Synonyms

This term is highly associated (but technically not synonymous) with mild traumatic brain injury and post-concussive symptoms.

Definition

A trauma sustained as a result of exposure to an explosion or its effects. Technically, blast injury can affect any physical system/function; its neurological effects are highlighted here.

Historical Background

Blast injuries can occur in any setting, civilian or military. However, exposure to the effects of explosive forces is much more associated with military populations and has been since the advent of modern warfare. Awareness of the effect of blast injuries began to emerge in earnest with the phenomenon of “shell shock” during the First World War. That war exposed a staggering number of soldiers to explosive injuries, far more than had previous conflicts. As a result, an ever-increasing number of military personnel presented with vague but incapacitating complaints that prevented them from returning to active (particularly front line) duty. Initially, these symptoms were considered to be secondary to organic central nervous system injury. Over time, however, others favored a more psychological or even intentional (i.e., malingering) explanation, citing the fact that many shell-shocked servicemen did not appear to have been as close to the explosion as would seem necessary to truly be negatively affected. The nature of the shell-shock symptoms was further obscured by the lack of diagnostic methods, absence of a clear definition of the syndrome, and even political factors (e.g., superiors being able to justify returning much-needed soldiers to frontline duty if their complaints reflected psychological or constitutional weaknesses rather than neurological/organic injuries). This debate of “psychological versus neurological” causes has continued throughout subsequent wars and is a particular focus of the recent conflicts in the Middle East, given the high incidence of explosives utilized by terrorists and non-Coalition combatants.

Current Knowledge

With improvements in medical care of trauma and the development of more effective defensive equipment (i.e., body and vehicle armor), a greater number of servicemen and women injured in combat are surviving than ever before: The mortality rate for wounded personnel has declined from approximately 30% during the Second World War to approximately 10% during the recent conflicts in Iraq and Afghanistan. As a result, a greater number of the wounded are surviving with traumatic brain injury than in the past – from under 20% during the Vietnam conflict to perhaps near 50% in the recent conflicts. Overall, it has been estimated that up to 30% of all combat troops in Operations Iraqi Freedom and Enduring Freedom (OIF/OEF) may have incurred an acquired brain injury of some degree. The majority of these combat-related brain injuries are sustained as a result of exposure to an explosion.

Explosions may cause injury through four mechanisms:

  1. 1.

    Primary Blast Injury

    A primary injury is one sustained from exposure to the shock/pressure waves initiated by the explosion. When explosive munitions are detonated, a shock-wave approaching a speed of 8000 m/s is generated. The waves generated from a blast can cause life-threatening injuries when they strike an individual directly or if they reflect off nearby surfaces and then come into contact with the person. The force generated is of such a magnitude that it often results in an instant fatality or in trauma to multiple body systems. Body organs that are relatively solid or fluid-filled tend to sustain a lesser degree of injury than those that are gas-filled or have a gas-liquid interface, such as the tympanic membrane, lung, colon, etc. Although not fully understood, research suggests that the explosion may injure the central nervous system directly, as in a concussion, but may also indirectly affect the brain. The latter case may occur when peripheral somatic areas are impacted by the blast, setting in motion events that ultimately impact the CNS, such as chemical/metabolic cascades, physical sequelae (i.e., cerebral infarction caused by an air embolism), and/or kinetic events (e.g., transfer of shock/pressure wave energy from the body, up the vasculature into brain tissue). It has been postulated that the severity and number of a person’s physical wounds from the primary blasts often overshadow symptoms of traumatic brain injury, delaying diagnosis and treatment for these injuries.

  2. 2.

    Secondary Blast Injury

    Secondary injuries occur when shrapnel, debris, or other objects are caught up by the blast and propelled against/into an individual. Many of these injuries are therefore penetrating in nature.

  3. 3.

    Tertiary Blast Injury

    This type of injury is sustained when the person is caught up and propelled by the blast wind that follows the initial shock wave and is thrown against an object, a structure, the ground, other individuals, etc., often resulting in blunt force wounds.

