The reports from covid-19 patients are disconcerting. Only a few hours before, they were enjoying a cup of pungent coffee or the fragrance of flowers in a garden. Then, as if a switch had been flipped, those smells disappeared.

Young and old alike are affected — more than 80% to 90% of those diagnosed with the virus, according to some estimates. While most people recover in a few months, 16% take half a year or longer to do so, research has found. According to new estimates, up to 1.6 million Americans have chronic smell problems due to covid.

Seniors are especially vulnerable, experts suggest. “We know that many older adults have a compromised sense of smell to begin with. Add to that the insult of covid, and it made these problems worse,” said Dr. Jayant Pinto, a professor of surgery and specialist in sinus and nasal diseases at the University of Chicago Medical Center.

Recent data highlights the interaction between covid, advanced age and loss of smell. When Italian researchers evaluated 101 patients who’d been hospitalized for mild to moderate covid, 50 showed objective signs of smell impairment six months later. Those 65 or older were nearly twice as likely to be impaired; those 75 or older were more than 2½ times as likely.

Most people aren’t aware of the extent to which smell can be diminished in later life. More than half of 65- to 80-year-olds have some degree of smell loss, or olfactory dysfunction, as it’s known in the scientific literature. That rises to as high as 80% for those even older. People affected often report concerns about safety, less enjoyment eating and an impaired quality of life.

But because the ability to detect, identify and discriminate among odors declines gradually, most older adults — up to 75% of those with some degree of smell loss — don’t realize they’re affected.

A host of factors are believed to contribute to age-related smell loss, including a reduction in the number of olfactory sensory neurons in the nose, which are essential for detecting odors; changes in stem cells that replenish these neurons every few months; atrophy of the processing center for smell in the brain, called the olfactory bulb; and the shrinkage of brain centers closely connected with the olfactory bulb, such as the hippocampus, a region central to learning and memory.

Also, environmental toxic substances such as air pollution play a part, research shows. “Olfactory neurons in your nose are basically little pieces of your brain hanging out in the outside world,” and exposure to them over time damages those neurons and the tissues that support them, explained Pamela Dalton, a principal investigator at the Monell Chemical Senses Center, a smell and taste research institute in Philadelphia.

Still, the complex workings of the olfactory system have not been mapped in detail yet, and much remains unknown, said Dr. Sandeep Robert Datta, a professor of neurobiology at Harvard Medical School.

“We tend to think of our sense of smell as primarily aesthetic,” he said. “What’s very clear is that it’s far more important. The olfactory system plays a key role in maintaining our emotional well-being and connecting us with the world.”

Datta experienced this after having a bone marrow transplant followed by chemotherapy years ago. Unable to smell or taste food, he said, he felt “very disoriented” in his environment.

Common consequences of smell loss include a loss of appetite (without smell, taste is deeply compromised), difficulty monitoring personal hygiene, depression and an inability to detect noxious fumes. In older adults, this can lead to weight loss, malnutrition, frailty, inadequate personal care, and accidents caused by gas leaks or fires.

Jerome Pisano, 75, of Bloomington, Illinois, has been living with smell loss for five years. Repeated tests and consultations with physicians haven’t pinpointed a reason for this ailment, and sometimes he feels “hopeless,” Pisano admitted.

Before he became smell-impaired, Pisano was certified as a wine specialist. He has an 800-bottle wine cellar. “I can’t appreciate that as much as I’d like. I miss the smell of cut grass. Flowers. My wife’s cooking,” he said. “It certainly does decrease my quality of life.”

Smell loss is also associated in various research studies with a higher risk of death for older adults. One study, authored by Pinto and colleagues, found that older adults with olfactory dysfunction were nearly three times as likely to die over a period of five years as seniors whose sense of smell remained intact.

“Our sense of smell signals how our nervous system is doing and how well our brain is doing overall,” Pinto said. According to a review published earlier this year, 90% of people with early-stage Parkinson’s disease and more than 80% of people with Alzheimer’s disease have olfactory dysfunction — a symptom that can precede other symptoms by many years.

There is no treatment for smell loss associated with neurological illness or head trauma, but if someone has persistent sinus problems or allergies that cause congestion, an over-the-counter antihistamine or nasal steroid spray can help. Usually, smell returns in a few weeks.

For smell loss following a viral infection, the picture is less clear. It’s not known, yet, which viruses are associated with olfactory dysfunction, why they damage smell and what trajectory recovery takes. Covid may help shine a light on this since it has inspired a wave of research on olfaction loss around the world.

“What characteristics make people more vulnerable to a persistent loss of smell after a virus? We don’t know that, but I think we will because that research is underway and we’ve never had a cohort [of people with smell loss] this large to study,” said Dalton, of the Monell center.

Some experts recommend smell training, noting evidence of efficacy and no indication of harm. This involves sniffing four distinct scents (often eucalyptus, lemon, rose and cloves) twice a day for 30 seconds each, usually for four weeks. Sometimes the practice is combined with pictures of the items being smelled, a form of visual reinforcement.

The theory is that “practice, practice, practice” will stimulate the olfactory system, said Charles Greer, a professor of neurosurgery and neuroscience at Yale School of Medicine. Although scientific support isn’t well established, he said, he often recommends that people who think their smell is declining “get a shelf full of spices and smell them on a regular basis.”

Richard Doty, director of the University of Pennsylvania’s Smell and Taste Center, remains skeptical. He’s writing a review of smell training and notes that 20% to 30% of people with viral infections and smell loss recover in a relatively short time, whether or not they pursue this therapy.

“The main thing we recommend is avoid polluted environments and get your full complement of vitamins,” since several vitamins play an important role in maintaining the olfactory system, he said.

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