Traumatic Brain Injury and Effects of Altitude - Human Performance ...

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Traumatic Brain Injury (TBI) and Effects of Altitude:An Analysis of the Literatureintracranial, acute, chronic, hydration, hyperbaric, hypertonic saline, acute mountain sickness, and AMS;as well as the tertiary search terms, hypertension, hypotension, hypothermia, hyperthermia, perfusion,oxygen, hemoglobin, edema, blood pressure, intra cranial pressure, ICP, hypoxia, military, defense, DoD,soldier, and veteran. In addition, we searched the reference lists of discovered articles for referencesthat may have been missed in the original search.The targeted 10-year search strategy described above yielded a total of 16 clinical articles describinghuman clinical studies (2000-2010) of factors that can threaten the outcome or survival of TBI patients:altitude-related brain damage, aeromedical evacuation, hypoxia, ICP/blood pressure, hydration andtemperature. In each case, we describe key findings from the available research literature and considerpotential interventions and management strategies that may be employed to monitor or reducenegative effects on neurological health. Finally, we consider implications unique to military TBI andidentify knowledge gaps that need to be addressed by future research.TBI at Altitude: Risk FactorsALTITUDE-RELATED BRAIN DAMAGEThere is a small but compelling literature that demonstrates lasting adverse neurological effects fromexposure to extremely high altitude, as evinced by irreversible structural MRI abnormalities. Garrido etal. (1993, 1996) found evidence of persistent cortical atrophy in elite climbers who had ascendedmultiple times to altitudes above 8000 meters without supplemental oxygen; interestingly, only oneamong a comparison group of seven Himilayan Sherpas showed similar evidence of neurological damage(Garrido et al., 1996).We also identified three more recent articles documenting evidence of brain damage after exposure toextreme altitude (Appendix: AppendixTable 2). One of these was a case study (Jersey, 2010) reporting a rare, near-fatal example ofdecompression sickness (DCS) in a U.S. Air Force pilot while flying a high-altitude surveillance aircraft(cabin pressure = 28, 000 feet). Decompression sickness is the result of exposure to changes inenvironmental pressure, either as the result of deep scuba diving or high-altitude aviation. In high-altitudesituations, DCS may occur if an unpressurized aircraft ascends rapidly or if its pressurization fails at highaltitude. Inert gas (nitrogen) in the body is released as bubbles, which can enter the arterial bloodstreamand damage the brain. In the case reported by Jersey, MRI images revealed that as a result of DCS, thepilot had developed bilateral frontal and right cerebellar abnormalities consistent with ischemia andhypoxia; he was also left with persistent cognitive impairments (confusion, amnesia, personality changes)and balance deficits (ataxia, impaired equilibrium). These clinical findings were described as similar tothose of traumatic brain injury or stroke.Fayed et al. (2006) and Paola (2008) observed MRI abnormalities in civilian mountain climbers who hadbeen exposed to very high/extreme altitudes without the benefit of supplementary oxygen. The largerof the two studies observed 35 climbers, finding irreversible frontal and parietal lesions in the amateurclimbers and diffuse cortical atrophy in professional climbers (Fayed et al., 2006). The smaller study,comprised only of world-class climbers, observed focal areas of cortical atrophy in climbers’ motorcortices (Paola, 2008).September 14, 2010 14
Traumatic Brain Injury (TBI) and Effects of Altitude:An Analysis of the LiteratureAEROMEDICAL TRANSPORTBy reducing the time between injury and definitive medical care, air transport can mitigate the impact ofsecondary cerebral injuries that would otherwise jeopardize TBI patient survival (Malacrida et al., 1993).Davis et al. (2005a) reported significantly better outcomes for moderate to severe TBI patients who aretransported by air vs. ground, with the clearest benefit observed among those with the most severeTBIs. However, air transport also introduces certain unique risks which have the potential to aggravateor increase the risk of secondary brain injury. Evacuation by air can expose head injury patients tomultiple sources of additional physiologic stress, including cold, hypobaric hypoxia, expansion of trappedgases, changes in temperature, noise, vibration, acceleration/deceleration and sometimes rapid tacticalascent (Barnes et al., 2008; Reddick, 1977). Military medical aircrews operate in austere environments,under demanding and sometimes dangerous circumstances. They must be prepared to ascend to higherthan-normalaltitudes not only to avoid weather systems and high terrain, but also to avoid detection orattack by military enemies on the ground.Letarte et al. (1999) reviews clinical recommendations for managing head injury in the flightenvironment, emphasizing that the two critical predictors of outcome from head injury, hypoxia andhypotension, may depend primarily on early and effective prevention and management by flight crews.Failure to avoid these well-established risks can have fatal consequences (Turkan et al., 2006).Supplemental oxygen treatment is indicated for patients with moderate to severe TBI (Glasgow ComaScale score/GCS < 14), hemorrhagic shock and/or other injuries that may be associated with impairedoxygenation such as chest trauma, neck/facial trauma and airway obstruction (Grissom et al., 2006;Sumann et al., 2009).Healthy individuals taken to an aircraft cabin altitude of about 2500 meters (8000 feet) can experiencean average drop in saturation of blood oxygen (SpO 2 ) of 4% or more (Cottrell et al., 1995; Muhm et al.,2007). Unfortunately, there is little empirical evidence available to inform risk assessment as relates toair transport of TBI casualties. We found two recent clinical studies that specifically address thisquestion (Appendix: Table 3). Neither study reported negative effects of pre-hospital care on TBIoutcome in a flight environment. To the contrary, findings in each case described overall potentialbenefits of aeromedical transport.Although some studies report adverse effects of pre-hospital ventilation by intubation on TBI patientoutcome (e.g., Davis et al., 2005b; Murray et al., 2000; Wang et al., 2004), airway management isgenerally considered fundamental to the acute care and oxygenation of severely head-injured patients(Winchell & Hoyt, 1997). Davis et al. (2005a) document improved survival among TBI patients who areintubated in flight compared to those who receive ground transport and subsequent intubation in thehospital emergency department. Recent evidence suggests that improved outcomes in patientstransported by air may depend on the effective use of monitoring procedures, resources, and avoidanceof hyperventilation (Argyros & Cassimatis, 2002; Austin, 2000; Barnes et al., 2008; Carrel et al., 1994;Davis et al., 2004; Poste et al., 2004).Donovan et al. (2008) examined the outcome of aeromedically transported patients with head injuries inrelation to intracranial air, which expands at high altitude and can lead to subdural hematoma and brainherniation. All patients underwent air transport without neurological deterioration. The authorsconclude that the presence of intracranial air in the head-injured patient is not an absolute contraindicationto air evacuation. However, it should be noted that only three of the military patientsSeptember 14, 2010 15
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