Research published this week in The Lancet Neurology investigates the brains of soldiers who survived high explosive blasts. The findings give new insight into the poorly understood effects of such injuries.
High explosives are responsible for around 60 percent of combat-related casualties.
An estimated 300,000 United States service personnel in Iraq or Afghanistan have received at least one traumatic brain injury (TBI) due to an explosive blast.
Blast-related injuries can cause a number of negative health outcomes.
This is not surprising; what is surprising is that routine imaging techniques cannot detect or explain why these deficits occur.
Despite no abnormalities being observed, soldiers commonly report debilitating neuropsychiatric symptoms, including headaches, trouble sleeping, erratic behavior, problems remembering, and depression.
These symptoms suggest there is structural damage to the brain but, often, none can be observed during standard brain scans.
Hidden neural damage
Researchers from the Uniformed Services University of the Health Sciences in Massachusetts have been investigating this phenomenon in more detail. Senior author Dr. Daniel Perl explains the problem: "Because the underlying pathophysiology is unknown, we have difficulty diagnosing and treating these 'invisible wounds.'"
Dr. Perl and his team examined the brains of eight deceased former military personnel who survived explosive attacks and later died of other causes.
The brains were compared with 15 male civilians who had experienced a TBI through other means, for instance, contact sports and motor vehicle accidents. They also examined brains affected by chronic opioid abuse and one with chronic traumatic encephalopathy - a progressive, degenerative disease affecting the brains of individuals with a history of repetitive brain trauma.
From the eight blast-survivors, five of the brains showed unique patterns of neural damage.
Brain areas involved in a number of functions - including memory, cognitive function, and sleep - displayed prominent, permanent scarring. In one of the brains, the area known to be involved in post-traumatic stress disorder (PTSD) also showed damage.
The complexity of blast damage
The neural scarring, referred to as astrogliosis, was observed in specific parts of the nervous system. The authors explain that the damage was measured "in tissues adjacent to cerebrospinal fluid, along boundaries between gray and white matter, and around blood vessel."
It seems that because of the complex interaction between extreme pressure and areas of differing tissue density, the areas where two biological materials meet were particularly at risk of damage.
This tell-tale damage was seen in the brains of soldiers who had survived more than 6 months after the blast and also in individuals who died 4-60 days after the explosion. This suggests that the damage was related to the blast.
"Our findings suggest, for the first time, that there might be a predictable pattern of physical damage to the human brain after blast exposure, which standard clinical neuroimaging techniques currently cannot detect," the authors write.
Because the research is a case series, it is not possible for the researchers to prove causation. In other words, they cannot say that the explosive blast was definitely the cause of the astrogliosis. But, because the pattern of damage was not found in the brains of the control subjects, the likelihood that the blast was to blame is increased.
"Although little is known about the effect of blast shockwave on the human brain, the unique pattern of damage that we found is consistent with known shockwave effects on the human body."
Dr. Daniel Perl, lead author
The findings are intriguing, but Dr. Perl is quick to mention the research's shortfalls. As he explains: "It is virtually impossible to obtain comprehensive head injury histories to rule out previous traumatic brain injury, or to get accurate data on injury severity such as the number of blast exposures, proximity to detonation, and power of explosion."
Exposure to blast shockwaves is steadily increasing among soldiers and civilians. The researchers hope that further studies will be able to compare the patterns of damage with a soldier's medical history to build up a stronger understanding of how these injuries impact the living brain.