I have heard that is caused from smoking to much, it may be just some rubbish but check it out oops got it wrong man

Relief for multiple sclerosis sufferers
There are a total of 85,000 people suffering from multiple sclerosis (MS) in Britain. This incurable debilitating disease manifests itself with a host of symptoms:

balance problems
muscle weakness and spasms
incontinence
pain
tremors

looks like that smokin help spasms :S