What causes decreased smell (hyposmia) and how is it treated?

Decreased Smell (Hyposmia): Causes and Treatment

Most Common Causes

Viral upper respiratory infections, including COVID-19, are the leading cause of decreased smell, accounting for 20-25% of specialist clinic presentations and affecting 59-86% of COVID-19 patients specifically. 1, 2

Infectious Etiologies

• Post-viral olfactory loss represents approximately 11% of community cases, with coronaviruses and influenza commonly damaging the olfactory neuroepithelium 1, 3
• COVID-19 causes olfactory dysfunction in 59-86% of infected patients, with smell/taste loss presenting as the initial or sole symptom in 11.9-22% of cases 1
• The pathophysiology involves loss of cilia of olfactory sensory neurons rather than true nasal obstruction—63% of COVID-19 patients with anosmia had no nasal congestion or rhinorrhea 4
• Recovery occurs in 73.3% of COVID-19 patients with a median duration of 7.6 days, though some cases persist beyond 28 days 4

Sinonasal and Structural Causes

• Chronic rhinosinusitis with nasal polyposis causes conduction loss from sinonasal passage obstruction, with CT-documented severity correlating with worse olfaction 1
• Inflammatory lesions including sarcoidosis and granulomatosis with polyangiitis impair olfaction 1

Traumatic Causes

• Head trauma is one of the most common causes, particularly frontobasal trauma causing neural damage to the olfactory nerve 1, 2
• The olfactory nerve is the most commonly disrupted cranial nerve in trauma 2

Neurological and Neurodegenerative Conditions

• Alzheimer disease and Parkinson disease are strongly associated with olfactory dysfunction, which can serve as an early diagnostic marker 1, 2
• Temporal lobe dysfunction and seizure disorders can cause olfactory disturbances 1, 2
• Depression is associated with olfactory dysfunction 1

Neoplastic Causes

• Tumors affecting the cribriform plate, including squamous cell carcinomas, meningiomas, and esthesioneuroblastomas, can impair olfaction 1, 2

Critical Clinical Evaluation

History and Physical Examination

• Determine unilateral versus bilateral involvement—unilateral examination is essential as bilateral testing may miss unilateral deficits that patients don't recognize 5
• Assess timing relative to viral illness, trauma, or other symptoms 4
• Distinguish true hyposmia from parosmia (distorted smell) or phantosmia (smell without stimulus) 6, 7
• Most "taste loss" in viral infections actually reflects loss of retronasal olfaction (flavor perception) rather than true taste dysfunction, as true taste only differentiates sweet, sour, salty, and bitter 1

Objective Testing

• Perform objective olfactory testing (UPSIT or Sniffin' Sticks) as patient self-assessment is unreliable—objective testing reveals 98.3% dysfunction versus 35% self-reported 1, 2
• Complete nasal endoscopy to identify sinonasal pathology 2
• Neurological examination focusing on cranial nerves and signs of neurodegenerative disease 2

Imaging Recommendations

• MRI orbits, face, and neck is the mainstay for directly imaging the olfactory apparatus when structural lesions are suspected 2
• CT maxillofacial for suspected sinonasal inflammatory disease, fractures, or bony abnormalities 2
• Do not order routine neuroimaging when there is a clear temporal relationship to viral infection 1

Treatment Algorithm

Post-Viral Hyposmia (Including COVID-19)

• Modified olfactory training (MOT) is the primary evidence-based treatment for post-infectious olfactory loss and COVID-19-induced parosmia 7
• Reassurance that spontaneous recovery occurs in 73.3% of COVID-19 cases within a median of 7.6 days 4
• Many post-viral cases naturally resolve without active therapy 6

Sinonasal Disease

• Treat underlying chronic rhinosinusitis with appropriate medical or surgical management 1
• Topical corticosteroids for inflammatory conditions 6

Traumatic Hyposmia

• Prognosis is generally poor for traumatic olfactory loss 3
• Olfactory training may provide benefit 7

Neurodegenerative Disease

• Address underlying condition, though olfactory dysfunction may be irreversible 1
• Counsel patients on safety measures regarding inability to detect warning odors 3

Nutritional Considerations

• Ensure adequate vitamin B12, iron, and zinc supplementation, as deficiencies can cause or exacerbate neurological symptoms including sensory disturbances 8
• Consider referral to registered dietitian for comprehensive nutritional assessment 8

Critical Pitfalls to Avoid

• Do not overlook COVID-19 as a potential cause, especially when smell loss is the primary symptom without traditional nasal congestion or rhinorrhea 1
• Do not rely on patient self-assessment of severity—objective testing reveals much higher rates of dysfunction 1
• Do not order unnecessary neuroimaging when there is a clear temporal relationship to viral infection 1
• Do not assume bilateral testing is sufficient—unilateral examination is necessary for definitive evaluation as patients may not recognize unilateral deficits 5
• Do not confuse hyposmia with parosmia or phantosmia, as these qualitative disorders require different management approaches 6, 7

Long-Term Prognosis

• Patients with olfactory dysfunction lasting more than 6 months from flu or cold present serious impairment of olfactory abilities with poor prognosis for recovery 9
• Olfactory dysfunction affects 3-20% of the general population and increases with age 3
• These disorders impair quality of life related to social interactions, eating, feelings of well-being, and ability to sense warning odors 3