Traumatic brain injury (TBI) is one of the predominant injuries of the current military conflicts in Iraq and Afghanistan, with prevalence ranging from 15 to 20 percent depending on the diagnostic criteria and patient population [1-7]. A majority of these injuries, 85 percent in one study, are mild concussions resulting from exposure to blasts [1-4,7]. Because military personnel with mild injuries are often returned to full duty status shortly after the injury-causing event, understanding the lasting effects on cognitive and physical functions, as well as the risks associated with repeat injury, is of significant importance .
Although research describing the effects of repeated concussions in military populations is limited, civilian literature points to several detrimental physical, cognitive, and emotional health effects of incurring multiple concussions [9-13]. Evidence exists that some aspects of neurocognitive function do not recover as quickly in those who have experienced multiple concussions, and some studies suggest a permanent reduction in cognitive performance [14-15]. One recent study showed greater reduction in neurological activity among persons with two concussions, with reduced time between concussive events acting as an important mediator .
The aims of the present study were to (1) provide a descriptive analysis of repeated concussion in U.S. military personnel, (2) identify whether decreased time between events is associated with increased severity of the second event, and (3) identify predictors of postinjury utilization of mental health and neurology outpatient services following the second concussive event.
The present study was an analysis of servicemembers with repeated concussions that were reported in the Expeditionary Medical Encounter Database (EMED) (formerly the Navy-Marine Corps Combat Trauma Registry). The EMED is a deployment health database maintained by Naval Health Research Center (NHRC), San Diego, California, and consists of documented clinical encounters of deployed military personnel from all service branches (a more extensive description of the EMED can be found elsewhere ).
Clinical EMED records were completed by medical providers stationed at forward-deployed Navy-Marine Corps military treatment facilities that were located in Iraq to treat Operation Iraqi Freedom casualties. Unique aspects of the EMED include detailed information regarding the injury incident, which is collected at or near the point of occurrence, as well as the inclusion of persons with otherwise mild injuries who are subsequently returned to duty. Clinical records are provided to NHRC, and professional coders review the records and assign codes using the Abbreviated Injury Scale (AIS); Injury Severity Score (ISS); and International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9CM) [18-20].
For the present study, eligible personnel were servicemembers who sustained two or more provider-diagnosed concussions during Operation Iraqi Freedom from March 2004 to April 2008. A concussion was defined by the presence of an ICD-9-CM code of 850.0 to 850.9. Severity of concussion was defined using the AIS, which is a scoring system that details the severity of each injury and is categorized into nine different body regions (i.e., head, neck, face, torso, abdomen, spine, upper limb, lower limb, and external) . All personnel in the study sustained concussions corresponding to a maximum head AIS of 1 (minor injury) or 2 (moderate injury). At the time of the present study, 113 of 14,653 individuals in the EMED with combat or noncombat injury met the inclusion criteria and comprised the study sample. Overall injury severity for each servicemember was determined with the ISS, which is derived from the AIS and ranges from 0 to 75 . …