Olfactory loss has been linked to a variety of causes and can profoundly influence a patient's quality of life.
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.    Prevalence increases with age and male gender, increasing to as much as 63% in people older than 80 years of age. 
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.  These 3 causes account for over 50% of patients.
Olfactory loss may occur as a sensorineural loss, following degenerative changes in the olfactory neuronal receptors or neuroepithelium in the olfactory cleft.  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.
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).
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).  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. 
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 work-up has been performed and a thorough explanation has been provided.  Some data suggest that olfactory training can increase the potential recovery after a postviral or post-traumatic loss. 
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 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. In the past, the likelihood for spontaneous recovery was believed in part to relate to the aetiology of the sensorineural loss.  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. 
Professor of Otolaryngology - Head and Neck Surgery
University of Cincinnati
AMS declares that he has no competing interests.
ENT Consultant and Honorary Senior Lecturer
University of Aberdeen
Department of Otolaryngology Head and Neck Surgery
Aberdeen Royal Infirmary
SKM declares that she has no competing interests.
Royal National Throat, Nose, and Ear Hospital
SG declares that he has no competing interests.
Massachusetts Eye and Ear Infirmary
Harvard Medical School
EHH declares that he has no competing interests.
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