Loss of smell: when the nose doesn't know

Harvard Health Letter, Feb, 1997 by Peta Gillyatt

Of the five senses, smell seems to get the least respect. Instead of getting credit for enabling a diner to tell the difference between key lime pie and lemon chiffon, the ability to make such subtle distinctions is often misattributed to taste. In fact, the nose can recognize about 10,000 scents, ranging from the yeasty aroma of baking bread to the smell of frying bacon, whereas the tongue lumps everything into four crude categories -- salty, sour, bitter, or sweet.

Appreciating the finer things in life, however, is only one of smell's functions. It is also essential for detecting smoke, gas leaks, and spoiled food -- not to mention when it's time to hit the shower. As with sight and hearing, however, the acuity of this sensory ability wanes with age. As many as half of people over the age of 65 have experienced some loss of smell, often as a result of diseases that become more commonplace later in life.

Odors, which pass either through the nostrils or through the mouth, are detected by hairlike receptors in the olfactory epithelium, a small patch of mucous membrane lining the upper nose. Nerve fibers connected to these receptors pass through tiny holes in the bony roof of the nasal cavity to enter the olfactory bulbs. These structures route information about smell to other parts of the brain via the olfactory tract. (See ON THE TRAIL OF A SCENT)

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Cause and defect

The sense of smell can be impaired for a variety of reasons. "The most common causes are nasal obstruction, trauma to the head or nose, or a viral illness," said Richard Gliklich, an assistant professor of otolaryngology at Harvard Medical School and a nasal and facial plastic surgeon at Massachusetts Eye and Ear Infirmary.

Septal deviations and chronic sinusitis are the most likely causes of persistent nasal obstruction. Healthy sinuses are sterile, air-filled spaces located above, behind, and below the eyes. These cavities produce mucus, which normally drains through a narrow opening. When sinusitis overstimulates mucus production, the opening can be blocked and the stagnant sinus becomes a breeding ground for bacteria. The resulting inflammation can cause a partial or total loss of smell.

Fortunately, "chronic sinusitis is highly treatable, and it's often possible for people to fully recover their sense of smell following therapy," said David Kennedy, professor and chair of the Department of Otolaryngology/Head and Neck Surgery at the University of Pennsylvania Medical Center and medical director of its Smell and Taste Center. Relief can often be obtained from antibiotics, decongestants, or steroids. Sometimes, an operation may be needed to drain infected sinuses or enlarge narrowed sinus openings. (For more information about sinusitis, see Harvard Health Letter; November 1995.)

Some physicians prescribe zinc supplements in the hope that they will boost the function of smell receptors, but so far there is no scientific proof that zinc can restore a lost sense of smell, said Dr. Kennedy.

The ability to smell can also be adversely affected if the nasal passages are blocked by polyps (protrusions of the nasal lining) that impede the passage of smells, by structural abnormalities in the nasal septum (the wall that divides the nostrils), or, in rare cases, by a tumor.

Although the loss of smell that accompanies a cold or bout of the flu is temporary for most people, some middle-aged and older folks permanently lose some or all of their ability to sense smells after a flu-like illness. Researchers have not yet determined whether a specific virus is to blame or figured out exactly how olfactory function is damaged, Dr. Gliklich said.

A fall, a fender-bender, or some other blow to the head can jar the brain enough to shear the fragile connection between the smell receptors and the brain. In older people, even minor bumps have been known to obliterate the sense of smell. Although the nerve cells sometimes grow back and re-establish connections with the olfactory bulbs, scarring often impedes total recovery.

When a rose is not a rose

Because progressive neurological disorders can affect the olfactory nerves, loss of smell can also be a warning sign for Alzheimer's disease, Parkinson's disease, or multiple sclerosis. People who are malnourished or have serious kidney ailments may have changes in smell and taste due to deficiencies in zinc, copper, thiamine, and vitamin [B.sub.12].

Certain prescription drugs can interfere with olfactory acuity. These include the high blood pressure medications diltiazem (Cardizem) and nifedipine (Adalat, Procardia), the antibiotic streptomycin, and the antidepressant amitriptyline (Elavil). Local anesthetics may temporarily impair the sense of smell, and radiation of the nasal area may have more lasting effects. In about 20% of all cases, however, no explanation can be found.

Sensory stimulation

When someone with smelling problems seeks medical treatment, the physician will first determine whether the sensory loss is complete or partial. This can be done informally by asking the patient to sniff a cup of coffee or another strong odor with closed eyes. More sophisticated tests involve scratching designated squares and correctly identifying common odors, such as bubble gum and root beer.