Traumatic brain injury and time to onset of Alzheimer's disease: A population-based study

Controversy continues as to whether traumatic brain injury is a risk factor for Alzheimer's disease. The authors examined a related hypothesis that am ong persons with traumatic brain injury who develop Alzheimer's disease, ti me to onset of the disease is reduced. They used data on all documented epi sodes of traumatic brain injury that occurred from 1935 to 1984 among Olmst ed County, Minnesota, residents. Community-based medical records were used to follow traumatic brain injury cases who were aged 40 years or older at l ast contact prior to June 1, 1988, for Alzheimer's disease until last conta ct, death, or June 1, 1988. The test of the hypothesis was restricted to th ose cases who developed Alzheimer's disease. The expected time to onset of Alzheimer's disease was derived from a life table constructed by using age- of-onset distributions within sex groups:for a previously identified cohort of Rochester, Minnesota, Alzheimer's disease incidence cases without a his tory of head trauma. The authors found that of the 1,283 traumatic brain in jury cases followed, 31 developed Alzheimer's disease, a number similar to that expected (standardized incidence ratio = 1.2, 95% confidence interval 0.8-1.7). However, the observed time from traumatic brain injury to Alzheim er's disease was less than the expected time to onset of Alzheimer's diseas e (median = 10 vs. 18 years, p = 0.015). The results suggest that traumatic brain injury reduces the time to onset of Alzheimer's disease among person s at risk of developing the, disease.