A Review of U.S. Military Traumatic Brain Injury Studies

Military service–related traumatic brain injury (TBI) is a signature wound of war, affecting more than 500,000 service members since 2000, with special operations forces (SOF) at particularly high risk because of frequent blast exposures and repetitive neurological stresses. Many TBIs go undiagnosed or are mild, yet they often co-occur with such conditions as posttraumatic stress disorder (PTSD) and depression and lead to significant long-term health impacts. Despite substantial federal and private investment, TBI research remains fragmented and siloed, highlighting the need for a comprehensive assessment of research efforts and funding in this critical area.

To address this, the Invisible Wounds Foundation asked RAND to conduct a scoping review of TBI research. Specifically, we mapped the progress of TBI research from January 2015 to June 2025 and identified overlaps and gaps in specific areas. We also highlighted the areas that received the most-focused investment. The results of the study can inform opportunities to better align research efforts and investments, ultimately addressing gaps in TBI knowledge and care for service members.

How the Study Was Conducted

We conducted a comprehensive search of peer-reviewed and gray literature on military service–related TBI from January 2015 to June 2025 across multiple databases, focusing on research involving human subjects and limiting in vivo and modeling approaches. Using literature review software, we screened and abstracted key data from 480 eligible articles, including type of TBI, comorbidities, study design, and funding information. We also mapped TBI research investments by extracting and analyzing funding data from major public and private sources. Award amounts were adjusted for inflation, and all inclusion criteria were applied iteratively to maintain consistency throughout the review process.

Key Findings

Type and Volume of Research on Military Service–Related TBI

There is limited research focused on TBI among SOF, although this population faces a higher risk of blast exposure and TBI. Of the papers reviewed, 83 percent focused on military populations, and most did not specify the branch of service or include details on SOF. Army and Marine Corps members were the most frequently studied populations; only seven papers focused solely on SOF, and 14 included SOF in mixed samples.

Most TBI research studies are observational and focused on diagnostic methods; far fewer papers focus on interventions or the root causes of blast injuries. Many studies combined approaches, such as imaging and biomarker analysis, and a smaller number of papers discussed modeling, product development, and expert recommendations. Our emphasis on human research may have excluded some relevant blast physics or in vitro studies. The small number of scale validation studies highlights the ongoing challenges in diagnosing TBI with existing tools.

Nonhuman research mainly uses murine models, with few studies involving larger animals or digital simulations. Most papers reported the source and severity of blasts, often including multiple types of TBI; however, cases of subconcussive injury were rare (31 papers). This low count may be due to the study's limited search terms and to underdiagnosis, suggesting an area for further study.

Most TBI studies examine TBI in the presence of other conditions. Sixty-eight percent of the articles reviewed reported findings on TBI with at least one comorbidity. Many studies examined multiple co-occurring issues, with PTSD being the most common, often appearing with neurological, psychological, behavioral, or sleep disorders. This subanalysis specifically focused on neurological conditions, the second most frequent comorbidity assessed in the literature.

Studies on short- and long-term outcomes of TBI focus on different issues. Studies that assessed the short- to medium-term period following TBI described general neurological symptoms, such as pain, inflammation, balance issues, and sensory changes, rather than any specific diagnoses. Longer-term studies examined neurodegenerative conditions, such as Alzheimer's disease, Parkinson's disease, and chronic traumatic encephalopathy (CTE), whereas others explored early biomarkers without clear clinical outcomes.

Most studies focus on one stage of TBI care, and few focus on prevention and rehabilitation. The analysis showed that 12 percent of the studies (58 papers) addressed prevention, mainly related to blast injuries; 46 percent (224 papers) focused on diagnosis, often using imaging and biomarkers; and 15 percent (77 papers) examined treatment, management, or mitigation strategies, including both animal and human studies. In contrast, only 9 percent (46 papers) addressed rehabilitation and long-term care.

Funding Sources and Amounts for Military Service–Related TBI Research

Since 2015, federal and private sources have invested more than $2.1 billion in TBI research. Most of this funding has come through grants, which make up 75 percent of all awards and more than 85 percent of total dollars spent, supporting a wide variety of research across the continuum of care.

The U.S. Department of War (DOW) and the U.S. Department of Health and Human Services (HHS) are leading research funders.¹ These federal agencies contributed $930 million and more than $1 billion, respectively, from January 2015 to June 2025. DOW funding primarily supports battlefield care, medical research for service members, and rehabilitation, whereas HHS funds basic science, diagnostics, prevention, and patient care, mainly through the National Institutes of Health. DOW's investment is less strongly tied to publication output than HHS's is. Additional research support comes from the U.S. Department of Veterans Affairs and the National Science Foundation for specialized research and treatment initiatives.

Grant funding peaked in 2022, reflecting a significant increase in investment and interest in TBI research. More specifically, federal grant funding peaked in 2022, at more than double the 2015 level. Since then, the number and scale of awards have fluctuated annually, highlighting evolving research priorities and growing interest in addressing gaps in TBI knowledge and care.

Philanthropic contributions have steadily increased to nearly $80 million cumulatively, supporting various initiatives within TBI research. These philanthropic funds have helped advance genetic studies, innovative treatments, caregiver support, and programs for specific groups, such as SOF, often at the community level.

Opportunities for Aligning Research Efforts and Funding

Substantial progress has been made in TBI research over the past ten years, especially in advanced imaging, biomarker discovery, and comorbidities, such as PTSD, depression, and sleep disorders. These advances have deepened our knowledge of TBI's mechanisms and its complex overlap with broader neurological and psychological health issues.

Yet in mapping TBI research with funding sources, we found that military service–related TBI research is siloed between agencies and specialties, which ultimately limits information-sharing, the translation of findings into practice, and the ability to address challenges that cut across different fields and areas of concern. Despite significant investment in TBI research, several critical gaps persist:

• Understudied populations: Little research since 2015 has focused on identifying and treating TBI among SOF, airmen, sailors, and members of the National Guard or reserve, even though these groups face high risks.
• Injury types: Studies and funding mainly address blast-related and mild TBI. Subconcussive and penetrating TBI has been less studied, highlighting an opportunity for targeted investment. Few studies examine the links between injury characteristics (e.g., blast pressure, soldier positioning, blast source) and clinical outcomes.
• Comorbidities: Although there has been growth in research on co-occurring PTSD, sleep disorders, and neurological conditions linked to TBI, such areas as substance misuse and sensory impairments (auditory and ocular conditions) receive little attention.
• Rehabilitation and recovery: There is little research on rehabilitation and long-term care for TBI and its comorbidities, leaving service members and veterans with few long-term, evidence-based recovery pathways.
• Research focus: Most studies are observational or imaging based. There is a need for more longitudinal, qualitative, and translational research to capture real-life experiences and long-term outcomes and for more etiological studies on injury mechanisms.

By highlighting both research achievements and persistent gaps, this study provides a road map for policymakers, funders, and researchers to align priorities to close knowledge gaps. A more coordinated research agenda is essential to advance prevention, diagnosis, treatment, and long-term recovery and ensure comprehensive care and support for all service members and veterans living with TBI.