Pain is private. Unlike blood pressure or temperature or other symptoms easily measured and defined, the physical reaction to unpleasant stimuli is hard to quantify or predict. It varies from person to person, with each individual describing pain — and its intensity — differently.
But that private perception can make the difference between a trip to the medicine cabinet for an aspirin or a trip to the doctor's office for something much stronger.
FOR THE RECORD:
Pain perception: An article in Monday's Health section about how people feel pain differently included a quotation that read, "If you took a portable radio that needs a AAA battery and you put in a large D battery, you could burn the circuit out or cause static." While in some instances changing the power supply in a circuit could damage the circuit, a device powered by a AAA battery would simply run for a longer time when powered by a D battery. —
Researchers study pain not to separate whiners from stoics but to understand why pain varies and, eventually, create individually tailored treatments for the many specific ailments that fall under the umbrella of pain.
If you place your hand on a metal plate that slowly heats up, you'll reach your pain threshold when the heat first feels painful. Keep your hand on the heating plate until you can't take the pain any more and you've found your pain tolerance. Threshold and tolerance are studied separately, and researchers have found consistent differences in how people report both.
The reasons for such differences are both psychological and biological — personal expectations, societal norms, genetics, hormones and even how much sleep you get. All have implications for pain treatment.
Genes determine much of how we perceive pain because all of the bits and pieces of the nervous system are built from instructions in the genetic code, which varies slightly from person to person.
But a boy in Pakistan who can pass knives through his arm without feeling pain and a woman in China who experiences the intense, burning pain of erythromelalgia have one thing in common — mutations in the same gene. This gene, SCN9A, which has been the focus of a flurry of recent pain research, holds the code for a type of molecule, called a sodium channel, on the surface of nerve cells. Touch something painful, such as a hot plate, and a sodium channel opens a gate that allows signals to run to your brain, which interprets these signals as pain.
In the body, sodium channels are the gatekeepers for many signals in many different kinds of tissues. Dr. Stephen Waxman, a neurology professor at Yale University and the Department of Veterans Affairs, likens the sodium channels of the nervous system to batteries: "If you took a portable radio that needs a AAA battery and you put in a large D battery, you could burn the circuit out or cause static."
Because such extreme differences in pain perception boiled down to the same gene, researchers decided to look at whether it affects pain perception in people without pain disorders. In a study published in January, they found that people who had a normal variation in the gene also had higher pain thresholds. For those with the variation in both of their copies of the gene, thresholds were even higher.
This gene has been particularly well-researched, but other studies are focusing on additional genes. "I think that there is going to be a number of other pain genes, but it may not be more than a handful that influence pain threshold, and I bet they'll all come out in the next two years," says Dr. C. Geoffrey Woods, a geneticist at the University of Cambridge and one author of the recent study.
Pain can be very different in infancy than in old age. Until the last two decades of the 20th century, it was thought that babies didn't feel pain. Infants were not given anesthesia during surgery because doctors considered it unnecessary for pain control and because they feared a potentially lethal anesthesia overdose. Medical opinion has since reversed.
"Not only do they experience pain, but the more pain they experience, that influences their perception of pain as they get older," says Allan Basbaum, professor and chairman of anatomy at UC San Francisco and editor in chief of the journal Pain. More pain early on means a higher sensitivity to pain later.
Studies of pain sensitivity in elderly patients show that they have a higher threshold for brief exposure to superficial pain, such as heat, in part because peripheral nerves become less sensitive as we age. However, the elderly are more sensitive to deep muscular pain, and some studies suggest that their biological pain control systems don't work as quickly as in younger patients, says Roger Fillingim, a psychologist and professor of community dentistry and behavioral science in the University of Florida College of Dentistry.