sections include: loss of the sense of smell, loss of smell as a sign of neurological disease, diagnosis and treatment
We rarely think about our sense of smell except when we sniff something particularly pleasant or unpleasant. But in fact our ability to detect and recognize odors does a great deal to shape our mental world. Smells can bring back vivid memories of places and people—and often the emotional states we associate with them. This may relate to the relative uniqueness of some smells, as well as to the brain regions to which the olfactory bulbs connect. Probably the sense of smell was even more important to our evolutionary ancestors. It may no longer be as vital for us as it is for animals that use their noses to hunt (or to avoid being hunted), but being able to smell rotten food, leaking natural gas, and smoke can still be a matter of life and death.
We are more aware of our sense of taste, at least at mealtimes. Our olfactory system works in tandem with our sense of taste to provide us with the flavors of foods and beverages. If people lose their sense of smell, they often feel that they have also lost some of their ability to taste, even though their taste buds are still responding normally. We call smell and taste the chemical senses because they rely on specialized cells in the nose and mouth that are responsive to the specific molecules they encounter, as opposed to physical changes in air or in light. Together, smell and taste can make eating, a basic animal behavior, into an aesthetic pleasure. It’s no coincidence that gusto, a word we associate with taking great pleasure in life, is derived from the Latin word for “taste.”
Doctors have written about disorders of smell and taste for many years. Indeed, in the third century B.C. the Greek naturalist Theophrastus recognized that a person could suffer anosmia—the loss of smell—due to a head injury or blocked nose. The great nineteenth-century British neurologist John Hughlings Jackson was among the first to note that aberrations in smell and taste often accompany neurological disturbances of the brain’s temporal and frontal lobes. He also recorded that strange smells and tastes can precede the paroxysms associated with epilepsy.
In recent years research into the sense of smell has benefited from the proliferation of commercial tests of this ability. These have provided scientific standards for measuring many different people’s olfactory capacities, opening the door to new discoveries. We now know more about why people lose their sense of smell, and how losing that sense can be a clue to a more far-reaching neurological disorder.
Loss of the Sense of Smell
The most common cause of permanent olfactory loss appears to be a severe upper respiratory infection, usually caused by a virus, which damages the layer of nerve cells (neuroepithelium) in the nasal cavity. It is rare for people under 45 to lose their sense of smell this way. People tend to experience such anosmia as they grow older, with men having greater and earlier loss than women. While we estimate that less than 2 percent of Americans under 65 have meaningful olfactory loss, approximately half of the population between the ages of 65 and 80 does. Over the age of 80, three quarters of the population experiences this problem. Age-related olfactory loss explains, in part, why many older people report that their food lacks flavor, and why a disproportionate number of elderly die from accidental gas poisoning.
There are probably a number of mechanisms responsible for age-related decreases in the ability to smell. It has been suggested that most loss of smell reflects cumulative damage to the olfactory epithelium from viruses, bacteria, and airborne toxins. When the nerve cells have already suffered damage over time, a sudden but minor problem, such as a bad cold, can produce perceptible and often permanent loss. Some cases of age-related olfactory loss may be caused by hardening of the sievelike bone through which the olfactory nerves pass on their way to the brain.
The second most common cause of olfactory dysfunction is head trauma. Between 7 percent and 15 percent of patients suffering such injuries appear to suffer measurable olfactory loss, usually anosmia or severe microsmia (being able to smell much less than normal). In most such cases, the injury has caused the nerves from the nose to be sheared off at the cribriform plate. Bruising particular areas of the brain can also cause olfactory problems; some people may regain their sense of smell over time as the brain swelling or hematomas caused by a head injury clear up.
The third most common cause of prolonged smell loss is a nasal or sinus disease such as polyposis (growths in the mucous membranes) or such inflammatory disorders as allergic rhinitis (hay fever). Usually, this simply means something is physically blocking odorant molecules in the air from moving to a person’s olfactory region—in other words, the person has a stuffed-up nose. Fortunately, most people benefit, at least to some degree, from such therapies as surgery to remove polyps or steroidal medication to reduce allergy symptoms. However, chronic inflammation of the nasal regions can result in permanent loss of a person’s smelling powers.
Disorders of taste and smell can arise from other sources as well, among them:
- congenital causes
- growths inside the nose and skull, such as olfactory groove meningiomas and frontal lobe gliomas
- radiation therapy for cancer
- nutritional and metabolic deficiencies—cirrhosis of the liver, thiamine deficiency
- endocrine problems—Addison’s disease, diabetes, Kallmann’s syndrome
- accidental damage to a cranial nerve during tonsillectomy or other surgery
- kidney disease and hemodialysis
Fortunately, these cases are rare.
Loss of Smell as a Sign of Neurological Disease
Because of our increased ability to test people’s smelling abilities in a standard fashion, we now know that decreased olfactory function can be among the first signs of some common neurological and psychiatric disorders.
New research suggests that psychosis may impair a person’s ability to identify odors. In schizophrenia, odor-identification test scores are the only objective measurement known to correlate with the disease’s duration. In other words, schizophrenics who have the most difficulty distinguishing odors also have the longest intense, or fulminant, period of illness. That finding suggests that there is a progressive, perhaps degenerative, component of schizophrenia that we have not yet recognized.
Interestingly, about 90 percent of people with Parkinson’s disease from an unknown cause suffer olfactory loss—more people than show the disease’s characteristic tremor. The loss of smell does not respond to medical therapy and, unlike the motor symptoms, does not change over time. Therefore, smell testing is useful in differentiating Parkinson’s from progressive supranuclear palsy (PSP), a similar disorder in which people usually suffer no major olfactory problems.
In the elderly, two other conditions that are often hard to distinguish at first are Alzheimer’s disease and depression. Many doctors use the Mini-Mental State Examination to unearth clues about which disorder might be affecting an individual. Recent studies have shown that scores on a simple three-item odoridentification test are even more effective. Moreover, an epidemiological study of 1,604 people 65 or older found that individuals’ scores on a 12-item odor-identification test were better at predicting who would show a cognitive decline over the next two years than their scores on a global cognitive test. People who were anosmic and who possessed a particular genetic marker had almost five times the risk of cognitive decline than people with ordinary smelling ability and without the gene. The difference was even more apparent for women: those with both anosmia and the genetic marker were almost ten times more likely to show cognitive decline than the other women. Though the individuals with the genetic marker and normal smelling ability were also more likely to suffer cognitive decline than people without the gene, the difference was not as striking as when anosmia was involved.
For most neurological diseases that affect a person’s sense of smell, we do not know why that change occurs. An exception is multiple sclerosis (MS) where people’s loss of smell is strongly correlated with the number of MS-related plaques in their subtemporal and subfrontal lobes—the regions involved in central olfactory processing. Brain plaques outside these two areas show no effect on olfactory test scores. Recent studies of people with MS have shown that over time their sense of smell waxes and wanes in direct association with the number of active plaques in these target regions.
Diagnosis and Treatment
In sum, it is worthwhile telling your physician about any smelling or tasting problems that you experience. It is important, however, that you receive quantitative testing of this sense to be certain you have a true problem. In some cases, loss or reduction of the olfactory sense can be an early sign of a serious neurological disorder, and early detection and treatment might improve your prognosis. In the best outcome, a smelling or tasting problem might be remedied with medication, surgery, or a change in whatever treatment might be causing the side effect. For many people, the loss of smelling ability is simply a problem that comes with age. The bad news is that in most cases physicians can do little to restore the sense of smell (and thus a full sense of the appreciation of food flavors), but the good news is that such anosmia usually does not lead to other problems.
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