Botulinum toxin is among the most powerful poisonous substances known; a teaspoonful would be enough to kill 100,000 people. It is a natural substance formed by the growth of Clostridium botulinum, one of a family of bacteria responsible for some very unpleasant diseases, including tetanus and gas gangrene. The botulinum bacterium causes a dangerous type of food poisoning, botulism, which is rare in Britain but occurs more often in countries such as the United States where home canning of food is widespread. If the canning process is faulty the food may be contaminated with the bacterium and so with its toxin, though the contents of the can may look and smell normal. Within 8-36 hours of the food being eaten the victim develops difficulty in swallowing and speaking, double vision and progressive paralysis. Even with prompt treatment with the antitoxin, the mortality rate may be as high as 25 per cent.
A lethal, tasteless, potent poison such as this was inevitably of interest to chemical warfare experts. The toxin was found to block nerve transmission and so to weaken or paralyse muscles, and have a remarkably long- lasting effect. This action is now being used as a treatment for a variety of muscular disorders, the most recent application being to help reduce handicap in children with cerebral palsy (spasticity).
Injection of botulinum toxin into a muscle blocks the release of the neurotransmitter acetylcholine and so prevents the muscle fibres from contracting. The effect of a single injection persists for at least three to four months - the time taken for new nerve endings to be formed.
Botulinum toxin was first used as a treatment for some types of squint, the condition in which one eye is pulled out of line with the other by lack of balance in the strength of the external eye muscles. Injection of a very small amount of the toxin into the muscle that was pulling too hard was found to restore the normal balance.
In the past 10 years this treatment has been extended to the muscle disorders known as dystonias, in which involuntary contraction of a group of muscles causes unwanted movements or abnormal postures. Examples are blepharospasm, in which the eyes are repeatedly screwed up; torticollis, in which the head is pulled over to one side by unequal contraction of the neck muscles; and involuntary twitching movements of the face muscles. All these conditions may be helped by injections of carefully measured doses of the toxin every few months.
Children born with cerebral palsy have damage to the brain present at birth which causes muscular weakness and sometimes paralysis. Much of the handicap, however, is the result of some muscles or groups of muscles contracting too strongly and being unable to relax. This spasticity is the main problem, making walking difficult or impossible. A research project in Belfast, reported in Developmental Medicine and Child Neurology, has described the treatment with botulinum toxin of 26 children with cerebral palsy. All had abnormal stiffness of the muscles at the back of the thigh or the back of the calf, making walking difficult or impossible. Injections of the toxin caused the muscles to become less stiff within a few days; the benefit lasted for several months before the muscles began gradually to return to their previous state.
Fourteen of the children were said by their parents to have shown 'marked' improvement, and 10 others 'moderate' improvement. Half of those who walked poorly showed a substantial improvement in mobility. Seven have sustained their improvement without further injections and 12 have had further injections between six and 18 months after the first. More research is needed, in particular to determine whether the benefits of treatment are sustained in the long term, but these early results are encouraging.Reuse content