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Disturbances of the Sense of Smell

Essentials

  • The most common causes include mucosal swelling associated with inflammation of the nasal cavities and paranasal sinuses, damage to the sense of smell caused by a viral infection, or a trauma to the head region.
  • Disturbances of the sense of smell caused by mucosal swelling can be alleviated with topical glucocorticoids.
  • In disturbances of the sense of smell, the patient often first complains of a weakened sense of taste.

Definition

  • Disturbances of the sense of smell can be caused by central (intracranial) disorders of the olfactory tract or peripheral (intranasal) ventilatory disorders of the nose.
  • Disorders include
    • anosmia (lack of the sense of smell)
    • hyposmia (decreased sense of smell)
    • hyperosmia (increased sensitivity to smells)
    • dysosmia/parosmia (distorted sense of smells).

Investigations

  • A bottle of tar (or other substance with a characteristic, easily recognizable intensive smell) should be available for the general practitioner in the clinical examination.
  • Clinical examination of the nose (anterior and posterior rhinoscopy)
  • In a specialized unit: nasoscopy, smell test, imaging studies (MRI or CT scan of the paranasal sinuses)
    • If there is a suspicion of an abnormality in the area of the olfactory bulb or nerve, or sequelae of a trauma, an MRI of the paranasal sinuses should be performed.
    • If the patient also has neurological symptoms, an MRI of the head should be performed.
    • In patients with nasal polyposis, a CT scan of the paranasal sinuses is performed mainly when planning operative treatment.

Aetiology

  • In most acute cases, patient history will reveal a rather certain diagnosis (infection or injury).
  • The most common causes: mucosal swelling and postviral damage in the nasal cavities and paranasal sinuses
    • Mucosal swelling associated with a viral or bacterial infection, i.e. a conductive disorder, but a viral infection may also damage the olfactory nerves thus causing a central defect.
    • Mucosal swelling associated with allergic rhinitis Allergic Rhinitis
    • Chronic rhinitis Nasal Stuffiness and particularly nasal polyposis Nasal Polyps are associated with a disturbance of the sense of smell that often fluctuates.
  • An injury to the head region
    • Anosmia caused by blows on the head, particularly on the back of the head, is often permanent.
  • Age, growing old
  • Rarer causes
    • Tumours
      • A tumour located at the bottom of the anterior cranial fossa or a tumour of the olfactory nerve may damage the olfactory bulbs or the olfactory tracts thus causing hyposmia or anosmia.
    • Neurological diseases
      • Migraine: hypersensitization to olfactory stimuli
      • Temporal epilepsy: paroxysmal sensations of strange odours
    • Neurodegenerative diseases
      • As a prodromal symptom in Parkinson's disease, Alzheimer's disease
      • In the more advanced stages of multiple sclerosis
    • Smoking
    • Congenital impaired/absent sense of smell (Kallmann's syndrome)
    • Endocrine diseases (diabetes, hypothyroidism)
    • Hormonal causes (variations in the menstrual cycle, pregnancy)
    • Psychological causes (schizophrenia, psychosis, olfactory hallucinations)
    • Toxic substances either ingested or inhaled (acrylates, methacrylates, cadmium, zinc)
    • Poor nutrition (deficiency of vitamin A, thiamine, zinc)
    • Intranasal cocaine
    • Drugs: antimicrobial agents, cytostatic drugs, different kinds of nasal sprays, mood-altering drugs, cardiovascular drugs, antipsychotic drugs, antithyroid drugs, muscle relaxants, antiparkinsonian drugs
    • Surgery of the nose or the base of the scull, laryngectomy (nasal airflow is ended)
    • Idiopathic in about 18%

Treatment

  • Causal treatment may be helpful in nasal diseases.
  • After a viral or bacterial infection, the sense of smell will be restored in most patients at least partly with time.
  • Nasal glucocorticoid therapy is worth trying if there is a suspicion of mucosal swelling as the cause of the decreased sense of smell.
  • The recovery of the sense of smell can be improved and accelerated by olfactory training when started as soon as possible after olfactory loss. However, even after more than a year, training can be useful.
  • In a coronavirus infection, the sense of smell usually recovers quickly, within 2-4 weeks. If not, olfactory training can be started immediately. Evidence on the effectiveness of olfactory training in coronavirus infections is not yet as clear as in other viral infections.
  • The patient can carry out the training independently at home.
    • Olfactory training is started with the smells of rose, eucalyptus, lemon and clove. The exercise is carried out twice daily, regularly for 3 months.
    • After 3 months the patient continues with 4 new smells that can be chosen according to availability and patient preferences. The exercises in the second phase are carried out in the same way as in the first 3-month phase.
  • Anosmia caused by nasal polyps Nasal Polyps may be corrected with glucocorticoids or operative treatment.
  • There is no treatment for injuries of the olfactory tract, but olfactory training has resulted in partial recovery of the sense of smell after mild head injuries.

Indications for ENT specialist consultation

  • Loss of the sense of smell without clear aetiology
  • Unilateral disorders, anosmia and dysosmia (erratic sense of smell) without evident intranasal cause, and olfactory hallucinations that may suggest an epileptic mechanism or a brain tumour.
  • The patient is not anymore capable of continuing in his/her occupation due to anosmia (retraining, statement for the insurance company).

    References

    • Fjaeldstad AW, Ovesen T, Stankevice D, et al. Olfactory training in long COVID-19 patients with lasting symptoms including olfactory dysfunction. Dan Med J 2023;70(3):. [PubMed]
    • Whitcroft KL, Hummel T. Olfactory Dysfunction in COVID-19: Diagnosis and Management. JAMA 2020;():. [PubMed]
    • Sedaghat AR, Gengler I, Speth MM. Olfactory Dysfunction: A Highly Prevalent Symptom of COVID-19 With Public Health Significance. Otolaryngol Head Neck Surg 2020;():194599820926464. [PubMed]
    • Hummel T, Whitcroft KL, Andrews P, et al. Position paper on olfactory dysfunction. Rhinol Suppl 2017;54(26):1-30. [PubMed]
    • Altundag A, Cayonu M, Kayabasoglu G, et al. Modified olfactory training in patients with postinfectious olfactory loss. Laryngoscope 2015;125(8):1763-6. [PubMed]
    • Konstantinidis I, Tsakiropoulou E, Bekiaridou P, et al. Use of olfactory training in post-traumatic and postinfectious olfactory dysfunction. Laryngoscope 2013;123(12):E85-90. [PubMed]