What are the most common causes of chronic anosmia?

Most Common Causes of Chronic Anosmia

The most common causes of chronic anosmia are post-viral infections (including COVID-19), chronic rhinosinusitis with or without nasal polyps, and head trauma, with other significant causes including neurodegenerative diseases and idiopathic anosmia. 1, 2

Primary Etiologies of Chronic Anosmia

1. Post-Viral Olfactory Loss

• Most notably associated with COVID-19 in recent years, with studies showing 35-85% of COVID-19 patients experiencing olfactory dysfunction 1
• Other respiratory viruses including rhinovirus, influenza, and other coronaviruses
• Mechanism involves inflammation of olfactory clefts and damage to olfactory epithelium 2
• Can persist for months to years, with varying rates of spontaneous recovery (13-44% complete resolution, 14-16% partial resolution) 2

2. Sinonasal Disease

• Chronic rhinosinusitis (CRS) with or without nasal polyps is a major cause 1
• Nasal polyps specifically associated with persistent nasal obstruction and anosmia (29% of patients with nasal polyps report anosmia) 1
• Mechanism is primarily obstructive, preventing odorants from reaching olfactory receptors
• Severity of sinus disease correlates with persistence of anosmia despite treatment 3

3. Head Trauma

• Post-traumatic anosmia results from shearing of olfactory nerve fibers as they pass through the cribriform plate
• Often permanent due to limited regenerative capacity of olfactory neurons
• Severity depends on the nature and force of impact

4. Neurodegenerative Diseases

• Early symptom in Parkinson's disease and Alzheimer's disease
• Can precede motor symptoms in Parkinson's by several years
• Results from pathological changes in central olfactory pathways

5. Sarcoidosis

• Granulomatous inflammation affecting the nasal mucosa
• Can cause both mechanical obstruction and sensorineural anosmia 1
• Often accompanied by nasal crusting, epistaxis, and other sinonasal symptoms

6. Atrophic Rhinitis

• Characterized by progressive atrophy of nasal mucosa
• Associated with crusting, dryness, and anosmia 1
• Can be primary (idiopathic) or secondary to other conditions

7. Idiopathic Anosmia

• Diagnosis of exclusion when no identifiable cause is found
• May represent undiagnosed viral damage or early neurodegenerative disease

Diagnostic Approach to Chronic Anosmia

Initial Assessment

• Duration and onset pattern (sudden vs. gradual)
• Associated symptoms (nasal obstruction, rhinorrhea, facial pain)
• History of viral infections, particularly COVID-19
• History of head trauma or neurodegenerative symptoms

Objective Testing

• Validated olfactory function tests (Sniffin' Sticks, UPSIT) are essential for quantifying degree of dysfunction 4
• Nasal endoscopy to evaluate for polyps, structural abnormalities, and signs of chronic inflammation
• Imaging (CT/MRI) indicated for:
  • Suspected sinonasal disease (CT preferred)
  • Suspected intracranial pathology (MRI preferred)
  • Persistent unexplained anosmia 4

Management Considerations

Post-Viral Olfactory Loss

• Olfactory training is first-line treatment (systematic exposure to 4 different odors twice daily for at least 12 weeks) 2, 5
• Intranasal corticosteroids may be beneficial, especially when combined with olfactory training 2, 5
• Systemic corticosteroids have limited evidence and potential side effects 2

Sinonasal Disease

• Medical management of underlying condition (intranasal corticosteroids, antibiotics if indicated)
• Surgical intervention (functional endoscopic sinus surgery) for cases refractory to medical management 1
• Prognosis depends on severity of disease and persistence of mucosal inflammation 3

Other Considerations

• Patient counseling regarding safety concerns (gas leaks, smoke detection, food spoilage)
• Quality of life impact is significant and should not be dismissed 2
• Referral to specialized smell and taste clinics for persistent anosmia (>6 months) 2

Common Pitfalls

• Assuming spontaneous recovery will occur in all cases of post-viral anosmia
• Inadequate duration of olfactory training (minimum 12 weeks required)
• Failure to address underlying sinonasal disease
• Dismissing the psychological impact of chronic anosmia on quality of life

Understanding the specific etiology of chronic anosmia is crucial for appropriate management and setting realistic expectations regarding prognosis and potential recovery.