Approach to loss of smell
- Approach to a complaint
Loss of smell may be conductive, from polyps or rhinosinusitis, or sensorineural, from viruses, trauma or neurodegenerative disease.
What it is
Loss of smell (hyposmia or anosmia) is divided into conductive, when odours do not reach the olfactory epithelium, and sensorineural, when the epithelium, the nerve or the central pathway is damaged. The most common causes are sinonasal disease, viral infection and craniofacial trauma. Many patients do not notice their deficit, so it is worth measuring. It may also herald neurodegenerative diseases such as Parkinson or Alzheimer disease, and it is a hazard at home: the person cannot detect smoke, gas or spoiled food.
Key questions in the history
- Did it start after a viral illness, a head injury or gradually?
- Nasal blockage, discharge or polyps? Does it fluctuate?
- Does the patient smell things that are not there, or are smells distorted?
- Tremor, slowness, memory problems?
- Normal puberty? How long without smell?
- Tobacco, chemical exposure, radiotherapy or new medicines?
Causes and classification
- Conductive: odours do not reach the epithelium (chronic rhinosinusitis with polyps, allergic rhinitis, tumours).
- Sensorineural: damage to the epithelium, nerve or central pathway (post-viral, including COVID-19, head injury, toxins, drugs).
- Neurodegenerative: may precede Parkinson disease and Alzheimer disease.
- Congenital: Kallmann syndrome (anosmia with hypogonadotropic hypogonadism).
- By degree: anosmia (absent) and hyposmia (reduced); parosmia (distorted smells) and phantosmia (smells that are not there).
Step-by-step approach
- Ask how it began: after a viral illness, after trauma or gradually.
- Look for nasal symptoms; if the loss fluctuates, think of a conductive cause.
- Do rhinoscopy and, if possible, nasal endoscopy to look for polyps or masses.
- Measure smell with a validated odour identification test.
- Do a neurological exam and look for signs of Parkinson disease, cognitive decline or absent puberty.
- If there is no sinonasal cause, or the loss is one-sided or there are neurological signs, order MRI.
Differential diagnosis of loss of smell
| Cause | Type | Key clue |
|---|---|---|
| Post-viral, including COVID-19 | Sensorineural | Onset after a respiratory infection; parosmia may occur |
| Chronic rhinosinusitis with polyps | Conductive | Blockage, discharge, fluctuating loss |
| Allergic rhinitis | Conductive | Sneezing, itching, watery discharge |
| Craniofacial trauma | Sensorineural | Onset after the blow; there may be a cribriform plate fracture |
| Neurodegenerative disease | Central | Parkinson or Alzheimer disease; may precede other symptoms |
| Kallmann syndrome | Congenital | Has never smelled; absent puberty |
Red flags
- One-sided loss with blockage or nosebleeds: rule out a sinonasal tumour.
- Neurological signs or cognitive decline.
- After head injury with clear nasal discharge: possible CSF leak.
- An adolescent with no puberty and anosmia.
- Hazard at home: cannot detect smoke, gas or spoiled food.
Next step / Initial management
- Conductive cause: an intranasal corticosteroid and saline rinses; polyp surgery is decided by ENT.
- Post-viral or post-traumatic: olfactory training (smelling several scents every day) and follow-up; many improve with time.
- Review drugs that affect smell and advise smoke and gas detectors at home.
- Routine ENT referral if it persists without a clear cause; priority referral if one-sided or with a mass, and to neurology if there are neurological signs.
Test yourself
Think of the answer, then tap the question to see it.
A 40-year-old woman lost her sense of smell after a viral respiratory infection 2 months ago; nasal endoscopy is normal. What do you recommend?
Olfactory training (smelling several scents every day) with follow-up: the non-drug option for post-viral smell loss.
A 16-year-old with no pubertal development has never been able to smell. Which syndrome do you suspect?
Kallmann syndrome: hypogonadotropic hypogonadism from GnRH deficiency with anosmia or hyposmia.
Which neurodegenerative diseases can be heralded by loss of smell?
Parkinson disease and Alzheimer disease.
What does a loss of smell that fluctuates and comes with nasal blockage and discharge suggest?
An inflammatory conductive cause, such as chronic rhinosinusitis with polyps or allergic rhinitis.
Connections
Educational content; it does not replace a medical assessment and is not a clinical instruction. For a real case, follow current guidelines and specialist advice.
Sources: Anosmia (StatPearls Publishing (NCBI Bookshelf), 2023); Olfactory Testing (StatPearls Publishing (NCBI Bookshelf), 2022); Olfactory Training (StatPearls Publishing (NCBI Bookshelf), 2024); Smell Disorders (National Institute on Deafness and Other Communication Disorders (NIDCD), 2023); Nasal Polyps (StatPearls Publishing (NCBI Bookshelf), 2025); Kallmann Syndrome (StatPearls Publishing (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.