Doctors say the drug, known as riluzole, appears to slow down the progress of the disease, and offers a "first glimmer of hope" for sufferers that a cure may one day be found.
Until now, the only treatment on offer has been physiotherapy and other palliative care in the later stages of the disease, when the patient is almost completely paralysed, unable to speak or swallow. Ultimately, breathing is affected: more than half of patients die of respir- atory failure within three or four years of diagnosis.
Tricia Holmes, director of care development at the MND Association, said that one of the most distressing features of the disease was that patients remained mentally alert even in the most advanced stages. "They really become prisoners of their own bodies," she said.
Riluzole, which is available on NHS prescription as Rilutek, does not halt or reverse MND but it can extend survival time. Why motor neurones - the large nerve cells which relay messages between the brain and muscles - start to die off in people with MND is not known, but one theory blames the accumulation of glutamate, a neurotransmitter.
High levels of glutamate are believed to kill motor-neurones by "over- exciting them," and riluzole may work by inhibiting its accumulation. A trial of more than 950 people with well-established MND showed that after 18 months of treatment, the number of patients dying was 35 per cent fewer than in those not taking the drug.
Neurologists are hopeful that if treatment is begun even earlier, at the time of diagnosis of MND, then its impact may be even greater. Professor Douglas Mitchell, a consultant neurologist at the Royal Preston Hospital, said such patients had only been receiving the drug through a special "early access" arrangement for a year, and it was too soon to assess the effects.
The severe disabling effect of the disease in its later stages has raised questions about the benefits of prolonging life, and about the cost of the drug which is around pounds 3,000 per patient per year. Professor Mitchell said that some of his patients had turned down the opportunity of taking the drug on the grounds that they did not want their lives extended. Others have welcomed the chance to gain some extra time.
"For many people time equates to quality of life," Professor Mitchell said. "Often they have some target or goal in mind, such as a birthday, or to see a son or daughter married. It is ultimately a decision for the patient to make, once fully informed by his or her doctor. It is not for able-bodied people to judge what is quality of life for the disabled."
The cost involved was "a drop in the ocean," Professor Mitchell said, compared to drugs such as beta-interferon, which was launched last year as a treatment for some types of multiple sclerosis.
There has been some opposition to riluzole from people who say that greater investment in physiotherapy, equipment and palliative care would have the same benefits, but this argument has been dismissed as illogical, given evidence of the beneficial effects of the drug.
There are several other new drugs for MND in the pipeline, including nerve growth factors, which help maintain healthy nerves. Another theory blames free radicals - highly toxic particles which occur in the body and the environment - for motor neurone damage. The cells are particularly vulnerable because they are very large and have just a single nucleus. Drugs which mediate this free radical damage are also a possibility, and ultimately a combination of therapies may provide the most effective means of treatment.
t British scientists say that several different genes are involved in a susceptibility to multiple sclerosis (MS), another debilitating disease of the nervous system. The findings, published in Nature Genetics, have been hailed as a significant step forward in understanding MS. An interplay of genes with environmental factors, possibly viruses, may be the cause. Relatives of people with MS have a slightly increased risk of contracting the disease, though many genetically susceptible individuals never develop it.
A progressively disabling disorder
There are about 5,000 people with MND in Britain. Average age of onset is about 55.
Survival time is usually about three years from diagnosis, but one in five patients lives for five years or more and a very few - such as the physicist and mathematician Professor Stephen Hawking - survive for more than 20 years.
Symptoms includeweakness, cramps, muscle twitches, fatigue, weight loss, and problems with speech, chewing and swallowing. The disease is progressive and patients reach full paralysis in the late stages.Reuse content