Assessment of olfactory loss

Olfactory loss has been linked to a variety of causes and can profoundly influence a patient's quality of life.

Epidemiology

It is estimated that functional anosmia has a prevalence of approximately 5% of the general population.[1]Hummel T, Whitcroft KL, Andrews P, et al. Position paper on olfactory dysfunction. Rhinology. 2016 Jan 31;56(1):1-30. https://www.rhinologyjournal.com/Rhinology_issues/manuscript_1593.pdf http://www.ncbi.nlm.nih.gov/pubmed/28623665?tool=bestpractice.com ​ Studies suggest that the prevalence of olfactory impairment in the adult population ranges from 0.6% in those aged <35 years to 13.9% among those aged ≥65 years (depending on the test method used, sample demographics and definitions of impairment).[1]Hummel T, Whitcroft KL, Andrews P, et al. Position paper on olfactory dysfunction. Rhinology. 2016 Jan 31;56(1):1-30. https://www.rhinologyjournal.com/Rhinology_issues/manuscript_1593.pdf http://www.ncbi.nlm.nih.gov/pubmed/28623665?tool=bestpractice.com [2]Schubert CR, Cruickshanks KJ, Fischer ME, et al. Olfactory impairment in an adult population: the Beaver Dam Offspring Study. Chem Senses. 2012 May;37(4):325-34. https://academic.oup.com/chemse/article/37/4/325/277450 http://www.ncbi.nlm.nih.gov/pubmed/22045704?tool=bestpractice.com [3]Desiato VM, Levy DA, Byun YJ, et al. The prevalence of olfactory dysfunction in the general population: a systematic review and meta-analysis. Am J Rhinol Allergy. 2021 Mar;35(2):195-205. https://journals.sagepub.com/doi/10.1177/1945892420946254 http://www.ncbi.nlm.nih.gov/pubmed/32746612?tool=bestpractice.com ​​​​​​

Prevalence of olfactory impairment increases with age and male sex, increasing to approximately 25% in adults aged 60-90 years.[2]Schubert CR, Cruickshanks KJ, Fischer ME, et al. Olfactory impairment in an adult population: the Beaver Dam Offspring Study. Chem Senses. 2012 May;37(4):325-34. https://academic.oup.com/chemse/article/37/4/325/277450 http://www.ncbi.nlm.nih.gov/pubmed/22045704?tool=bestpractice.com [4]Pinto JM, Wroblewski KE, Kern DW, et al. The Rate of Age-Related Olfactory Decline Among the General Population of Older U.S. Adults. J Gerontol A Biol Sci Med Sci. 2015 Nov;70(11):1435-41. https://www.doi.org/10.1093/gerona/glv072 http://www.ncbi.nlm.nih.gov/pubmed/26253908?tool=bestpractice.com [5]Seubert J, Laukka EJ, Rizzuto D, et al. Prevalence and Correlates of Olfactory Dysfunction in Old Age: A Population-Based Study. J Gerontol A Biol Sci Med Sci. 2017 Aug 1;72(8):1072-1079. https://academic.oup.com/biomedgerontology/article/72/8/1072/3746132 http://www.ncbi.nlm.nih.gov/pubmed/28444135?tool=bestpractice.com ​​

In a cross-sectional sample of older American adults (mean age 68 years), approximately 22% were unable to identify four or more odours using a validated odour identification task.[6]Pinto JM, Schumm LP, Wroblewski KE, et al. Racial disparities in olfactory loss among older adults in the United States. J Gerontol A Biol Sci Med Sci. 2014 Mar;69(3):323-9. https://www.doi.org/10.1093/gerona/glt063 http://www.ncbi.nlm.nih.gov/pubmed/23689829?tool=bestpractice.com Older African Americans and Hispanics were more likely to suffer from olfactory dysfunction.[6]Pinto JM, Schumm LP, Wroblewski KE, et al. Racial disparities in olfactory loss among older adults in the United States. J Gerontol A Biol Sci Med Sci. 2014 Mar;69(3):323-9. https://www.doi.org/10.1093/gerona/glt063 http://www.ncbi.nlm.nih.gov/pubmed/23689829?tool=bestpractice.com

Aetiology

In patients presenting primarily for loss of smell, the most commonly identified aetiologies include a prior viral upper respiratory infection, head trauma, and underlying chronic sinus inflammation.[7]Deems DA, Doty RL, Settle RG, et al. Smell and taste disorders, a study of 750 patients from the University of Pennsylvania Smell and Taste Center. Arch Otolaryngol Head Neck Surg. 1991;117:519-528. http://www.ncbi.nlm.nih.gov/pubmed/2021470?tool=bestpractice.com [8]Seiden AM, Duncan HJ. The diagnosis of a conductive olfactory loss. Laryngoscope. 2001 Jan;111(1):9-14. http://www.ncbi.nlm.nih.gov/pubmed/11192906?tool=bestpractice.com [9]Hoekman PK, Houlton JJ, Seiden AM. The utility of magnetic resonance imaging in the diagnostic evaluation of idiopathic olfactory loss. Laryngoscope. 2014 Feb;124(2):365-8. http://www.ncbi.nlm.nih.gov/pubmed/23775878?tool=bestpractice.com These 3 causes account for over 50% of patients.

