The condition, named macrophagic myofasciitis, causes inflammation and damage to the connective tissue between strands of muscle.

It was first identified in France in 1993 and appears to be spreading.

Details of 18 cases of are reported by French doctors in the Lancet medical journal.

Severe fatigue

Research published in The Lancet

Patients developed muscle and joint pains, weakness, fever and fatigue, which was sometimes severe.

There was no sign of wasting, but in some individuals muscles were tender to the touch.

Initially doctors suspected two well-known inflammatory conditions, polymyalgia rheumatica or polymyositis, which cause similar symptoms.

But an examination of muscle tissue from the patients under a microscope revealed something never seen before.

Large numbers of immune system cells called macrophages were densely packed around the connective tissue sheaths that bundle the muscles together.

Normally macrophages defend the body by engulfing and digesting microbes. Why they should be congregating around the sheathing tissue and causing damage was a puzzle.

Unique disorder

The doctors, led by Professor R K Gherardi, of the Hopital Henri Mondor in Creteil, France, said: "At present the cause of this unique disorder is unknown."

They speculated that an infection may be responsible. The prime suspects were mycobacteria which are related to the microbes which cause TB and leprosy.

Mycobacteria have been linked to other inflammatory muscle diseases. In addition, many of the patients had conditions which might reduce their ability to fight off a mycobacterial infection.

One patient recovered after being given anti-mycobacterial therapy. Combinations of anti-inflammatory steroid drugs and antibiotics proved effective for many of the others.