The most common causes include mucosal swelling associated with inflammation of the nasal cavities and paranasal sinuses, which can be alleviated with topical glucocorticoids, damage to the sense of smell caused by a viral infection, or a trauma to the head region.
General neurological diseases and tumours adjacent to the olfactory nerves or the olfactory tract should be kept in mind as rare causes.
Definition
Disorders of smell include anosmia (lack of the sense of smell), hyposmia (decreased sense of smell), hyperosmia (increased sensitivity to smells) and dysosmia/parosmia (distorted sense of smells). They can be caused by central (intracranial) disorders of the olfactory tract or peripheral (intranasal) ventilatory disorders of the nose.
Remember that the patient often first complains of a weakened sense of taste!
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 the cause of altered sense of smell is suspected to be 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 another neurological disease is suspected, 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.
During the COVID-19 pandemic, in some of the patients infected with the new coronavirus (SARS-CoV-2), changes in the senses of smell and taste have even constituted the first symptoms (see also Covid-19 and other Coronavirus Infections).
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 has a fluctuating character.
The second most common cause is an injury to the head region.
Blows on the head, particularly on the back of the head, may cause axonal damage in the olfactory receptor cells and in such case anosmia is often permanent.
Physiological cause
Age, growing old
Rarer causes
Tumours
A tumour located at the bottom of the anterior cranial fossa, e.g. so-called olfactory groove meningioma or a tumour of the olfactory nerve, i.e. esthesioneuroblastoma 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)
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 many patients at least partly with time.
According to the latest research, the recovery of the sense of smell can be significantly improved and accelerated by olfactory training when started as soon as possible particularly after olfactory loss caused by upper respiratory tract infection 12. The patient can carry out the training independently at home.
Olfactory training is started with the smells of rose, eucalyptus, lemon and clove. The patient sniffs one smell intensively for 10 seconds, then pauses for 10 seconds and sniffs the next smell. When all the 4 smells have been sniffed the series is repeated once. The exercise is carried out twice daily, in the morning before breakfast and in the evening before bedtime. This program is performed regularly for 3 months.
After 3 months the patient continues with 4 new smells that can be chosen according to availability and patient preferences. These could include e.g. menthol, tangerine, dried or fresh thyme, rosemary and sage, or loose-leaf tea, e.g. jasmine, green or bergamot (Earl Grey) tea. The exercises in the second phase are carried out in the same way as in the first 3-month phase, twice daily.
Fragrances/oils for the training can be bought in ordinary food shops (clove, fruits, herbs, teas), health food shops, ethnic food shops or in pharmacies.
Nasal glucocorticoid therapy is worth trying if there is a suspicion of mucosal swelling as the cause of the decreased sense of smell.
Anosmia caused by nasal polyps may be corrected with glucocorticoids or operative treatment.
There is no treatment for injuries of the olfactory tract, but according to the latest research, olfactory training has resulted in partial recovery of the sense of smell after mild head injuries.
Indications for 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.
Irreversible disturbance of the sense of smell after infection or injury even if the patient had started olfactory training.
The patient is not anymore capable of continuing in his/her occupation due to anosmia (retraining, statement for the insurance company).
References
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]
Altundag A, Cayonu M, Kayabasoglu G et al. Modified olfactory training in patients with postinfectious olfactory loss. Laryngoscope 2015;125(8):1763-6. [PubMed]
Hummel T, Whitcroft KL, Andrews P ym. Position paper on olfactory dysfunction. Rhinol Suppl 2017;54(26):1-30. [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]
Whitcroft KL, Hummel T. Olfactory Dysfunction in COVID-19: Diagnosis and Management. JAMA 2020;():. [PubMed]