Sense of Smell Can Go Off Either Way

Barbara Giurlando’s troubles began several years ago after her office was remodeled and she was suddenly surrounded by new printers, faxes and copiers that gave off a weird odor that she now thinks was ozone.

All of a sudden, says Giurlando, 62, who lives in Boston and works for an architectural firm, the smell of perfumes, deodorants or after-shave became sickening. She couldn’t even read newspapers without feeling queasy from the ink smell.

Susan Rioff, 54, a Lexington, Mass., woman who volunteers in the newborn intensive care unit at a Boston hospital, has the opposite problem.

Seven years ago, she was thrown from a horse and suffered a severe brain injury that left her with no sense of smell. Because smell is central to taste, food also lost its flavor. “You can stay alive without smelling,” she says ruefully, “but you’re missing a lot of pleasures. There’s a whole part of experience that I cannot share. It makes me very sad.”

By conservative estimates, 4 million Americans suffer from the complete or partial loss of the sense of smell, sometimes, as in Rioff’s case, after a head injury, but more often as a result of aging or Alzheimer’s disease. In fact, half of all people lose at least part of their sense of smell by age 65 and 75% do so by age 80, probably because of lower levels of key brain chemicals, repeated colds or cumulative exposure to toxins or medications. Until recently, there was little that doctors could do to help.

Thousands of others, like Giurlando, develop a hyper-acute sense of smell that drives them to distraction, though researchers do not know what triggers it. One woman became as sensitive to smells as cockroaches, whose noses are 100,000 times more sensitive than humans’. She could walk around a room and trace by smell where different people had stood, a sensitivity that was so disabling she became housebound.

Still others suffer from “phantom” smells, or olfactory hallucinations--imagined foul odors such as that from feces or strong chemicals--that may be caused by nasal polyps or damage to olfactory or taste nerves.

To those not afflicted, such problems, particularly the loss of the sense of smell, may seem trivial. But as many as 90% of people who lose the sense wind up depressed or anxious, says Dr. Alan Hirsch, neurological director of the Smell and Taste Treatment and Research Foundation in Chicago.

The sense of smell is one of the most ancient parts of our nervous system, so old that it forms part of the “reptilian brain,” neural circuits that evolved eons ago. In fact, the sense of smell is the only part of the human sensory system that has a direct, neurological link from the outside world to the limbic system in the brain, which controls fear and other emotions.

This special wiring--a kind of neurological shortcut--is vital to survival. It allows an animal to detect immediately any nearby predators, or to avoid other potentially fatal dangers such as rotting food. This quick link to the brain also allows an animal to detect the presence of an eager mate, through odor molecules called pheromones. The primitive nature of the link between smell and the limbic system is also the reason smells can trigger long-buried memories.

Laypeople often assume that there is only one way to detect odors--by sniffing, or orthonasal olfaction. But there is another way--retronasal olfaction--by which smell plays its central role in taste, says Linda Bartoshuk, a psychologist at Yale Medical School.

“When you chew,” says Bartoshuk, tiny puffs of air containing odor molecules drift “up the backward route to the nose. You are not truly aware that this is olfaction--it’s what most people mistakenly call ‘taste.’ ” But in fact there are only five true tastes--sweet, salty, sour, bitter and umani, a Japanese term for a savory, delicious taste. All the other, subtle flavors that we can detect are combinations of these basic tastes plus the sense of smell.

Basic though the sense of smell is, it’s only in the last few years that scientists have begun to understand how it works--and how to treat it when things go wrong.

Though humans can detect thousands of distinct odors, the nose only has about 1,000 olfactory receptors that lie on specialized nerve cells lining the top of the nasal cavity, notes Linda Buck, a Howard Hughes neurobiologist at Harvard Medical School. She and others have recently discovered that particular odors are detected by particular combinations of these receptors.

When an olfactory receptor captures an odor molecule, it triggers electrical signals carried by nerves that run from the nose to the olfactory bulbs inside the brain. Along this path, wispy projections of these nerves, called axons, pass through a porous, paper-thin piece of bone called the cribiform plate.

The reason someone with a head injury loses the sense of smell is because, as the brain gets slammed around inside the skull, this bony plate sheers off the delicate axons. This breaks the connection between the olfactory receptors and the brain, notes Marcia Levin Pelchat, a biological psychologist at the Monell Chemical Sense Center, a nonprofit research organization in Philadelphia.

If the sheering of olfactory nerves is complete, there is little that doctors can do to help. But if some neural connections remain intact, there is a growing array of drugs that may enhance whatever sense of smell is left, says Hirsch of Chicago.

Steroids may help in some cases, particularly if the problem involves nasal polyps, benign growths that can cause nasal swelling, blocking odor perception.

Another approach now being tested in Hirsch’s lab is to give patients 9 milligrams a day of phosphatidylcholine, a drug sold over the counter as Phoschol, which boosts levels of a brain chemical called acetylcholine. The olfactory bulbs need lots of acetylcholine, which suggests that increasing this chemical might restore the sense of smell. So far, however, the results have been mixed.

Hirsch is also studying the possible efficacy of 880 milligrams a day of zinc, as tablets to be swallowed. Zinc, which appears to help the transmission of signals along olfactory nerves, seems to be safe, he says, though if taken as a nasal spray, it can actually cause a loss of the sense of smell.

Researchers are also experimenting with Ritalin (the drug used to quiet hyperactive children) because it can raise levels of a brain chemical called norepinephrine, which is important to olfactory bulb function.

Other drugs being tested as ways to increase olfactory bulb function include theophylline (a stimulant in coffee, chocolate and asthma medications); amantadine (a drug used to treat Parkinson’s disease); amitriptyline (an antidepressant); thiamine (a B vitamin); Neurontin (an anti-epileptic drug); and Depakote (an anti-convulsant).

For people like Giurlando whose sense of smell is too acute, “desensitization” training may help. This means learning to pair a non-offensive odor, such as pepper, with an offensive one. This doesn’t really change the sense of smell, but does allow people to be less conscious of the noxious odor. Another solution is to sniff tubes of peppermint, which also can inhibit the conscious recognition of the offensive smell.

Phantom smells, or phantosmias, often respond to treatment with anti-convulsant or anti-depressant drugs.

In the future, higher-tech solutions may become available, including transplantation of neural stem cells.

The bottom line, say those who treat smell disorders, is to see a doctor if you’re troubled about smelling too much or too little. But there’s also something else you can do: Wear your seat belt in the car and a helmet if you ride a bike or a horse. You probably can’t avoid losing some sense of smell because of aging--but you can reduce the odds of losing it through a head injury.

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Judy Foreman writes a syndicated column on health issues. She is a fellow in medical ethics at Harvard Medical School. She can be reached by e-mail at foremanj@brandeis.edu. Her column runs occasionally in Health.