Explosions and Blast Injuries: A Primer for Clinicians

As the risk of terrorist attacks increases in the US, disaster response personnel must understand the unique pathophysiology of injuries associated with explosions and must be prepared to assess and treat the people injured by them.

Key Concepts

• Bombs and explosions can cause unique patterns of injury seldom seen outside combat.
• The predominant post explosion injuries among survivors involve standard penetrating and blunt trauma. Blast lung is the most common fatal injury among initial survivors.
• Explosions in confined spaces (mines, buildings, or large vehicles) and/or structural collapse are associated with greater morbidity and mortality.
• Half of all initial casualties will seek medical care over a one-hour period. This can be useful to predict demand for care and resource needs.
• Expect an “upside-down” triage — the most severely injured arrive after the less injured, who bypass EMS triage and go directly to the closest hospitals.

Background

Explosions can produce unique patterns of injury seldom seen outside combat. When they do occur, they have the potential to inflict multi-system life-threatening injuries on many persons simultaneously. The injury patterns following such events are a product of the composition and amount of the materials involved, the surrounding environment, delivery method (if a bomb), the distance between the victim and the blast, and any intervening protective barriers or environmental hazards. Because explosions are relatively infrequent, blast-related injuries can present unique triage, diagnostic, and management challenges to providers of emergency care.

Few U.S. health professionals have experience with explosive-related injuries. Vietnam-era physicians are retiring, other armed conflicts have been short-lived, and until this past decade, the U.S. was largely spared of the scourge of mega-terrorist attacks. This primer introduces information relevant to the care of casualties from explosives and blast injuries.

Classification of Explosives

Explosives are categorized as high-order explosives (HE) or low-order explosives (LE). HE produce a defining supersonic over-pressurization shock wave. Examples of HE include TNT, C-4, Semtex, nitroglycerin, dynamite, and ammonium nitrate fuel oil (ANFO). LE create a subsonic explosion and lack HE’s over-pressurization wave. Examples of LE include pipe bombs, gunpowder, and most pure petroleum-based bombs such as Molotov cocktails or aircraft improvised as guided missiles. HE and LE cause different injury patterns.

Explosive and incendiary (fire) bombs are further characterized based on their source. “Manufactured” implies standard military-issued, mass produced, and quality-tested weapons. “Improvised” describes weapons produced in small quantities, or use of a device outside its intended purpose, such as converting a commercial aircraft into a guided missile. Manufactured (military) explosive weapons are exclusively HE-based. Terrorists will use whatever is available – illegally obtained manufactured weapons or improvised explosive devices (also known as “IEDs”) that may be composed of HE, LE, or both. Manufactured and improvised bombs cause markedly different injuries.

Blast Injuries

The four basic mechanisms of blast injury are termed as primary, secondary, tertiary, and quaternary (Table 1). “Blast Wave” (primary) refers to the intense over-pressurization impulse created by a detonated HE. Blast injuries are characterized by anatomical and physiological changes from the direct or reflective over-pressurization force impacting the body’s surface. The HE “blast wave” (over-pressure component) should be distinguished from “blast wind” (forced super-heated air flow). The latter may be encountered with both HE and LE.

Table 1: Mechanisms of Blast Injury

LE are classified differently because they lack the self-defining HE over-pressurization wave. LE’s mechanisms of injuries are characterized as due from ballistics (fragmentation), blast wind (not blast wave), and thermal. There is some overlap between LE descriptive mechanisms and HE’s Secondary, Tertiary, and Quaternary mechanisms.

Table 2: Overview of Explosive Related Injuries

Note: Up to 10% of all blast survivors have significant eye injuries. These injuries involve perforations from high-velocity projectiles, can occur with minimal initial discomfort, and present for care days, weeks, or months after the event. Symptoms include eye pain or irritation, foreign body sensation, altered vision, periorbital swelling or contusions. Findings can include decreased visual acuity, hyphema, globe perforation, subconjunctival hemorrhage, foreign body, or lid lacerations. Liberal referral for ophthalmologic screening is encouraged.

Selected Blast Injuries

• Lung Injury: “Blast lung” is a direct consequence of the HE over-pressurization wave. It is the most common fatal primary blast injury among initial survivors. Signs of blast lung are usually present at the time of initial evaluation, but they have been reported as late as 48 hours after the explosion. Blast lung is characterized by the clinical triad of apnea, bradycardia, and hypotension. Pulmonary injuries vary from scattered petechae to confluent hemorrhages. Blast lung should be suspected for anyone with dyspnea, cough, hemoptysis, or chest pain following blast exposure. Blast lung produces a characteristic “butterfly” pattern on chest X-ray. A chest X-ray is recommended for all exposed persons and a prophylactic chest tube (thoracostomy) is recommended before general anesthesia or air transport is indicated if blast lung is suspected.
• Ear Injury: Primary blast injuries of the auditory system cause significant morbidity, but are easily overlooked. Injury is dependent on the orientation of the ear to the blast. TM perforation is the most common injury to the middle ear. Signs of ear injury are usually present at time of initial evaluation and should be suspected for anyone presenting with hearing loss, tinnitus, otalgia, vertigo, bleeding from the external canal, TM rupture, or mucopurulent otorhea. All patients exposed to blast should have an otologic assessment and audiometry.
• Abdominal Injury: Gas-containing sections of the GI tract are most vulnerable to primary blast effect. This can cause immediate bowel perforation, hemorrhage (ranging from small petechiae to large hematomas), mesenteric shear injuries, solid organ lacerations, and testicular rupture. Blast abdominal injury should be suspected in anyone exposed to an explosion with abdominal pain, nausea, vomiting, hematemesis, rectal pain, tenesmus, testicular pain, unexplained hypovolemia, or any findings suggestive of an acute abdomen. Clinical findings may be absent until the onset of complications.
• Brain Injury: Primary blast waves can cause concussions or mild traumatic brain injury (MTBI) without a direct blow to the head. Consider the proximity of the victim to the blast particularly when given complaints of headache, fatigue, poor concentration, lethargy, depression, anxiety, insomnia, or other constitutional symptoms. The symptoms of concussion and post traumatic stress disorder can be similar.

