DESCRIPTION

• Anosmia means partial or complete loss of the sense of smell.
• Partial anosmia has always been a common symptom of acute and chronic nasal inflammation.
• Total anosmia was rare prior to 2020, but SARS-CoV-2 infection has dramatically changed its frequency.
• Much remains unknown about the prognosis for COVID-19–induced anosmia or how it has changed the proper anosmia workup.

EPIDEMIOLOGY

• Congenital absence of olfactory fibers is extremely rare. Almost all smelling disorders are acquired.
• Alteration in olfactory ability can be caused by viral infection, trauma, or anterior skull base neoplasms. Before the COVID-19 pandemic, when anosmia was a rare condition, non–COVID-19 viral infections were by far the most common cause.
• In the initial wave of SARS-CoV-2 infections, anosmia was the first symptom in >25% of patients, contributed as a reason for testing in 40% of patients, and was noted in 73% of patients prior to diagnosis.
• The propensity of SARS-CoV-2 infection causing anosmia (especially with earlier strains) has completely changed anosmia epidemiology. Anosmia is now a common chief complaint and can be assumed to be caused by SARS-CoV-2 infection unless proven otherwise.

ETIOLOGY

Since 2020, almost all anosmia has been acquired via SARS-CoV-2 infection.

RISK FACTORS

Young patients are more likely to experience anosmia after COVID than older patients.

GENERAL PREVENTION

Vaccination, and when indicated mask wearing and social distancing, decreases the risk of COVID-19 infection and therefore anosmia.

PATHOPHYSIOLOGY

• Congenital anosmia is caused by absent, not defective, olfactory fibers.
• Severe nasal inflammation (e.g., nasal polyps caused by cystic fibrosis or primary ciliary dyskinesia) blocks the nose so that aromas cannot reach olfactory fibers.
• Traumatic and neoplastic invasion cause anosmia via nerve destruction. Viral-induced anosmia is thought to occur via injury through one of three possible mechanisms: release of tumor necrosis factor, death of olfactory receptor cells, or damage to intracranial olfactory neurons.

COMMONLY ASSOCIATED CONDITIONS

• Gustatory (taste) dysfunction was noted in 43% of the first wave of COVID-19 patients.
• The Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS) epidemics were also characterized by anosmia.