What Your Nose Knows

iStock

ANOSMIA OR “smell blindness” has recently emerged as a possible early indication of coronavirus infection – and it also may be for neurodegenerative diseases, including Alzheimer’s and Parkinson’s.

But until something goes wrong, smell is an underappreciated sense—not tested regularly like vision or hearing.  Most “olfies,” people with a typical sense of smell, don’t realize that “anosmics” miss out not only dangerous gas leaks and bad body odor but also on emotions and memories associated with smell.

Of those testing positive for COVID-19, more than 60% of German patients and about 30% in China described losing their sense of smell for several days, according to STAT. While widespread testing has not yet confirmed the association, anyone noticing a diminished sense of smell should self-isolate, wear a mask in public and try to contact a medical professional and to get tested.

Nasal/sinus infections and respiratory disease are the most common causes of olfactory dysfunction along with head trauma. Smell also declines with aging, called presbyosmia.  On the University of Pennsylvania Smell Identification Test (UPSIT), a typical average score of 37 declines to around 34 in one’s 70s and 26 in the 80s—with lower values classified as dysfunction.

In addition to anosmia, hyposmia is a diminished sense of smell; in parosmia or distorted perception, odors like rotten eggs can replace those of flowers; and phantosmia involves smelling something that’s not there, most commonly something unpleasant like dead animals.

Infection in the nasal cells can produce a dead-animal odor. And a simple head cold can completely destroy the sense of smell. Respiratory viruses inflame or damage cells in the nose, and that blocks the passage of odor molecules to neural receptors located in the mucosa.

Qualitative odor sensations—of flowers or spoiled food—arrive in the brain via the first cranial nerve (CN1).  A second facial nerve (CN5) relays somatosensory information from the eyes and skin as well as the nose —stimulated for example by irritation from a cut onion, also by foods like ginger and mint.

In addition to its smell sensors, blood vessels in the nose provide a route into the bloodstream and in some cases across the blood/brain barrier: for vaccines, such as FluMist; and for drugs—both legal, including those for psychiatric disorders, and illegal like cocaine.

In olfaction, detection is the first step—and the first tested when a problem arises. Discrimination, the second step, involves comparing scents against each other. The final step, identification, involves memory and diminishes with age and with age-related diseases. For Alzheimer’s patients, olfactory identification deficit is most often related to delayed memory processing.

In clinical research, patients receive odor stimuli via scratch ‘n sniff strips, perfumed paper strips, glass vials and felt-tip pens. But sensitivity to “monomolecular odorants,” such as the rose-like odor of phenylethyl alcohol, varies greatly even among those with a normal sense of smell.

A bigger challenge for assessing olfactory function can be a patient’s familiarity, based on personal or cultural experience, with test odorants. Adapting the UPSIT for Taiwanese subjects, for example, involved replacing scents like clover, cinnamon and gingerbread with others like sandalwood, jasmine and grapefruit.

In the case of head injury, even minor impact-collisions that cause the brain to lurch back and forth in the skull can block or sheer the CN1. Following a bicycle accident and subsequent surgery to relieve bleeding in his brain, Philadelphia artist JC had about 50% permanent loss of smell.  He can detect a person smoking but not whether the smoke is marijuana—and he relies on a carbon monoxide monitor to detect smoke at home.

Smell contributes about 30% to taste, according to JC’s surgeon.  While other variables like texture help him distinguish strawberries from bananas and good wine from bad, JC cannot discriminate among fine wines.

“Many people with anosmia believe they have also lost their sense of taste,” according to Philadelphia’s Monell Chemical Senses Center.  But what they have usually lost is the ability to sense flavor: while sugar has a sweet taste, strawberry and banana flavors “are actually smells.”

Combining aroma with the five basic tastes—via the airway connecting the mouth and nose—creates thousands of flavors. To experience taste in the absence of smell, place salt, sugar, vinegar and/or black coffee on the tongue; alternatively, to assess flavor alone, eat two flavors of the same brand of ice cream, such as vanilla and chocolate.

The inability to distinguish such odors as the smell of lilacs or a baby’s head can diminish emotions and memory associated with these experiences—and can lead to “symptoms of depression, anxiety and isolation.  [Those with anosmia] describe the condition as like experiencing the world through a frosted window,” according to the Monell Center.

The connection of olfactory function to neurodegenerative diseases remains unclear.  The Center states: “There is no evidence” that unexplained smell loss—not caused by head trauma or rhinosinusitis—will increase risk or is a sign of these diseases in the absence of other symptoms.

On the other hand, among patients with Parkinson’s disease, olfactory dysfunction occurs in 80 to 90% and can precede motor symptoms by several years. And, of 3,000 participants in one study, those unable to name at least four of five common smells (peppermint, fish, rose, orange and leather) were more than twice as likely to develop dementia in the next five years.

Among the rest, 78% were able to name at least four scents; 14% just 3; and 5% only 2. Those able to name just one had an 80% risk of a dementia diagnosis; and, of those unable to name any, almost all received this diagnosis.

Also, Swedish research looking at adults aged 40 to 90 over a 10-year period found that those who had lost their sense of smell had a 19% higher risk of early death.

As Philadelphia artist JC explained, a cause of olfactory dysfunction among painters is the fumes from paint thinner in closed studios with bad ventilation, especially for those starting out, with little money to afford something better. But for JC, the effects of his head injury have an upside: in the belching smoke of a city bus, instead of exhaust fumes, he sometimes smells night-blooming jasmine, which he remembers from evenings in his grandmother’s Florida garden.

—Mary Carpenter

Every Tuesday, well-being editor Mary Carpenter delivers health news you can use.