Olfactory loss has been linked to a variety of causes and can profoundly influence a patient's quality of life.
Epidemiology
Estimates suggest that the incidence of olfactory loss in the general adult population ranges from 19% in people older than 20 years of age, to 25% in people older than 50 years of age.[1]Bramerson A, Johansson L, Ek L, et al. Prevalence of olfactory dysfunction: the Skovde population-based study. Laryngoscope. 2004 Apr;114(4):733-7.
http://www.ncbi.nlm.nih.gov/pubmed/15064632?tool=bestpractice.com
[2]Murphy C, Schubert CR, Cruickshanks KJ, et al. Prevalence of olfactory impairment in older adults. JAMA. 2002 Nov 13;288(8):2307-12.
http://www.ncbi.nlm.nih.gov/pubmed/12425708?tool=bestpractice.com
[3]Hoffman HJ, Ishii EK, MacTurk RH. Age-related changes in the prevalence of smell / taste problems among the United States adult population: results of the 1994 disability supplement to the National Health Interview Survey. Ann NY Acad Sci. 1998 Nov 30;855:716-22.
http://www.ncbi.nlm.nih.gov/pubmed/9929676?tool=bestpractice.com
Prevalence increases with age and male sex, increasing to as much as 63% in people older than 80 years of age.[2]Murphy C, Schubert CR, Cruickshanks KJ, et al. Prevalence of olfactory impairment in older adults. JAMA. 2002 Nov 13;288(8):2307-12.
http://www.ncbi.nlm.nih.gov/pubmed/12425708?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 odors using a validated odor identification task.[4]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.[4]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
Etiology
In patients presenting primarily for loss of smell, the most commonly identified etiologies include a prior viral upper respiratory infection, head trauma, and underlying chronic sinus inflammation.[5]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
[6]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
[7]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.[8]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 odor 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 flavor. Taste sensation includes salt, sour, sweet, bitter, and umami (savory).[9]Lindemann B, Ogiwara Y, Ninomiya Y. The discovery of umami. Chem Senses. 2002 Nov;27(9):843-4. Flavor 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.[10]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 May;117(5):519-28.
http://www.ncbi.nlm.nih.gov/pubmed/2021470?tool=bestpractice.com
Treatment
It is important to determine whether the loss is due to an inflammatory process, since this is the only situation where therapy will be effective. 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 workup has been performed and a thorough explanation has been provided.[11]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 loss.[12]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.[13]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.doi.org/10.1002/alr.21669
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. 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 get little sympathy from friends and family and sometimes even from their physicians.
For most patients who experience a loss of smell, no restorative therapy is available (unless the loss is secondary to inflammatory rhinosinusitis). Therefore, while most patients do benefit from counseling regarding safety issues and flavor enhancement methods, any information regarding prognosis becomes very important. Studies suggest prognosis relates more to the severity of the initial loss, the age and sex of the patient, smoking, and the presence of dysosmia, than to the etiology of sensorineural loss.[14]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
[15]London B, Nabet B, Fisher AR, et al. Predictors of prognosis in patients with olfactory disturbance. Ann Neurol. 2008 Feb;63(2):159-66.
https://www.doi.org/10.1002/ana.21293
http://www.ncbi.nlm.nih.gov/pubmed/18058814?tool=bestpractice.com