Pathophysiology

Olfactory loss may occur as a sensorineural loss, following degenerative changes in the olfactory neuronal receptors or neuroepithelium in the olfactory cleft.[10]Jafek BW, Hartman L, Eller PM, et al. Postviral olfactory dysfunction. Am J Rhinol. 1990 May;4(3):91-100. It may also occur as a conductive loss following an obstruction or inflammation within the nasal vault that prevents access of odorants to the olfactory receptors.

Types of olfactory loss

The loss may be complete (anosmia) or partial (hyposmia). It may be associated with perceived odour distortions (dysosmia) that are related to actual environmental odorant stimulation (parosmia) or occur spontaneously (phantosmia).

Presentation

Patients may present complaining of both smell and taste loss, or isolated taste loss, but on investigation their taste function is intact. This relates to the common confusion between taste and flavour. Taste sensation includes salt, sour, sweet, bitter, and umami (savoury).[11]Lindemann B, Ogiwara Y, Ninomiya Y. The discovery of umami. Chem Senses. 2002 Nov;27(9):843-4. Flavour perception is based upon olfactory, tactile, and thermal sensations as well as taste, with olfactory being arguably the most important of these sensations. The loss of olfactory input makes food generally unappealing, something most patients notice very quickly. However, a true measurable taste loss is distinctly uncommon.[7]Deems DA, Doty RL, Settle RG, et al. Smell and taste disorders, a study of 750 patients from the University of Pennsylvania Smell and Taste Center. Arch Otolaryngol Head Neck Surg. 1991;117:519-528. http://www.ncbi.nlm.nih.gov/pubmed/2021470?tool=bestpractice.com ​​

Treatment

For patients with olfactory loss due to an inflammatory process, reducing inflammation and obstruction in the nasal vault can effectively restore the ability to smell.[12]Kohli P, Naik AN, Farhood Z, et al. Olfactory outcomes after endoscopic sinus surgery for chronic rhinosinusitis: a meta-analysis. Otolaryngol Head Neck Surg. 2016 Dec;155(6):936-48. http://www.ncbi.nlm.nih.gov/pubmed/27576679?tool=bestpractice.com

Unfortunately, in most cases of olfactory loss, no specific therapy is available. However, patients will continue to seek medical opinion until they feel a thorough work-up has been performed and a thorough explanation has been provided.[13]Harris R, Davidson TM, Murphy C, et al. Clinical evaluation and symptoms of chemosensory impairment: 1000 consecutive cases from the nasal dysfunction clinic in San Diego. Am J Rhinol. 2006 Jan-Feb;20(1):101-8. http://www.ncbi.nlm.nih.gov/pubmed/16539304?tool=bestpractice.com ​​ Some data suggest that olfactory training can increase the potential recovery after a postviral or post-traumatic olfactory loss.[14]Konstantinidis I, Tsakiropoulou E, Bekiaridou P, et al. Use of olfactory training in post-traumatic and postinfectious olfactory dysfunction. Laryngoscope. 2013 Dec;123(12):E85-90.[15]Pekala K, Chandra RK, Turner JH. Efficacy of olfactory training in patients with olfactory loss: a systematic review and meta-analysis. Int Forum Allergy Rhinol. 2016 Mar;6(3):299-307. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4783272 http://www.ncbi.nlm.nih.gov/pubmed/26624966?tool=bestpractice.com

Adverse sequelae

Safety issues related to olfactory loss include the inability to detect gas leaks, fire, or spoiled foods. However, the most significant impact for each patient is typically the loss in quality of life related to eating and drinking.[1]Hummel T, Whitcroft KL, Andrews P, et al. Position paper on olfactory dysfunction. Rhinology. 2016 Jan 31;56(1):1-30. https://www.rhinologyjournal.com/Rhinology_issues/manuscript_1593.pdf http://www.ncbi.nlm.nih.gov/pubmed/28623665?tool=bestpractice.com [16]Patel ZM, Holbrook EH, Turner JH, et al. International consensus statement on allergy and rhinology: olfaction. Int Forum Allergy Rhinol. 2022 Apr;12(4):327-680. https://onlinelibrary.wiley.com/doi/10.1002/alr.22929 http://www.ncbi.nlm.nih.gov/pubmed/35373533?tool=bestpractice.com ​ This can be devastating. In some cases it may result in diminished appetite and inadequate nutritional intake, and in other cases it can cause increased oral intake and weight gain as patients seek chemosensory satisfaction. These consequences can have a negative impact on associated medical conditions such as diabetes and high blood pressure. Invariably, patients receive little sympathy from friends and, on occasion, family and sometimes even from their physician.

For most patients who suffer a loss of smell, no restorative therapy is available (unless the loss is secondary to inflammatory rhinosinusitis). Therefore, while most patients do benefit from counselling regarding safety issues and flavour enhancement methods, any information regarding prognosis becomes very important. Studies suggest prognosis relates more to the severity of the initial loss (as confirmed on objective olfactory testing), the age and sex of the patient, smoking, and the presence of dysosmia, than to the aetiology of sensorineural loss.[17]Hummel T, Lötsch J. Prognostic factors of olfactory dysfunction. Arch Otolaryngol Head Neck Surg. 2010 Apr;136(4):347-51. http://archotol.jamanetwork.com/article.aspx?articleid=496192 http://www.ncbi.nlm.nih.gov/pubmed/20403850?tool=bestpractice.com [18]London B, Nabet B, Fisher AR, et al. Predictors of prognosis in patients with olfactory disturbance. Ann Neurol. 2008 Feb;63(2):159-66. http://www.ncbi.nlm.nih.gov/pubmed/18058814?tool=bestpractice.com ​​