Doctors now believe that some people's bodies behave symmetrically so that when one joint is attacked by arthritis it sends a 'warning' to its partner (or spare) on the other side of the body.
The second joint then starts to mount its own defences, prematurely, since there is no enemy present. The inflammation provoked gives rise to the pain.
Professor David Blake, of the Bone and Joint Research Unit at the London Hospital Medical College, said yesterday that early trials of a drug developed in Sweden which may interrupt this process began 12 weeks ago on four adults. Results are promising enough to extend the tests.
Professor Blake has been investigating the phenomenon for five years. His interest was triggered by an effect of German measles on his 10-year-old daughter. He saw how an attack of arthritis in one knuckle of a finger joint was, after a few hours, repeated in the opposite knuckle. This happened twice.
'We are now fairly confident that we have the pieces of the jigsaw in place,' he said. The work has been done in collaboration with Hammersmith Hospital.