Smell disorders may seem to be a rare or minor problem, but they could be affecting more patients than internists expect. Conditions like anosmia and hyposmia are estimated to occur in between 1% to 2% of the North American population, and these rates may increase to as high as 13.5% among people ages 40 years or older, according to a 2016 study published in BMJ Open.
“This is a very common but underrecognized problem,” said Robert I. Henkin, MD, PhD, founder of the Taste and Smell Clinic in Washington, D.C. “It is important for the general practitioner to recognize that these problems occur in about 20 million people in the United States and that without treatment, they will persist.”
According to the National Institute on Deafness and Other Communication Disorders (NIDCD), smell disorders can include a decreased (hyposmia) or complete inability to smell (anosmia), a change in the normal perception of odors (parosmia), or the sensation of an odor that is not there (phantosmia).
In addition to causing immediate problems, smell disorders have been linked to poor outcomes in several studies. A 2015 study published in the American Journal of Rhinology & Allergy found that depressed mood and suicidal ideation were reported by about one in five adults with olfactory dysfunction. Additionally, a 2014 review of olfactory disorders and quality of life published in Chemical Senses showed that loss of sense of smell is associated with decreased food enjoyment, inability to detect harmful food and smoke, and negative effects on social situations and working life.
Awareness and early diagnosis are key to improving outcomes, said experts, who offered more information and tips on sniffing out these tricky conditions.
Two main categories
Smell disorders can be classified into two main categories, according to George Scangas, MD, clinical fellow in otolaryngology at Massachusetts Eye and Ear in Boston. Conductive defects are a breakdown in the transmission of odorants in the air to the olfactory epithelium in the nose. Sensorineural defects involve central neural structures and a lack of reception or processing of smell signals.
The most common cause of conductive smell disorders is chronic sinusitis or nasal polyps, Dr. Scangas said. With these conditions, inflammatory processes may cause olfactory defects or polyps may cause a physical blockage of the nasal cavity, preventing the flow of odorants.
A second common cause of conductive smell disorders is head trauma, Dr. Scangas said.
“For everyone who presents to the emergency room with a head trauma, 20% to 30% will develop some type of olfactory dysfunction and 5% will develop complete smell loss of anosmia,” Dr. Scangas said. He noted that about 35% of people who have olfactory loss or dysfunction as a result of trauma experience spontaneous improvement.
The most common causes of central or sensorineural defects are upper respiratory infections or viral infections.
“Twenty percent to 30% of people who present with a new smell complaint or dysfunction have a recent upper respiratory infection,” Dr. Scangas said, adding that which viruses, which patients, and the likelihood of recovery related to these infections is not yet well understood. “The spontaneous recovery rate in patients with upper respiratory infection as the cause is about 35% to 65%, but it can take up to one year for smell to improve.”
Another common central or sensorineural cause of smell loss is age. Dr. Scangas pointed out that age-related loss, a natural phenomenon, is often a diagnosis of exclusion but is still a “major player” in smell dysfunction. According to Dr. Scangas, about 50% of people ages 65 years or older and about 80% of people ages 80 years and older are affected. Less often, neurodegenerative diseases such as Alzheimer's disease or Parkinson's disease, changes in medications, or congenital diseases can also cause smell dysfunction.
Questions aid diagnosis
A careful patient history and differential diagnosis can help pinpoint the diagnosis of a smell disorder and determine whether the loss of smell is isolated or associated with other nasal symptoms. Dr. Scangas said important questions to ask include “How long has this been happening? How severe is it? Did it have a gradual onset or was it sudden? Were you sick recently? Do you have chronic sinus issues? Is there a history of head trauma or have you recently had surgery? Have you recently changed medications?”
Whether or not smell loss is a patient's primary symptom may depend on how suddenly the loss occurred, Dr. Scangas said. “A sudden loss has tremendous and sudden negative impact on quality of life and, thus, is normally the focus of the patient's visit,” he said. “Alternatively, patients with chronic sinusitis who gradually lose smell also have other symptoms, like nasal congestion and drainage, which tend to be the main complaints.”
Physicians should inquire about smell with any nasal-related symptoms or diseases, Dr. Scangas said. However, because there is no good evidence in the literature to suggest that early medical intervention improves outcomes in the case of viral-related sudden smell loss, early diagnosis in those cases is not as imperative, he noted.
There are also commercially available “scratch and sniff” tests to diagnose smell disorders, according to Edmund D. Pribitkin, MD, MBA, professor of otolaryngology-head and neck surgery at Thomas Jefferson University in Philadelphia. The University of Pennsylvania Smell Identification Test (UPSIT) is a 40-item test that can be administered in 10 to 15 minutes. During the test, a patient releases a scent by scratching with a pencil and then tries to identify the scent from four choices. Internists can administer the full UPSIT or a shorter version that includes only 12 items, Dr. Pribitkin said.
“These tests are very accurate but they measure only whether someone can appreciate certain odors, not the threshold at which they appreciate the odors,” Dr. Pribitkin clarified. Patients who score poorly on a test like the UPSIT should be referred to a specialist. However, because smell is subjective, physicians shouldn't rely solely on tests to make the diagnosis but should consider the results in conjunction with the history and physical, he said.
Some other red flags for referral include an inability to breathe through one side of the nose, recurrent bleeding episodes, or cognitive changes. Although referral to an otolaryngologist with experience with nasal endoscopy and treatment of smell loss is ideal, most ear, nose, and throat specialists (ENTs) should be able to perform a nasal endoscopy, which can go a long way in ruling out some of the less likely causes of smell dysfunction, such as tumors, Dr. Scangas said. Endoscopy can also determine whether a patient has nasal polyps, another potential cause. And ENTs trained in rhinology or with a specific focus on sinuses or smell disorders should be able to conduct more extensive analysis that could include lab tests, taste and smell function tests, saliva and nasal mucus analysis, and nutritional evaluation, according to Dr. Henkin.
Most smell disorders are initially treated by addressing the underlying cause. For example, if there is an obstruction or blockage of the nasal passage, removal of the obstruction can result in return of sense of smell, Dr. Pribitkin explained.
With viral-related loss, a virus can affect the nerve endings in the nose that sense odor, and the body is often left to rebuild the connections from the nerves to the brain. In these cases, sense of smell can return after the virus is gone, but it may take months or years, Dr. Pribitkin said.
Another common issue, sinusitis, can be treated using antibiotics or steroids, resulting in a reduction of the inflammation and swelling of the nose. “When this happens, odorants can get to the nerve ending in the olfactory groove, and many people will experience improvements,” Dr. Scangas said.
However, he explained, there is a caveat. “Smell nerves have a ‘use it or lose it’ phenomenon. If a patient has had chronic inflammation in the nose their entire life and has not been able to smell for years, then even by treating sinusitis, smell may not come back.”
There are ways being tested to help “retrain” the brain, though, Dr. Pribitkin said. These smell training methods involved smelling a series of smells three to four times a day for a few minutes; however, success varies. Physicians can recommend that patients try smell training, which can be learned by anyone, doctor or patient, through the Fifth Sense website, said Dr. Pribitkin. The Fifth Sense is a nonprofit organization in the United Kingdom dedicated to smell and taste disorders. Dr. Mainland, meanwhile, recommended Smell Training, a comprehensive patient resource in the U.K.
“Results vary depending on circumstances, but studies show definite improvement over doing nothing or a random smell rehabilitation,” Dr. Pribitkin said. If this type of smell rehabilitation does not work, any improvement in smell is not a given, he noted.
“Only about one-third to one-half of people with smell disorders get better,” he said. “Many will not recover their sense of smell at all.”