Chronic Effects of Traumatic Brain Injury in Military Service Members

Traumatic brain injury (TBI), both in combat and in the home and training environment, represents an important concern for the US military. Although TBI associated with military service shares many characteristics with typical civilian-associated TBI across the injury severity spectrum, there are substantial differences that may affect the course of recovery and long-term outcomes. Some of these differences are related to contextual factors, such as the typical mechanisms of injury, which may differ from what is experienced commonly in the civilian setting. Others are related to potentially emotionally traumatic circumstances in which the injuries are acquired as well as other aspects of the military or characteristics of military service members. These differences are important to consider when evaluating or treating military service members with a history of TBI, regardless of the severity of the initial injury.

Between 2000 and the second quarter of 2022, 463,392 service members sustained at least 1 TBI, with 82.3% of these TBIs classified as mild, 11% as moderate, 1% as severe, 1.2% as penetrating, and 4.5% not further classified.¹ In recent wars, explosive blast has played a substantial role as an injury mechanism. Blast, as a mechanism of injury for TBI, shares some characteristics with more typical brain injury causes; there are also some aspects that differ greatly. Blast injury, in both combat and training, largely occurs only in military settings. In many cases, both causes may be present (eg, an explosion causing a motor vehicle collision). In a Department of Veterans Affairs polytrauma system study, the pattern of blast-related injuries noted was different compared with other mechanisms. Injuries to the face (including eye, ear, oral, and maxillofacial), penetrating brain injuries, symptoms of posttraumatic stress, and auditory impairments were more common in people with blast injuries than in those with other injuries.² With polytrauma, what might have been an uneventful recovery from a mild concussion can be complicated by pain, lengthy rehabilitation, sensory impairment, or other factors. In people who experience combat injury, behavioral health concerns, especially posttraumatic stress disorder (PTSD), also are common.

Chronic, low-level blast exposure also is a concern for the military. Low-level exposure can occur during training with the use of breaching charges and high-caliber weapons. Concerns over such exposures were one factor that led the US Department of Defense to commission a Warfighter Brain Health Initiative in 2022.

In most circumstances, recovery from an isolated mild TBI (concussion) follows a predictable course and recovery. Whereas symptoms are common in the days and weeks following concussion, permanent cognitive or physical effects are rare. In the military population, more persistent symptoms than might be seen in the civilian population are more common and are related to a variety of psychological and trauma-related factors.³ Mild TBI is the most common type of TBI experienced in the military population, but more severe and penetrating TBI also is seen, which can lead to lengthy rehabilitation and persistent difficulties across the lifespan.^4,5 Estimates of long-term disability among service members with TBI are about 56% to 66% in those with severe or penetrating TBI and about 12% in those with mild TBI. About 25% of all service members hospitalized for TBI will develop long-term disability.⁶

PTSD and Other Mental Health Conditions

In the military TBI population, posttraumatic stress symptoms are the most critical behavioral health concern. In one meta-analysis, in those with a military-related TBI, the frequency of PTSD was 48.8%, with a relative risk of PTSD of 2.32.⁷ For those with a mild TBI, PTSD is a substantial contributor to the persistence of symptoms over time and may be a stronger predictor of persistent symptoms in the presence of a mild TBI history itself.⁸ Whereas individuals with a mild TBI typically recover over the course of weeks or months, individuals with PTSD or PTSD with sleep disturbance had a markedly increased rate of poor neurobehavioral outcomes.^9,10 Adverse outcome after TBI is not isolated to the mild TBI population. One recent study examining quality of life and neurobehavioral symptoms across the spectrum of TBI severity demonstrated a stronger influence of PTSD than the severity of the TBI itself on outcomes 3 to 5 years after injury.¹¹ PTSD may be an independent risk factor for cardiovascular disease, cognitive decline, and premature death.^12,13

Other Health Concerns

Recent long-term studies of individuals with a history of mild TBI have demonstrated elevated risk for a number of health concerns, including chronic pain, increased opioid usage, sleep disturbances, and Parkinson disease.¹⁴ Health risks also exist for family caregivers, including difficulties with sleep, fatigue, pain, and overall reduced quality of life.¹⁵ Emerging data suggest that unhealthy family functioning may have a reciprocal effect and is associated with poor neurobehavioral outcome in the service member after mild TBI.

Chronic Traumatic Encephalopathy

One potential concern for this population is the development of a deteriorating neurologic condition related to repetitive subconcussive trauma such as been described in professional athletes. Chronic traumatic encephalopathy (CTE) is a progressive neurologic deterioration characterized by an accumulation of abnormal phosphorylated tau in the depths of the cortical sulci hypothesized to result from repetitive blows to the head. Accompanying these brain changes, a number of clinical correlates have been described, including cognitive dysfunction, behavioral derangements, and, in some cases, dementia.¹⁶ This has been an area of active interest for the Department of Defense, although the Institute of Medicine has stated that there is limited evidence to support an association between recurrent blast-related TBI and CTE with progressive cognitive or behavioral decline, or both.¹⁷ Some researchers have questioned whether CTE findings are unique to repetitive trauma and the specific link between neurobehavioral symptoms and specific neuropathology.¹⁸ One recent study described the neuropathologic examinations of 225 brains of deceased service members. Neuropathologic findings of CTE were present in <5% of the sample, with more than half of the CTE cases having only a single pathognomonic lesion. Relatively higher risk was noted in the brains of those individuals who sustained an injury during military service caused by either the head striking the physical object or from blast exposure. Of the decedents with CTE, 60% had received at least one diagnosis of a psychiatric disorder, with PTSD being the most common diagnosis in the group. Sixty percent of those with CTE also had a reported history of alcohol or substance abuse, or both. However, pathologic features of CTE were not found in the brains of most of the individuals who died of suicide, had a substance abuse history, or had been diagnosed with a psychiatric disorder. The authors noted that, despite concerns around CTE in the military population, especially among people exposed to blast, CTE was found infrequently. Some limitations were acknowledged given the relatively small sample size.¹⁹

Longitudinal Studies

There have been multiple studies of longer-term outcome after military TBI. As noted previously, long-term persistent symptoms can occur at high rates in people who experience military-related mild TBI. One ongoing prospective study of a cohort of military service members with a history of combat concussion showed a 37 to 49 times greater likelihood of disability (as measured by the Glasgow Outcome Scale Extended) than a combat-deployed control group. People who were younger at the time of injury and had lower education were at the greatest risk.²⁰ Another recent study examined neurobehavioral symptoms in a group of military service members and veterans at 5 and 10 years after TBI (all severities) as well as 2 control groups (noninjured and injury other than TBI). At both 5 and 10 years post TBI, high numbers of neurobehavioral symptoms were reported, with 50% to 75% reporting a wide range of vestibular, cognitive, and affective symptoms, including anger, depressive symptoms, anxiety, feelings of grief and loss, headaches, and more generalized pain. High rates of symptom reporting were not isolated to the TBI group; even the injured control group reported high rates of symptoms, albeit to a lesser degree.²¹

Conclusion

TBI associated with military service, regardless of the severity, puts military service members and veterans at risk of long-term adverse outcomes. Clinicians treating these individuals should assess a broad range of potential long-term sequelae, including physical, emotional, cognitive, and family concerns. Assessment of posttraumatic stress symptoms is of particular importance because outcome after TBI is influenced heavily by PTSD. Only through a comprehensive approach to care can the clinician address potential long-term negative consequences of the injury properly.