Loss of smell (anosmia)
Complete or partial loss of the sense of smell. Most cases are post-viral (COVID-19, common cold, influenza) or chronic sinonasal disease. New unilateral loss or progressive loss with neurological features needs ENT / neurology workup.
What it means
Anosmia is complete loss of the sense of smell; hyposmia is partial loss. Two anatomical mechanisms: (1) conductive — odour molecules can't reach the olfactory epithelium (chronic rhinosinusitis, nasal polyps, deviated septum); (2) sensorineural — the olfactory neurons or their central connections are damaged (post-viral, head trauma, neurodegenerative disease, drug-induced). Since 2020, COVID-19 has been the single largest cause of acute anosmia globally; most patients recover smell within 2-4 weeks, but 5-15% have persistent loss for >6 months. Other infections cause similar damage. Head trauma (especially anterior cranial fossa fracture) shears the olfactory nerves crossing the cribriform plate. Anosmia is one of the EARLIEST signs of Parkinson's disease and Alzheimer's disease, often preceding motor / cognitive symptoms by years. Other causes: chronic sinonasal disease, intranasal drugs (cocaine, zinc gluconate sprays — withdrawn), Kallmann syndrome (congenital, with hypogonadism), brain tumour (rarely; meningioma of olfactory groove).
Common causes
- Post-viral (COVID-19, common cold, influenza) — by far the commonest acute cause.
- Chronic rhinosinusitis, nasal polyps — gradual, often with congestion / discharge.
- Head trauma — sudden, after a fall / accident; shears olfactory nerves.
- Smoking — long-term smokers have reduced smell.
- Ageing — gradual decline after 60.
- Parkinson's disease, Alzheimer's disease — early non-motor / non-cognitive sign.
- Drug-induced — zinc gluconate intranasal sprays, intranasal cocaine, certain chemotherapies.
- Kallmann syndrome — congenital anosmia + hypogonadism.
- Brain tumour — olfactory groove meningioma, frontal lobe lesion (rare).
- Vitamin B12 deficiency, severe hypothyroidism — modest contributors.
Lab work-up approach
Bloodwork has a supporting role: B12 (deficiency contributes; easy to fix), TSH (severe hypothyroidism affects smell), CBC. SARS-CoV-2 antigen / PCR if acute. Sinonasal endoscopy / CT for chronic sinusitis. MRI brain if unilateral, progressive without rhinitis, or paired with neurological features. Mediora.AI flags B12 + TSH + anaemia patterns; the diagnostic workup is ENT + neurology.
Tests Mediora.AI can interpret
Related conditions
When to see a doctor
Sudden loss of smell during a viral illness usually resolves within 2-4 weeks; persistent loss >6 weeks warrants ENT review and 'smell training' (structured exposure to 4 strong odours twice daily for 3+ months — restores smell in ~50% of post-viral cases). Unilateral or progressive anosmia without rhinitis needs MRI brain (exclude tumour) and neurology review (early Parkinson's / Alzheimer's). Sudden anosmia + headache + visual changes is an emergency. Mediora.AI flags reversible lab contributors; smell-loss diagnosis is ENT + neurology.