Anosmia

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Basics

Description

  • Anosmia is defined as the inability to detect smells or odors. This condition can be temporary or permanent depending on the underlying pathology.
  • Hyposmia is defined as a decrease in sense of smell.
  • Olfaction, the sense of smell, is necessary in determining the taste and flavor of foods and beverages. It is also important in the detection of environmental hazards such as spoiled food, leaking natural gas, smoke, or airborne pollutants.
  • Patients with olfactory disturbances may describe a loss of flavor or taste.
  • The human sense of smell is chiefly mediated by cranial nerve I (CN I; olfactory nerve) with contribution from cranial nerve V (CN V; trigeminal nerve).
    • Qualitative odor sensations (e.g., the smell of a flower or of spoiled food) are mediated by CN I.
    • The somatosensory components of odorants (e.g., warmth, coolness, sharpness, and irritation) are mediated by the ophthalmic and maxillary divisions of CN V.

Epidemiology

Incidence
There are >200,000 physician visits each year for chemosensory complaints.

Prevalence
  • Prevalent sex: male > female
  • Age-related deficits in olfaction are well documented (1), and the ability to detect odors gradually declines with age.

Etiology and Pathophysiology

  • Olfactory decline associated with aging (presbyosmia) is the most common cause of anosmia (1)[A].
  • Anosmia not associated with presbyosmia is most often caused by nasal/sinus disease, an acute upper respiratory infection (URI), or head trauma. Just as in hearing loss, these deficits can be divided into conductive loss (more common) and sensory loss.
  • Conductive defects:
    • Obstruction in the nasal cavity, caused by inflammation or tumors, may prevent flow of odorants to the olfactory epithelium:
      • Acute or chronic rhinosinusitis (most common)
      • Nasal polyps in CRS
      • Benign sinonasal papillomas
      • Certain developmental abnormalities (e.g., encephaloceles, dermoid cysts) may also cause obstruction.
      • Malignancy
    • In addition, patients with tracheostomies may experience hyposmia because of a reduced or absent nasal airflow.
  • Central/sensorineural defects:
    • Most olfactory defects are caused by inflammatory processes in the nasal cavity itself.
    • Chronic or recurrent rhinosinusitis can lead to progressive damage of the olfactory mucosa, leading to a permanent decline in olfaction.
      • Viral infections
      • Sarcoidosis (affecting neural structures)
      • Granulomatosis with polyangiitis (Wegener granulomatosis)
      • Multiple sclerosis
    • Genetic causes (e.g., Kallmann syndrome)
    • Endocrine disorders
      • Hypothyroidism
      • Hypoadrenalism
      • Diabetes mellitus
    • Head trauma, stroke, brain surgery, or subarachnoid hemorrhage
      • Shearing damage to olfactory bulbs or cortical lesions at the olfactory-related brain regions (e.g., amygdala, temporal lobe, and frontal lobe) can lead to posttraumatic anosmia.
    • Iatrogenic: toxicity from systemic or inhaled drugs (e.g., aminoglycosides, formaldehyde, alcohol, nicotine, organic solvents, or zinc nasal sprays)
    • Degenerative processes of the central nervous system (CNS) must always be considered, and smell loss may be the first presenting symptom (2)[A].
      • Parkinson disease
      • Alzheimer disease
      • Normal aging, also known as presbyosmia

Genetics
Anosmia may present as a symptom in certain genetically inherited disorders (Kallmann syndrome, migraine syndromes, rheumatologic conditions, endocrine disorders, neurodegenerative disease).

Risk Factors

  • Age >65 years
  • Poor nutrition
  • Tobacco abuse
  • Allergy

General Prevention

  • Smoking cessation
  • Good oral hygiene and awareness/avoidance of noxious chemicals
  • Consuming a well-balanced diet
  • Treatment of sinonasal disease and allergies

Commonly Associated Conditions

Chronic rhinosinusitis, URI, and head trauma

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