The effect, which as yet has no explanation, is similar to an altered pain perception effect in fibromyalgia patients recently reported by the same research team.
In fact, the study finds, people with lower back pain say they feel severe pain, and have measurable pain signals in their brains, from a gentle finger squeeze that barely feels unpleasant to people without lower back pain. People with fibromyalgia felt about the same pain from a squeeze of the same intensity.
But the squeeze's force must be increased sharply to cause healthy people to feel the same level of pain -- and their pain signals register p in different brain areas.
The results, which will be presented Oct. 27 at the annual meeting of the American College of Rheumatology in New Orleans, may help lead researchers to important findings on lower back pain, and on enhanced pain perception in general.
Senior authors Richard Gracely, Ph.D., and Daniel Clauw, M.D., did the study at Georgetown University Medical Center and the National Institutes of Health, but are now continuing the work at the University of Michigan Health System. In May, they and their colleagues published a paper in the journal Arthritis and Rheumatism on pain perception in fibromyalgia patients.
To correlate subjective pain sensation with objective views of brain signals, the researchers used a super-fast form of MRI brain imaging, called functional MRI or fMRI. They looked at the brains of 15 people with lower back pain whose body scans showed no mechanical cause, such as a ruptured disk, for their pain. They also looked at 15 fibromyalgia patients and 15 normal control subjects.
As a result, they say, the study offers the first objective method for corroborating what lower back pain patients report they feel, and what's going on in their brains at the precise moment they feel it. And, it continues to give researchers a road map of the areas of the brain that are most -- and least -- active when patients feel pain. The researchers hope that further study on larger groups of patients will yield more information on altered pain processing.
"The fMRI technology gave us a unique opportunity to look at the neurobiology underlying tenderness, which is a hallmark of both lower back pain and fibromyalgia," says Clauw. "These results, combined with other work done by our group and others, have convinced us that some pathologic process is making these patients more sensitive. For some reason, still unknown, there's a neurobiological amplification of their pain signals."
Lower back pain affects nearly all Americans from time to time, especially those who are overweight, sedentary or work in physically demanding jobs. The pain can interfere with life and work; problems stemming from lower back pain are the second most frequent cause of lost work days in adults under the age of 45, ranking below only the common cold.
Much of the pain may be due to pulled muscles, strained ligaments, damaged joints or small tears in the disks that act as cushions between the bones of the spine -- all causes that don't show up well on X-rays but often can be seen on CT or conventional MRI scans. These physical causes often disappear after a few weeks, but many patients have chronic or recurring lower-back pain.
In the study, the lower-back pain patients were examined by CT scan to rule out mechanical causes of their pain. Then they, the fibromyalgia patients and the healthy control subjects had their brains scanned by fMRI for more than 10 minutes while a small, piston-controlled device applied precisely calibrated, rapidly pulsing pressure to the base of their left thumbnail. The pressures were varied over time, using painful and non-painful levels that had been set for each patient prior to the scan.
The study's design gave two opportunities to compare patients and controls. The subjective comparison measured the pressure levels at which the pain rating given by back pain patients, fibromyalgia patients and control subjects was the same. The objective comparison looked at the rating that the three types of participants gave when the same level of pressure was applied.
The researchers found that it only took a mild pressure to produce self-reported feelings of pain in the lower-back pain and fibromyalgia patients, while the control subjects tolerated the same pressure with little pain.
"In both the back pain patients and the fibromyalgia patients, that same mild pressure also produced measurable brain responses in areas that process the sensation of pain," says Clauw. "But the same kind of brain responses weren't seen in control subjects until the pressure on their thumb increased substantially."
Though brain activity increased in many of the same areas in both patients and control subjects, there were striking differences, too. All the subjects had increased activity in eight areas of their brains, but lower-back pain patients showed no increased activity in two areas that were active in both fibromyalgia patients and normal control subjects. Meanwhile, fibromyalgia patients showed increased activation in two other areas not active in back pain patients and healthy subjects.
This response suggests that lower-back pain patients have enhanced response to pain in some brain regions, and a diminished response in others, Clauw says.
The study was supported in part by the National Fibromyalgia Research Association, the U.S. Army and the NIH. In addition to Clauw and Gracely, the research team included Thorsten Giesecke and Masilo Grant of UMHS, Karen Munoz of NIH, Reshma Kumar of Georgetown, and Alf Nachemson of the University of Gotenberg, Sweden.