Introduction

Traumatic brain injury (TBI) is likely as old as warfare. In modern military medicine much of the focus has been on the effects of bullets and metallic fragments upon the brain. In World War I, for example, the English neurologist Sir Gordon Holmes detailed his observations on over 2,000 cases of head injury, including a detailed analysis of 23 cases involving penetrating injury to the visual cortex.¹ Much of that work was influenced by the work of the Japanese ophthalmologist Tatsuji Inouyewho created the first relatively accurate map of the primary visual cortex; the map was based on his correlational observations of visual field defects following penetrating injuries to the occipital cortex during the Russo-Japanese war of 1904.² In later years, Teubermade significant contributions to our understanding of the effects of penetrating brain injury in warfare by studying those injured in World War II.³ Alexander Luria, whose work contributed much to the beginnings of what is now known as neuropsychology, also studied injured soldiers during World War II.^4,5 His rehabilitation work centered on focal brain injury and how it affected cognition, language, and motor functioning. In addition, the work of Grafman et al.^6,7 and Carey et al.⁸ during the Vietnam era helped increase our understanding of both the acute effects and the late neurobehavioral changes of brain injuries. These contributions have allowed for further developments in modern military medicine and provided a strong foundation for our investigations today.

TBI in Iraq and Afghanistan

In the current conflicts in Iraq and Afghanistan, the focus on severe and penetrating brain injuries has shifted; the attention is now being placed on closed TBI and those brain injuries at the milder end of the spectrum, especially mild TBI (mTBI) or concussion, as it is also known. TBI severity is based on such measures as the Glasgow Coma Scale,⁹ duration of loss of consciousness or coma, and duration of posttraumatic amnesia. Current United States Department of Defense (DoD) ICD-9 derived diagnoses of TBI in the DoD Health Care System show that for 2009, penetrating brain injury accounted for just 1.4% of the total brain injuries, while severe closed brain injury accounted for less than 1% of the total. Of the of 27,862 TBIs counted in the year 2009, about 78% (21,859) were classified as mild (available at http://www.dvbic.org/TBI-Numbers.aspx). These percentages are consistent in the period 2003–2009, where a total of 134,476 brain injuries were reported. It should be noted that these numbers are limited to those that presented to the military health care system. There are likely others, largely mTBI, who never sought medical treatment or came to the attention of health care providers. Data suggest that during deployment as many as 20% or more may have suffered a concussion. Those with TBI during deployment are more likely to report postinjury and postdeployment somatic and/or neuropsychiatric symptoms than those without such an injury history.¹⁰

In Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF), from October 2001 to January 2005, the Joint Theater Trauma Registry reported that of those with battle injuries, a total of 1,566 combatants sustained 6,609 combat wounds. The wounds were to the head (8%), eyes (6%), ears (3%), face (10%), neck (3%), thorax (6%), abdomen (11%), and extremities (54%). The proportion of head and neck wounds from2001 to 2005 was higher than the proportion suffered in World War II, Korea, and Vietnam wars. Furthermore, while gunshot wounds accounted for 18% of the injuries from 2001 to 2005, those sustained from explosions accounted for 78% of the injuries, the highest proportion seen in any large-scale conflict.¹¹

Blast injury — the “newest” mechanism of injury

As a result of OIF/OEF, injury due to blast has received significant attention,^12-15 leading one to believe that this is a “new” injury mechanism. However, the effects of explosions on the brain were described as early as 1916 in the medical literature. ¹⁶ Explosions were also a significant source of injury in World War I and World War II, accounting for 35–73% of the injuries, respectively.¹¹ The cluster of symptoms that became known as “shell shock” was originally thought to be related to blast exposure, although the idea was controversial even during World War I and in the years after.¹⁷

The results of the meta-analysis18 published by the World Health Organization collaborating center task force on mTBI suggest that the vast majority of adults have good outcomes following uncomplicated mTBI, typically recovering in full within months. In terms of cognition, any decrements that are apparent in neuropsychological functioning after mTBI typically resolve in 1–3 months.¹⁹ While this suggests that the majority of those sustaining mTBI, even under combat conditions, will have good recovery over the longer term, even transient symptoms may have military operational consequences. Slowed reaction time, headache, dizziness, or inattention may have implications for combat readiness or troop welfare. Even among military personnel that are not injured, there is recognition that combat operations may have cognitive consequences due to stress, sleep deprivation, or other factors.^20,21 Troop commanders are increasingly aware of these issues and have been supportive of the military’s effort to screen form TBI on the battlefield, including use of the Military Acute Concussion Evaluation (MACE)²² and standardized DoD clinical practice guidelines around management of mTBI.