Parkinson's disease affects millions of old people. The symptoms include progressive tremor, rigidity of limbs and difficulty in body coordination. The cause of Parkinson's is unknown, the pathology poorly understood. The main physiological change is the depletion of the neurotransmitter -- a chemical that helps transmit nerve impulses -- dopamine.

Since the '60s, drugs have been available for boosting the amount of dopamine in the brain. Although they have given most patients a new lease of life, there are several side-effects. Parkinson's patients suffer from common problems like nausea and hypotension (low blood pressure) during the first weeks of therapy. After a few years, many experience erratic mobility and mounting confusion.

Since 1988, a new way of treating Parkinson's -- brain grafting with foetal tissue -- has been seriously debated. Surgeons in Mexico, Sweden, Britain and the US had transplanted brain tissue from aborted foetuses on to Parkinson's patients, some of whom claimed spectacular recovery. There are 200 transplant patients the world over.

Says Olle Lindvall, who leads the Swedish team at the University Hospital in Lund, "We now have good evidence that grafts can survive and have functional effects. But the functional improvements are incomplete and the challenge is to find out if it is possible to obtain more complete recovery." Another line of treatment that is being tried out is pallidotomy -- driving a probe into the brain and burning out hyperactive brain cells in an area called the pallidium.

Although there have been over 500 pallidotomies in the US, with patients reportedly regaining control of their limbs, many researchers remain sceptical, saying that the surgery poses the risk of bleeding in the brain that can paralyse or kill.