A review of long-term outcomes of repetitive concussive and subconcussive blast exposures in the military and limitations of the literature.

Abstract

Objective: The purpose of this review is to summarize the long-term cognitive, psychological, fluid biomarker, and neuroimaging outcomes following repetitive concussive and subconcussive blast exposures sustained through a military career. Method/Results: A review of the literature was conducted, with 450 manuscripts originally identified and 44 manuscripts ultimately included in the review. The most robust studies investigating how repetitive concussive and subconcussive exposures related to cognitive performance suggest there is no meaningful impact. Although there are minimal studies that suggest some small impacts on neuroimaging and fluid biomarkers, most findings have been in very small samples and fail to replicate. Both repetitive blast mTBI and subconcussive blasts appeared to be associated with increased self-reported symptoms. Many of the studies suffered from small sample size, failure to correct for multiple comparisons, and inappropriate control groups. Conclusions: Overall, there is little evidence to support that repetitive blast mTBIs or subconcussive blast exposures have a lasting impact on cognition, neuroimaging, or fluid biomarkers. In contrast, there does appear to be a relationship between these exposures and self-reported psychological functioning, though it is unclear what mechanism drives this relationship. Small sample size, lack of correction for multiple comparisons, limited control groups, lack of consideration of important covariates, limited diversity of samples, and lack of reliable and valid measures for assessment of blast exposure are major limitations restricting this research. Patients should be encouraged that while research is ongoing, there is little to currently suggest long-term cognitive or neurological damage following repetitive blast exposure.