A treatment already used for immune diseases like rheumatoid arthritis appears to also work for chronic pain, scientists have discovered.

One small dose of intravenous immunoglobulin (IVIG) reduced pain in just under half of patients treated.

The pain relief lasted five weeks, on average, with few or no side effects, Annals of Internal Medicine reports.

The Liverpool University experts now plan bigger trials on more patients with Complex Regional Pain Syndrome.

Also known as Reflex Sympathetic Dystrophy, Complex Regional Pain syndrome (CRPS) involves a malfunction of the nervous system that causes often unrelenting pain.

It usually develops after an injury or trauma to a limb, and continues after the injury has healed.

Experts are not entirely sure why some people develop CRPS, but the latest discovery of how to treat it suggests it might be partly down to inflammation and a heightened immune response to the damage.

The immunoglobulin treatment contains blood antibodies that help dampen inflammation.

The team at Liverpool's Pain Research Institute tested the treatment on 13 of their patients who had been experiencing chronic pain for the past six months at least.

Although the treatment did not work for every single patient, for many it provided significant relief.

Lead researcher Dr Andreas Goebel said the real effect of this treatment in clinic may turn out to be even greater because the therapy can be given in higher doses, and repeated treatment may have additional effects.

"IVIG is normally repeated every four weeks and we are working to develop ways which would allow patients to administer the treatment in their own home," he said.

Professor Franz Blaes, of the University of Giesseu in Germany, has also been trialling the treatment in CRPS patients.

He said: "We have seen the same in our patients in more acute stages of the disease. Some of the patients really do benefit - probably between thirty and fifty percent of them.

"It may be that stopping the inflammation stops the problem.

"It is quite an expensive treatment and, as yet, we are not able to tell who will respond until we try it. But we are working on that."

Longstanding CRPS affects about 1 in 5,000 people in the UK.