Fewer Sensors, More Pain

by Tom Valeo

May, 2007

Pain is a private, subjective experience, but researchers at Harvard have found a way to quantify it somewhat by counting the number of pain sensors found in a tissue biopsy no larger than the head of a pin.

Paradoxically, the people suffering the greatest pain don’t have extra pain sensors—they have fewer than normal.

“When you lose neurons, it makes the remaining neurons go berserk and fire inappropriately,” says Anne Louise Oaklander, associate professor of neurology at Harvard Medical School. “No one had been able to find tests to prove a cause of pain in patients with reflex sympathetic dystrophy, which causes chronic pain, so we took biopsies and counted the number of nerve endings.

“What we found was an average 29 percent drop in nerve endings in chronic pain patients. It’s paradoxical, but the hallmark of neuropathic pain is actually the degeneration of pain fibers.”

Oaklander learned the technique from Jack Griffin, now the chairman of the Department of Neurology at Johns Hopkins Hospital in Baltimore. He would like to know how the surviving pain sensors have changed to produce such terrible pain.

“The fact that you can lose fibers and have spontaneous pain implies that they have undergone a change of function that drives a persistent state of pain, or an increased response to pain,” Griffin says.

Biopsies of malfunctioning pain sensors will help detect the changes that cause their heightened activity, he adds. “The biopsy could also be used to identify regenerating nerve fibers and the characteristics of surviving fibers. There’s a long list of things we’d like to know. I would assume that it will all get sorted out.”