Some people may suffer from mysterious back pain because their brains are ultra-sensitive and wired up in a different way, say researchers.

They have found that some people with lower back problems that appear to have no obvious physical cause seem to register pain much more easily than most people.

A gentle squeeze so soft that it was not registered by healthy people was enough to trigger pain signals in their brains.

A similar effect was also seen in patients with fibromyalgia, who suffer pain in the muscles, ligaments and tendons.

But healthy people had to be squeezed a lot more sharply to feel the same level of pain - and it registered in different areas of the brain.

The researchers, from the University of Michigan, say they do not know why this effect happens, but hope their work will help explain lower back pain.

Pressure

They studied 15 patients who had lower back pain, which had no "mechanical" cause, such as a ruptured disk or pulled muscle, 15 patients with fibromyalgia and 15 healthy people.

Even if you see there's an increase in brain activity, lots of things could contribute

Dr Jan van der Merwe, St Thomas's Hospital,

Brain scans were carried out on all the patients while rapidly pulsing pressure was applied to the base of their left thumbnail.

It only needed a mild pressure - which had little impact on the healthy patients - to produce feelings of pain in the lower-back pain and fibromyalgia patients

The scans showed that while all the patients showed increased activity in some areas of the brain, the pattern of stimulation differed between the three groups.

Processing pain

The researchers say this suggests lower-back pain patients have an enhanced response to pain in some brain regions, and a diminished response in others.

They say their work helps provide a "road map" of which parts of the brain are most and least active when patients feel pain.

Dr Daniel Clauw, who led the study, said: "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 is a neurobiological amplification of their pain signals."

But Dr Jan van der Merwe, head of the Input Pain Management unit at St Thomas's Hospital, London, told BBC News Online it was difficult to take conclusive findings from looking at brain activity because so many things were happening at the same time.

"Even if you see there's an increase in brain activity, lots of things could contribute to the different signals.

"There could be some activity because they are experiencing pain or because they are thinking about something else."

The research was presented to the American College of Rheumatology conference in New Orleans.