Problems with our sense of smell, including phantom odors or a loss of smell, can be a warning sign of serious illness.
Since the beginning of the pandemic, Covid infection has been the main culprit for causing a loss of smell or taste. Although most recover within a month or so, about 5% of people with a confirmed case of Covid report smell and taste dysfunction six months later, according to a study published in July.
Before a full recovery, many patients who have Covid-related loss of smell describe a period when they experience phantom smells like burning rubber or smoke or other foul odors that aren't really there, the researchers report. Covid is also linked to a condition known as parosmia, which turns a pleasant odor such as coffee into an unpleasant one.
For people who have mostly recovered from Covid but are still coping with a loss of smell, scientists from Duke Health found some new clues from biopsies taken deep inside nasal cavities. There's evidence of continued inflammation and an immune response persisting months after an infection.
Normally, we experience smell when the olfactory sensory neurons in the nose pick up an odor and then transmit a message to the brain, which identifies the odor. Another way for the brain to receive information about an odor is through a channel that connects the top of the throat to the nose. When we chew food, aromas are released that pass through the channel to the olfactory sensory neurons, then to the brain.
Viruses including flu, parainfluenza and other coronaviruses, along with other health issues, can lead to smell dysfunction, said rhinology expert Dr. Jonathan Overdevest, an assistant professor of otolaryngology and head and neck surgery at Columbia University Irving Medical Center in New York.
“Alterations in one’s sense of smell can be the result of chronic sinusitis,” said Overdevest. "The ongoing inflammation can in time impair the sense of smell or cause smell loss. If a dental infection is causing the sinusitis, people may sense a foul smell.”
Beyond Covid, smelling something that isn't there can indicate a serious condition.
A 2018 study found that millions of Americans may have some kind of olfactory disorder, reporting unpleasant, bad or burning odor when no actual odor is there. The researchers from the National Institute on Deafness and Communication Disorders, part of the National Institutes of Health, found a link to depression, migraine auras and head trauma.
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“A head trauma that shears the connection between the brain and the peripheral nerves in the nose can lead to a loss of smell," Overdevest said.
Neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, can also lead to problems with smell, including phantosmia, according to the National Institute on Deafness and Communication Disorders (NIDCD).
“When someone is smelling something and there is no source of that smell, similar to when people have phantom limb pain, it’s called phantosmia,” said Stephanie Hunter, a postdoctoral fellow at the Monell Chemical Senses Center in Philadelphia. “It’s thought that this might be caused by overactive neurons."
People who suffer with a smell disorder may lose weight, since smell and taste are so closely linked, although others may eat too much and gain weight or use too much salt.
Commonly reported phantom smells are bad, such as rotten eggs or burning hair.
Types and causes of smell disorders
The causes of smell disorders are not well understood, but women seem to be affected twice as often as men, according to the NIDCD study. Normal aging is also associated with some loss of smell or taste.
About 2% of Americans have some type of olfactory problem, including:
- Anosmia, the inability to smell.
- Hyposmia, a decreased ability to smell.
- Parosmia, a distorted perception. Coffee may smell like sewage, for example.
- Phantosmia, sensing an odor that isn't there.
Loss of taste comes when a person loses the sense of smell and then is left only with the basics that come through the tongue: sweet, sour, salty, bitter and umami. Some of the taste disorders, include:
- Ageusia, the inability to taste.
- Hypogeusia, a decreased ability to taste.
- Dysgeusia, a distorted ability to taste.
Sometimes, people with a smell disorder can interpret something like urine or feces as a pleasant odor.
"We don’t hear about this as much as unpleasant smells," said Hunter. "Maybe people smelling pleasant smells are less likely to look for help.”
Smell disorders can be diagnosed and treated by an otolaryngologist, or ENT, who specializes in ear, nose, throat, head and neck diseases.
Smell training can help someone with a disorder get back to normal, Hunter said. During the training, which should last at least three months, the patient is introduced to four strong odors — rose, eucalyptus, lemon and cloves.
"You smell the odor and then try to remember what it smelled like before you lost your sense of smell," Hunter said. "It can be really frustrating and a lot of people give up. But it’s important to stick with it.”