinson disease)
  • Infections
• Smell-specific disturbance:
  • Nasal and sinus disease (e.g., allergies, rhinitis, rhinorrhea, URI)
  • Cigarette smoking
  • Cocaine abuse (intranasal)
  • Hemodialysis
  • Neoplasm (e.g., brain tumor, nasal polyps, intranasal tumor)
  • Systemic lupus erythematosus (SLE)
  • Bell palsy
• Taste-specific loss:
  • Oral appliances, procedures
  • Intraoral abscess, gingivitis
  • Damage to CNs VI, IX, or X
  • Stroke (especially frontal lobe)
• Selected medications:
  • 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
Unknown

Risk Factors

• Poor nutritional status
• Smoking

General Prevention

• Well-balanced diet
• Maintain good oral and nasal health.
• Avoid tobacco products, chemical exposures.

Geriatric Considerations
Anosmia also may be an early sign of degenerative disorders and has been shown to predict increased 5-year mortality (6)[B].

Pediatric Considerations

• 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
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