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A commonly held belief is that the pressure alone from an explosive blast injury is sufficient to cause brain injury in human beings. While this would intuitively seem to make sense, what many people do not realize is that there are no published, peer reviewed, prospective research studies with human subjects that have demonstrated this to be the case. The evidence to date is based on a few old single case studies, military documents that were not scientific research studies, and data from animal research.
There is no doubt that explosive blasts are associated with brain injury. However, there are many components to a blast that can cause injury to the body. First, there is the primary pressure wave injury, which is injury caused by the changes in the atmosphere caused by the explosion. The organs that are most vulnerable to this type of injury are those with air-fluid interfaces, such as the lungs, intestines, or inner ear. These tend to be hollow body parts. The most common type of injury from a primary pressure wave explosion is an eardrum rupture.
Then there are secondary blast injuries, in which the force of the explosion causes objects to fly through the air and strike someone. There is no doubt that this can cause a brain injury, since an object can fly through the air at considerable force and cause blunt trauma to the skull and its underlying contents. A tertiary injury is when the force of the explosion causes the person to be thrown into solid object. Clearly, this can also cause brain injury, if the person is thrown forwards with enough force. Lastly, quaternary blast injuries are injuries caused by the aftermath of the blast. These types of injuries usually include burns, chemical and toxic dust inhalation poisoning, radiation exposure, and crush injuries due to building collapse.
One of the problems in stating that primary pressure waves causes brain injury in humans is that it is often impossible to know whether or not someone (such as a soldier) was only exposed to a primary blast injury or whether secondary or tertiary injuries also occurred. As Hurley and colleagues (2006) stated, “A still unresolved controversy is whether primary blast forces directly injure the brain” (p. 143, emphasis added). Similarly, Bochicchio and colleagues (2008) noted “...it is difficult to clearly distinguish between primary versus secondary or tertiary blast injury” (p.270). It is important to emphasize that no one denies this is possible, or even that it is likely, but caution needs to be taken before it is stated as a scientific fact in humans.
The word “humans” is emphasized because there is evidence that primary pressure waves alone can cause brain injury in animals. However, generalizing from animal studies to humans is not always possible. To begin with, the brains of mice, which is the animal group most often researched in these types of studies, are extremely small, structurally different, and not at all identical to the brains of humans. Secondly, the animal studies typically involve exposing them to blast forces at the end of a giant shock tube. While this may be something you see in an episode of a Tom and Jerry cartoon, it is not necessarily something that can be generalized to what soldiers experience during war time. To quote Hurley and colleagues, “The vulnerability of the human brain to primary blast injury is controversial and an area of active research” (p. 145).
The main problem with relying on information from single studies with humans on this issue is that there is often insufficient information to determine exactly what happened during the injury to draw firm and generalizable conclusions. A good example is an often cited case study from 1979 by Murthy et al. The case study describes a 26-year-old pipe fitter who presented to a hospital about 40 days after a pipe connection (5-cm in diameter) exploded under high pressure (80 lb/square feet). He reported feeling “dazed” and could not hear for a few hours after the explosion. He stated he did not sustain any physical injury and did not lose consciousness. Ten days later, he reported headache, vomiting, and slight blurry vision on the right eye. The problem with concluding that this case proves that primary pressure waves cause brain injury is that there were apparently no witnesses to the event and therefore we do not actually know if the force of the explosion thrust him forwards and caused him to hit his head on a solid object (e.g., a shelf). If this occurred, the bleed could have been caused by a tertiary injury and not a primary injury. Since he did not present to the hospital for 40 days, there was no physical examination performed at the time to examine his head for external signs of head injury. In addition, blows to the head do not always cause outward signs of trauma. Lastly, since 40 days elapsed between the time the patient was injured and arrived to the ER, we do not know if there were any intervening events that occurred which may explain his presentation.
As the committee of the Institute of Medicine (IOM) reported in December 2008, "There is a paucity of information in the scientific literature regarding the sequelae of blast injury, and there is a need for prospective, longitudinal studies to confirm reports of long-term effects of exposure to blasts." The IOM report was commissioned by the Department of Veterans Affairs and based on an analysis of 1,900 peer-reviewed studies.
MTBIFacts.com fully supports future research in this area but cautions against definitive statements on this topic until such research has been performed, replicated, and accepted by the general scientific community.
Another point that bears mentioning is that when humans are exposed to violent explosive forces that could potentially maim, disemember, or kill them, they tend to be psychologically traumatized by this and develop signs and symptoms of this posttraumatic stress disorder (PTSD). Some of the signs and symptoms of PTSD in the Diagnostic and Statistical Manual of Mental Disorders-IV (APA, 1994) are identical to those of post-concussional disorder. These symptoms include irritability, sleep disturbance, and concentration problems. In addition, postconcussional disorder criteria include apathy and lack of spontaneity, which is very similar to the PTSD symptoms of restricted affect and diminished interest or participation in significant activities. Also, many patients with PTSD develop another anxiety-based problem known as panic attacks. Dizziness is a symptom of panic attacks and is also a symptom of postconcussional disorder. Whereas PTSD and panic attacks have enough research behind them to be considered legitimate diagnoses in DSM-IV, this is not the case for postconcussional disorder, which is listed in the section entitled “Criteria Sets and Axes Provided for Further Study.” According to the text, diagnoses are listed in that section because “…there was insufficient information to warrant inclusion of these proposals as official categories or axes in DSM-IV.”
Too often, clinicians do not inquire about other PTSD symptoms such as nightmares, intrusive thoughts of trauma, physical arousal upon reminders of trauma, attempts to avoid such reminders, and emotional numbing. As a result, PTSD is often overlooked, an overemphasis is placed on a brain injury diagnosis which may or may not be accurate, and patients continue to suffer with a condition (i.e., PTSD) that is generally responsive to treatment. In addition, patients may continue to incorrectly believe that PTSD symptoms are brain injury symptoms. Of course, PTSD and mild traumatic brain injury can co-occur but an early focus should be on treating PTSD to improve outcome.
REFERENCES
American Psychiatric Association (1994). Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. Washington, DC, American Psychiatric Association.
Bochicchio et al. (2008). Blast injury in a civilian trauma setting is
associated with a delay in diagnosis of traumatic brain injury. The American Surgeon, 74, 267-270.
Hurley et al. (2006). Blast-related traumatic brain injury: What is known? J Neuropsychiatry Clin Neurosci, 1, 141-145.
Murthy et al. (1979). Subdural hematoma in an adult following a blast injury: case report. Journal of Neurosurgery, 50, 260-261.
Dr. Carone offers paid lectures on MTBI, “post concussion syndrome,” and symptom validity testing upon request. He can be contacted at info@mtbifacts.com.
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