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According to a recent report, head injuries do not seem to trigger the development of multiple sclerosis, a degenerative neurological disease. The hypothesis that trauma to the head might influence the development of multiple sclerosis in genetically predisposed individuals has been debated for several years, but the link has not been studied conclusively.
Researchers from Oxford University studied data for a group of over 110,000 people admitted to a hospital with head injury and analysed the patterns of their trauma. They then compared the occurrence of multiple sclerosis in this group to a reference cohort of over 500,000 individuals.

Researchers failed to find any evidence linking head injury to the development of multiple sclerosis.

The ratio of multiple sclerosis after head injury, when compared with the reference cohort, did not increase, or decrease, with time. Moreover, the severity of the injury - defined by the length of stay at the hospital - did not seem to influence outcomes either.

Journal of Neurology, Neurosurgery, and Psychiatry, April 2006

that's funny coz i cracked myself in the head fiercely 2 days before my first attack swept through. mind u i was not hospitalized so i never would have made it into that study.