Emergency Management Options

• Follow your hospital’s and regional disaster system’s plan.
• Expect an “upside-down” triage - the most severely injured arrive after the less injured, who by-pass EMS triage and go directly to the closest hospitals.
• Double the first hour’s casualties for a rough prediction of total “first wave” of casualties. (See Mass Casualties Predictor.)
• Obtain and record details about the nature of the explosion, potential toxic exposures and environmental hazards, and casualty location from police, fire, EMS, ICS Commander, regional EMA, health department, and reliable news sources.
• If structural collapse occurs, expect increased severity and delayed arrival of casualties.

Medical Management Options

• Blast injuries are not confined to the battlefield. They should be considered for any victim exposed to an explosive force.
• Clinical signs of blast-related abdominal injuries can be initially silent until signs of acute abdomen or sepsis are advanced.
• Standard penetrating and blunt trauma to any body surface is the most common injury seen among survivors. Primary blast lung and blast abdomen are associated with a high mortality rate. “Blast Lung” is the most common fatal injury among initial survivors.
• Blast lung presents soon after exposure. It can be confirmed by finding a “butterfly” pattern on chest X-ray. Prophylactic chest tubes (thoracostomy) are recommended prior to general anesthesia and/or air transport.
• Auditory system injuries and concussions are easily overlooked. The symptoms of mild TBI and post traumatic stress disorder can be identical.
• Isolated TM rupture is not a marker of morbidity; however, traumatic amputation of any limb is a marker for multi-system injuries.
• Air embolism is common, and can present as stroke, MI, acute abdomen, blindness, deafness, spinal cord injury, or claudication. Hyperbaric oxygen therapy may be effective in some cases.
• Compartment syndrome, rhabdomyolysis, and acute renal failure are associated with structural collapse, prolonged extrication, severe burns, and some poisonings.
• Consider the possibility of exposure to inhaled toxins and poisonings (e.g., CO, CN, MetHgb) in both industrial and criminal explosions.
• Wounds can be grossly contaminated. Consider delayed primary closure and assess tetanus status. Ensure close follow-up of wounds, head injuries, eye, ear, and stress-related complaints.
• Communications and instructions may need to be written because of tinnitus and sudden temporary or permanent deafness.

Selected Readings

Auf der Heide E. Disaster Response: Principles of Preparation and Coordination Disaster Response: Principles of Preparation and Coordination.

Quenemoen LE, Davis, YM, Malilay J, Sinks T, Noji EK, and Klitzman S. The World Trade Center bombing: injury prevention strategies for high-rise building fires. Disasters 1996;20:125–32.

Wightman JM and Gladish SL. Explosions and blast injuries. Annals of Emergency Medicine; June 2001; 37(6): 664-p678.

Stein M and Hirshberg A. Trauma Care in the New Millinium: Medical Consequences of Terrorism, the Conventional Weapon Threat. Surgical Clinics of North America. Dec 1999; Vol 79 (6).

Phillips YY. Primary Blast Injuries. Annals of Emergency Medicine; 1986, Dec; 106 (15); 1446-50.

Hogan D, et al. Emergency Department Impact of the Oklahoma City Terrorist Bombing. Annals of Emergency Medicine; August 1999; 34 (2), pp

Mallonee S, et al. Physical Injuries and Fatalities Resulting From the Oklahoma City Bombing. Journal of the American Medical Association; August 7, 1996; 276 (5); 382-387.

Leibovici D, et al. Blast injuries: bus versus open-air bombings—a comparative study of injuries in survivors of open-air versus confined-space explosions. J Trauma; 1996, Dec; 41 (6): 1030-5.

Katz E, et al. Primary blast injury after a bomb explosion in a civilian bus. Ann Surg; 1989 Apr; 209 (4): 484-8.

Hill JF. Blast injury with particular reference to recent terrorists bombing incidents. Annals of the Royal College of Surgeons of England 1979;61:411.

Landesman LY, Malilay J, Bissell RA, Becker SM, Roberts L, Ascher MS. Roles and responsibilities of public health in disaster preparedness and response. In: Novick LF, Mays GP, editors. Public Health Administration: Principles for Population-based Management. Gaithersburg (MD): Aspen Publishers; 2001.