The exact relationship of injuries to onset of multiple sclerosis (MS) has long been debated. The patient will naturally try to relate onset to a definable incident. Many case studies are very convincing - injuries are dramatic events that will easily be remembered by patients - but they are not truly valid statistical evidence. McAlpine believed, "Trauma to a limb or any part of the body, slight or severe, including operations, may occasionally precipitate the disease in a predisposed person or cause a relapse."

In a 1972 study of 250 MS patients, McAlpine's researchers found that 14 percent had an injury within three months prior to onset. In a parallel study of 250 patients with other diseases, only 5.4 percent had an injury within three months prior to onset. Although the studies showed a significant difference, most authorities do not believe that injury actually causes MS. It is more likely that injury brings out symptoms of an existing lesion that was previously silent.