Anosmia

Anosmia (also known as anosphresia or olfactory anesthesia) is the complete absence of the sense of smell. 

Terminology

In addition to anosmia, there is also hyposmia (a.k.a. microsmia or olfactory hypoesthesia) representing a diminished sense of smell and parosmia (a.k.a. dysosmia or paraosmia) referring to any pathology of the olfactory system ^1,5. Phantosmia is the experience of a smell being present without an actual stimulus, i.e. an olfactory hallucination ⁵. For completion, normosmia is the state of having normal olfactory function.

Epidemiology

It is estimated that the point prevalence of an impaired sense of smell is as much as 19.1% adult population and four-fifths of over 75-year-olds ².

Pathology

Etiology

The underlying causes of anosmia have been traditionally subdivided into ^2,5:

• conductive: an obstruction to the passage of odorants, or
• sensorineural: dysfunction of the olfactory neurons or olfactory nerve (I)
• central: dysfunction of the olfactory processing systems of the CNS

Conductive

Common

• allergic rhinitis
• rhinosinusitis +/- sinonasal polyposis
• recreational drugs, e.g. cocaine

Uncommon

• systemic pathologies with olfactory complications, e.g. granulomatosis with polyangiitis
• postoperative, for example nasal surgery
• medications, e.g. antibiotics (aminoglycosides, penicillins, macrolides, tetracyclines), amino ester anesthetics (e.g. procaine), propylthiouracil

Sensorineural/central

Common

• viral infections, e.g. coronaviruses
• neurological disease
  • neurodegenerative disorders, e.g. Parkinson disease, Alzheimer disease
  • stroke
  • myasthenia gravis
• psychiatric
  • depression
  • bipolar disorder
  • schizophrenia
• head injury

Uncommon

• intracranial lesions, e.g. olfactory meningioma
• toxins, e.g. heavy metals, solvents (e.g. toluene, ammonia, benzene)
• genetic or malformative, e.g. arhinencephaly or Kallmann syndrome

It is increasingly clear that this anatomical approach to the etiology of olfactory dysfunction is limited, and ultimately unhelpful, as underlying diseases, e.g. chronic rhinosinusitis, may cause loss of smell via both obstructive and neural mechanisms ⁵.

History and etymology

Anosmia is derived from Classical Greek roots. "οσμη" (osme) means an aroma/scent, and "αν" (an) is a prefix used to indicate negation ⁴.