  4. 4.

    Quaternary Blast Injury

    Quaternary blast injuries arise from the aftereffects of an explosion. Examples include being exposed to radiation, fire, chemicals, dust, or toxic substances that were precipitated by the explosive event.

Typically, an individual is exposed to more than one mechanism, making the contributions of one particular mechanism difficult to separate from others.

Approximately 60% of explosion-related injuries in combat lead to an acquired brain injury. As is the case with other etiologies, the majority of brain injuries resulting from explosions are classified as mild in nature. “Mild” traumatic brain injury (mTBI) has not been consistently defined in the literature, which is a substantial limitation in making meaningful comparisons between studies. Despite this, definitions such as that proposed by the American Congress of Rehabilitation Medicine (ACRM) are coming into wider acceptance and have largely been adopted by the military. The ACRM definition of mTBI includes at least one of the following symptoms: less than half an hour of loss of consciousness, less than 24 h of posttraumatic amnesia, any retrograde or anterograde amnesia, mental status changes immediately after injury, and transient/permanent neurological impairments. The literature cautions that a mild TBI from an explosion may not be equivalent to mild TBI from other etiologies (e.g., motor vehicle accidents, sports injuries), as the former may affect the brain more diffusely and tend to involve trauma to other organ systems, thereby complicating the patient’s clinical presentation and recovery. However, many studies have indicated that factors such as loss of consciousness, resultant symptom profiles, and recovery courses do not appear to substantially differ between blast victims and those injured by other means, tentatively suggesting that knowledge gleaned from studying these other etiologies has at least some applicability to blast injury survivors.

Cognitive (e.g., memory, attention), somatic (e.g., dizziness, headache, sleep initiation/maintenance difficulties), and emotional (e.g., nervousness, irritability) symptoms are commonly seen initially after blast injuries. In civilian mTBI samples, these symptoms usually resolve quickly, with most individuals showing rapid recovery within the first week. However, over one-third may continue to experience significant post-concussion symptoms, and as many as 15% may continue to experience persistent symptoms after 12 months (“Persistent Post-Concussion Syndrome”). Unfortunately, these persistent symptoms have not been consistently defined, and many point out that the constellation of symptoms present are vague and lack specificity needed to identify them as constituting a true syndrome.

There is debate over whether the more chronic symptom constellation after mild TBI reflects a true neurological condition; this has particular relevance in blast injury, as the brief history of “shell shock” above illustrates. Those favoring a more neurological position cite animal models in which direct and indirect exposure to primary blasts causes structural, chemical, and electrophysiological changes in the brain. Additionally, some studies using functional MRI and diffuse tensor imaging in humans have reported cerebral alterations in some (albeit not all) persons who have sustained blast injuries. Conversely, those weighing psychological factors more heavily in terms of causation point to the mTBI literature that indicates non-neurological factors, such as premorbid psychological coping resources and external stressors, appear to influence the development of concussion symptoms in some individuals. The fact that mTBI symptoms overlap considerably with symptoms seen in disorders such as post-traumatic stress disorder (PTSD) is particularly noteworthy, given the high incidence of PTSD in military personnel who have experienced combat: Gaylord et al. (2008) found that nearly 20% of military persons who incurred blast and burn injuries were appropriate for both mild brain injury and PTSD diagnoses. Hoge et al. (2008) reported that approximately 15% of soldiers surveyed after being returned home might meet criteria for both mTBI and PTSD; these servicemen and women were more likely to have been exposed to a blast injury. In addition, their survey indicated that the presence of affective distress might be the major factor in maintaining chronic health difficulties, including mTBI symptomatology. A compromise position of sorts posits mTBI symptoms are likely neurological in origin but are subsequently maintained by emotional/psychological factors and that the presence of PTSD and similar affective disturbances can complicate healing from and coping with mTBI (and, concomitantly, mTBI symptoms can exacerbate and prolong PTSD symptoms). The fact that PTSD symptoms can arise long after the actual trauma indicates that these emotional disturbances may influence a person at virtually any point in his/her brain injury recovery.

