Frequency of Mild Traumatic Brain Injury in Iraq and Afghanistan: Are We Measuring Incidence or Prevalence?

Objective: Rates of mild traumatic brain injury (mTBI) differ considerably between US and UK forces, despite similar methodology and similar exposure risks. We assessed, in the UK forces, if the differences in rates based on last deployment can be explained by differences in deployment length, given that US forces deploy for approximately twice as long as UK forces. Participants: A total of 3763 personnel deployed to Iraq or Afghanistan who completed a questionnaire between 2007 and 2009. Main Outcome Measures: Mild traumatic brain injury in the last deployment contrasted to current posttraumatic stress disorder, psychological distress (General Health Questionnaire-12), multiple physical symptoms, and alcohol misuse. Results: In the Army and Royal Marines, there was an association between length of deployment (per month) and mTBI (odds ratio: 1.31, 95% confidence interval: 1.13-1.51), which remained significant after adjustment for confounders (odds ratio: 1.25, 95% confidence interval: 1.08-1.45). No other outcome was associated with length of deployment. Results based on the total sample were similar to those in the Royal Marine and Army sample, except for adjusted multiple physical symptoms. Conclusions: Comparisons of mTBI rates should take account of length of deployment when based on last deployment. Doing so reduces but does not eliminate the differences between UK and US forces.