Risk of multiple sclerosis after head injury: record linkage study.

BACKGROUND: The possibility that head injury may influence the development of multiple sclerosis (MS) has been studied inconclusively in the past. OBJECTIVE: To determine whether head injury is associated with an increased risk of MS. METHOD: Analysis of database of linked hospital and death records, comparing the occurrence of MS in a cohort of people admitted to hospital with head injury and a reference cohort. RESULTS: The rate ratio for MS after head injury, compared with the reference cohort, was 1.1 (95% confidence interval, 0.88 to 1.36). There was no significant increase in the risk of MS at either short or long time periods after head injury. Using length of hospital stay as a proxy for severity of injury, there was no significant increase in the rate ratio for MS after head injuries with hospital stays of less than two days (rate ratio = 1.1 (0.71 to 1.57)), two or more days (rate ratio = 1.0 (0.68 to 1.45)), or seven or more days (rate ratio = 1.3 (0.64 to 2.34)). CONCLUSIONS: The method used, record linkage, ensures that patients' recollection of injury, or any tendency to attribute MS to injury, cannot have influenced the results. Injuries to the head were not associated with either the aetiological initiation or the clinical precipitation of onset of multiple sclerosis.