Treatment of blast injuries begins with a thorough diagnostic assessment. The armed services have made significant improvements in their endeavors to standardize comprehensive screening and interviewing methods to identify service personnel who may have experienced an acquired brain injury, beginning on the battlefield and continuing throughout the military’s medical system. Efforts have been made to carefully screen every wounded individual for other symptoms (e.g., tinnitus) that place them at higher risk for having sustained a TBI in a blast, to help ensure mTBIs are not underdiagnosed. The assessment process includes a thorough medical evaluation of the patient’s current condition and a comprehensive interview that elicits historical information about past psychological treatment/coping, substance use, and combat exposure.

Neuropsychological evaluation is recommended to occur as early as possible to help identify post-concussive symptoms and clarify the diagnostic picture, enabling education and treatment efforts to proceed more quickly. Whereas there are cognitive deficits commonly seen after most mTBIs (e.g., slowed attention and information processing speed, motor slowness, executive dysfunction, and memory difficulties), the profile of cognitive weaknesses can be quite variable, necessitating a broad-based neuropsychological assessment (i.e., a sampling of all major cognitive domains). Additionally, as is the case in sports concussions, the symptom picture for many blast survivors may evolve relatively rapidly, arguing for use of tests that have alternate forms (e.g., California Verbal Learning Test-2nd Edition, Hopkins Verbal Learning Test-Revised). Tracking somatic symptoms (e.g., Neurobehavioral Symptom Inventory, Post-Concussion Scale-Revised) over time may also have utility. Because of the high degree of PTSD and other affective disorders, a thorough psychological evaluation should always be performed (including objective personality measures such as the MMPI-2 or PAI and instruments such as the PTSD Checklist), and observation for these symptoms should be an ongoing effort, not simply one restricted to an initial evaluation. Given the high degree of lowered effort present in civilian mTBI cases, effort testing (e.g., Test of Memory Malingering) is often advocated, with the caveat that poor performance on an effort test should not automatically be interpreted as an indicator of intentional feigning of symptoms, but as a sign that further investigation is warranted as to the cause of the lowered effort.

After a thorough diagnostic assessment has been performed, treatments generally have proceeded along the lines advocated for mild brain injuries attributed to non-blast causes. Specifically, reassurance and education regarding the nature and general recovery of cognitive and other symptoms after mTBI is delivered. Specific treatments need to be tailored to the individual, in recognition that not all mTBIs are expressed identically. For instance, in a cluster analysis performed on 1341 servicemen who had sustained an mTBI in combat over the previous 2 years, Bailie et al. (2016) found four different subtypes: good recovery (low overall cognitive and affective symptoms; 37.8% of the sample), high PTSD but few cognitive symptoms (21.9%), elevated cognitive but few affective symptoms (21.5%), and mixed symptoms (18.6%). These results suggest an array of treatment strategies that is necessary to effectively address appropriate needs. Such treatment strategies would include medication (e.g., analgesics for pain, soporific medication for insomnia, antidepressants for affective symptoms), relaxation strategies for anxiety symptoms, psychotherapy for PTSD symptoms, and evidence-based cognitive rehabilitation.

Future Directions

The research literature in blast injury is still evolving. The following is a partial list of necessary future research efforts: clarifying definitions of mTBI and post-concussive symptom constellations; separation of the effect of different blast mechanisms (e.g., primary, secondary) on the brain; standardization of research methodology with respect to inducing blast injuries in animal subjects; comparison of mTBI symptoms, course, and recovery between blast injury survivors and those who have injuries from other sources; investigation of the effect of multiple blast exposures; and investigation of how PTSD/affective distress differs from and interacts with mTBI. Studies should take into account situational factors such as combat exposure, combat intensity, length and number of deployments, as well as potentially moderating variables on symptom expression and recovery (e.g., substance use, pain intensity, sleep integrity). More prospective research is clearly needed. Within neuropsychology, development of alternate forms for many tests is encouraged.

See Also