Anosmia
- is a relatively common presentation
- the prevalence of anosmia is expected to increase over secondary to its association with COVID-19 infection
- overall prevalence of olfactory dysfunction has been estimated was 19.1%, composed of 13.3% with hyposmia and 5.8% with anosmia (1)
- olfactory dysfunction is (2):
- more common in advancing age
- about 50% of the population report disturbance in olfaction between 65 and 80 years; the incidence increases if aged >80 years
Most of the cases of anosmia - lack of the sense of smell - are bilateral.
The causes of anosmia can be considered in terms of unilateral or bilateral lack of sensation.
Another pathological sieve is considering causes of anosmia in terms of conductive and sensorineural causes (2,3)
Conductive causes include:
- chronic rhinosinusitis, allergic rhinitis
- diseases causing congestion and inflammation of the nasal mucosa, such as chronic rhinosinusitis and allergic rhinitis, can obstruct this pathway
- nasal polyps
- if a nasal polyp obstructs the olfactory cleft can cause a physical barrier
- gross septal deviation
- loss of sense of smell from a deviated septum is uncommon but gross deviations could lead to a reduced sense of smell
- intranasal tumours
- granulomatous disease of the nose
- iatrogenic
- e.g. endoscopic sinus surgery, especially extended skull base surgery, may cause an iatrogenic loss of sense of smell, due to damage of the olfactory mucosa intraoperatively; diversion of the upper respiratory tract following a total laryngectomy will bypass the nasal cavity so will result in olfactory dysfunctionl
Sensorineural causes include:
- viral/post viral (including COVID-19)
- estimated around 55% of COVID-19 patients may present with anosmia
- history of a previous upper respiratory tract infection occurs in 20-30% of cases olfactory dysfunction (2)
- post-viral anosmia has a wide range of recovery, estimated to be between 35% and 67%
- COVID- 19-related losses significantly improve within 4 weeks in 90% of patients
- head trauma
- most common cause of olfactory dysfunction (along with rhinosinusitis) is head injury
- up to 30% of patients who sustain a significant head injury will have a resultant olfactory dysfunction (2)
- about 30% of patients with traumatic olfactory dysfunction resolve, with the majority within 12 weeks
- medication related (for example, ACE inhibitors, diuretics, calcium channel blockers, statins, tobacco
- alcohol and illicit drug (especially cocaine) abuse
- neurological (temporal lobe epilepsy, multiple sclerosis, cerebrovascular disease)
- anosmia may be a presenting symptom in neurodegenerative disorders such as Parkinson's and Alzheimer's disease
- space-occupying lesion
- anterior cranial fossa tumours, meningiomas, and frontal lobe lesions
- congenital (for example, Kallmann's)
Red-flag features to be aware of in olfactory dysfunction include (2):
- unilateral nasal symptoms
- bleeding
- crusting/scabbing within the nasal cavity
- cacosmia (perceived malodorous smell)
- orbital symptoms (swelling, visual symptoms, or ophthalmoplegia)
- severe frontal headaches
- frontal swelling
- the onset of new neurological/meningitic symptoms
Reference:
- Bramerson A, Johansson L, Ek L, et al. Prevalence of olfactory dysfunction: the Skovde population-based study. Laryngoscope 2004; 114(4): 733-737.
- Deutsch PG et al. Anosmia: an evidence-based approach to diagnosis and management in primary care British Journal of General Practice 2021; 71: 135-138. DOI: https://doi.org/10.3399/bjgp21X715181
- Boesveldt S et al. Anosmia- A Clinical Review Chem Senses. 2017 Sep; 42(7): 513-523.