Anosmia (the inability to smell) and hyposmia (a decreased ability to smell) describe the range of olfactory dysfunction, or smell disorders. The sense of smell is a complex process that involves the nose and the brain. When air passes into the nose, odor molecules bind to the receptors of the olfactory nerves. These nerves are found in a specialized lining at the top of the nasal cavity called the olfactory epithelium. The stimulation of the olfactory receptors and the nerves cause them to transmit signals to the brain, where it is processed into a scent that a person can recognize and identify. The ability to smell is directly linked to the sense of taste, and often anosmia will cause a loss of the ability to taste.
Smell disorders such as anosmia affect about 15 out of 1000 people in the U.S. and are more common with older age. Some common causes include sinonasal disorders such as allergic rhinitis (hay fever) and nasal polyps, head trauma, and infections such as viral illnesses. Anosmia may be congenital (present at birth), idiopathic (no known cause), or related to dementia such as that seen in Parkinson’s disease or Alzheimer’s disease. There have been reports of sudden anosmia, sometimes in the absence of other symptoms, as a marker of COVID-19.
Smell disorders may occur suddenly, such as with a viral illness or head trauma, or gradually. Diagnosis of a smell disorder may start with the patient self-reporting it, however not all patients who have a measurable olfactory dysfunction realize it. Examination of a patient with a smell disorder may include nasal endoscopy to look for obstructive causes such as polyps or swelling, or imaging using a CT scan or MRI. Testing for the ability to recognize common odors can be performed and, given our current pandemic, COVID-19 testing is appropriate.
Treatment of olfactory dysfunction depends on the cause. Up to two thirds of cases associated with a viral infection resolve on their own. Cases related to nasal obstruction, for example from nasal polyps or allergic rhinitis, require treatment of those underlying causes. Olfactory training, which involves daily exposure to a set of common odors, is another treatment option.
Research is being done into the ability of damaged olfactory neurons (nerve cells) to regenerate and the role of medications to support this. The loss of the ability to smell can significantly impact a person’s quality of life. If you feel that your quality of life has been negatively impacted by the loss of the ability to smell, talk to your physician who can refer you to an ENT specialist. For safety, people who have lost their sense of smell should maintain fire and natural gas alarms and avoid eating foods that are beyond their expiration dates.
For more information go to: www.nidcd.nih.gov/health/smell-disorders
By Peter Galvin, MD
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