Incidence rates for mild traumatic brain injury among active duty US Military personnel between 1997 and 2007

Mild traumatic brain injury (mTBI) has been described as the most common form of traumatic brain injury within military populations; however, few epidemiologic studies have examined incidence rates for mTBI in this population. The objective of this study was to examine trends in the incidence of mTBI among active duty US service members between 1997 and 2007. Specifically, we were interested in comparing incidence rates prior to the initiation of combat operations in Iraq and Afghanistan with subsequent years. A retrospective cohort study was conducted utilizing data extracted from the Defense Medical Surveillance System to identify all incident cases of mTBI within the study population. The primary outcome of interest was the incidence rate of mTBI per 1000 person-years. Multivariable Poisson regression was used to analyze the data. There were 98,012 mTBI cases and 14,956,955 person-years of follow-up, for an overall incidence rate of 6.55 (95%CI: 6.51, 6.59) per 1,000 person-years. There was a steady increase in the mTBI rate over time. The average change in the mTBI rate was 8.5% (95%CI: 8.2%, 8.8%) per year; however, the rate rose dramatically in the last two years of the study period. Overall, for 2006-2007 vs. 1997-2005, the rate ratio was 1.61 (95%CI: 1.58, 1.65). The greatest increase in the rate of mTBI was observed among those serving in Iraq, who experienced a 38.4% (95%CI: 35.4%, 41.1%) annual increase in new cases. The observed increase in the incidence of mTBI in this population has significant policy implications in terms of allocating appropriate healthcare resources.