A simple sniff test may help doctors pick up the scent of debilitating brain diseases.
How the nose knows when the brain's in pain. I'm Bob Hirshon and this is Science Update.
In the future, neurologists may stockpile canisters of nasty odors, like those of pungent cheese, rotten meat, or skunk. They'd be used to screen for degenerative brain diseases like Alzheimer's. University of Cincinnati psychologist Robert Frank says the sense of smell is often a first casualty of these diseases. So he and biologist Robert Gesteland invented a simple sniff test.
And it's all based on the observation that if you're sniffing and you don't encounter a smell, you take bigger sniffs than when you're sniffing and you do encounter a smell.
Their device measures the air pressure created by a sniff and compares how hard a patient sniffs at a strong odor versus an empty canister. If they're roughly the same, further testing would determine whether it's a mere nasal problem or an early sign of brain damage. I'm Bob Hirshon, for AAAS, the science society.
Making Sense of the Research
There are different kinds of tests for diseases. Some are diagnostic tests, designed to say with relative certainty whether or not you have a particular illness. These tests may sometimes be expensive, time-consuming, painful, or even risky, but the primary goal is to get the right answer. Examples of diagnostic tests include a blood test for H.I.V., or a biopsy (sample of body tissue) to test for cancer. They are often, but not always, given to people who have some reason to suspect they might have a particular illness, either because they've been exposed to it, they have symptoms, or they have a genetic risk.
On the other end of the spectrum, there are screening tests. These tests are intended to identify people, often from large groups of patients, who might have an illness, or one of several possible illnesses, and who require more specific diagnostic tests to find out for sure. Often, the goal of screening tests is to catch illnesses early, while they're easier to treat. Therefore, they're often given to people who have no obvious symptoms, or any reason to suspect they're sick. So the tests need to be fast, easy, and relatively cheap. Of course, they should also be as accurate as possible, but the goal is not a “yes” or “no” answer, but a “probably” or “probably not.”
This test, called the Sniff Magnitude Test, falls into the second category. It's not meant to diagnose a specific disease. In fact, the idea behind it is that many different brain diseases cause problems with smelling in their early stages, so the sniff test serves as a good all-in-one, first-round test.
So why not make it simpler, and stick some old socks under someone's nose? Well, just seeing the socks might make you think you smell them, when you really don't. That's why Frank's device uses canisters: so the patients don't know what they're getting. Also, with a canister, you can make sure you put exactly the same amount of the same kind of stink in each jar, so all the patients get the same test. If people aren't smelling the exact same thing, how can you compare one to another?
Now, why not just ask the patient “Smell that"? Because you wouldn't get a very scientific answer: just a “yeah,” “kinda,” or “not really.” If you can't turn it into a number, it's hard to draw the line between those who need further testing and those who don't. You could ask the patient to rate the strength of a smell, on a scale from 1 to 10. That would be an improvement, but the personality of the patient would still play a huge role in the result. A “7” to one person might be a “2” to another.
That's why they decided to use a more objective measure: the strength of the sniff. Or rather, the strength of the second sniff compared to the first. As Frank says, it's been proven that if you sniff and smell something, you don't sniff as hard the second time as you would if you smelled nothing. That's especially true of bad-smelling smells: if you walk by a nasty-smelling pile of garbage, you're not going to breathe very deeply until you're safely past. This is something that we do automatically, without thinking, which makes it a very good thing to study objectively.
Finally, Frank's team reduces uncertainty by comparing the patient to himself. In other words, they compare the difference between a patient's first and second sniffs on empty canisters, to the difference between his first and second sniffs on stinky canisters. If the patient's sense of smell is working normally, the difference in strength between the two sniffs on the smelly canisters will be greater than the difference in sniff strength on the empty canisters. If the sniff differences are about equal, it may suggest that the sense of smell isn't up to snuff.
Of course, further testing may reveal that it's not Alzheimer's, but a sinus problem that's stifling the patient's sense of smell. But that's okay, because screening tests aren't meant to be the final word. In fact, it's better to set off a false alarm than to miss a real problem.
Now try and answer these questions:
- What is the Sniff Magnitude Test?
- Why is it meant to screen for brain disorders?
- Why did the designers choose the sense of smell?
- Why does the device measure sniff strength?
- Why is this device used as a screening test and not a diagnostic test?
You may want to check out the May 11, 2007, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: replacing your skeleton with metal, a sniff test for neurological diseases, a decline in baby boy births, how your brain puts on the brakes, and the Robin Hood inside many of us.
The following resources provide students with a brief introduction to various diagnostic imaging technologies used in brain research.
Watch Your Thoughts! Diagnostic Imaging and the Brain
Mosquito Smell Cycle
6-12 | Audio
Grandfathers and Telomeres
6-12 | Audio