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;114:733-737.
http://www.ncbi.nlm.nih.gov/pubmed/15064632?tool=bestpractice.com
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Murphy C, Schubert CR, Cruickshanks KJ, et al. Prevalence of olfactory impairment in older adults. JAMA. 2002;288:2307-2312.
http://www.ncbi.nlm.nih.gov/pubmed/12425708?tool=bestpractice.com
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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;855:716-722.
http://www.ncbi.nlm.nih.gov/pubmed/9929676?tool=bestpractice.com
Prevalence increases with age and male gender, 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;288:2307-2312.
http://www.ncbi.nlm.nih.gov/pubmed/12425708?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.
[4]
Seiden AM. The initial assessment of patients with taste and smell disorders. In: Seiden AM, ed. Taste and smell disorders. New York, NY: Thieme; 1997:4-19.
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.
[5]
Jafek BW, Hartman L, Eller PM, et al. Postviral olfactory dysfunction. Am J Rhinol. 1990;4:91-100.
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).
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Lindemann B, Ogiwara Y, Ninomiya Y. The discovery of umami. Chem Senses. 2002;27:843-844.
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.
[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
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.
[8]
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;20:101-108.
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.
[9]
Konstantinidis I, Tsakiropoulou E, Bekiaridou P, et al. Use of olfactory training in post-traumatic and postinfectious olfactory dysfunction. Laryngoscope. 2013;123:E85-E90.
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. In the past, the likelihood for spontaneous recovery was believed in part to relate to the etiology of the sensorineural loss.
[10]
Seiden AM, Duncan HJ. The diagnosis of a conductive olfactory loss. Laryngoscope. 2001;111:9-14.
http://www.ncbi.nlm.nih.gov/pubmed/11192906?tool=bestpractice.com
However, 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.
[11]
Hummel T, Lötsch J. Prognostic factors of olfactory dysfunction. Arch Otolaryngol Head Neck Surg. 2010;136:347-351.
http://archotol.jamanetwork.com/article.aspx?articleid=496192
http://www.ncbi.nlm.nih.gov/pubmed/20403850?tool=bestpractice.com
