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Primary etiologies of acute brain injury in children include traumatic brain injury (TBI), hypoxic-ischemic encephalopathy (HIE), stroke, cerebral hemorrhages, infections, inflammatory conditions, seizures, tumor or mass lesions, metabolic abnormalities, and toxins (Table 105-1). TBI is the leading cause of death and disability in both children older than 1 year of age and young adults. Following the primary injury, the tissues surrounding the injury and the entire brain are vulnerable to further secondary injuries. The purpose of the initial management, investigation, monitoring, and treatment of acute brain injuries is aimed at preventing this secondary injury.
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Pathophysiology of Secondary Injuries
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Secondary injuries evolve in the minutes to days after the primary event and include both endogenous responses to the primary injury and secondary insults that occur in the field or during the course of care, such as hypoxemia or hypotension. Mechanisms involved in endogenous secondary injuries include energy failure, excitotoxicity and apoptosis, oxidative stress, mitochondrial dysfunction, inflammation, and multiple cell death pathways.
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Hypoxia-Ischemia, Energy Failure, Excitotoxicity, and Oxidative Stress
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In severe injury with cessation of blood flow, as occurs in ischemic stroke or cardiac arrest, a characteristic pattern of injury ensues. Oxygen stores are depleted very rapidly (< 20 seconds), and adenosine triphosphate (ATP) stores are depleted within 5 minutes. No further ATP can be generated to fuel energy-dependent cellular processes, and cell membranes depolarize, resulting in influx of sodium and calcium, efflux of potassium, cellular swelling, oxygen free radical production, and release of excitatory neurotransmitters such as glutamate from astrocytes and neurons. The release of glutamate triggers further depolarization of adjacent cells, and this potent combination of increased cellular metabolism and ischemia accelerates hypoxic injury. The same cellular dysfunction also occurs in situations of altered blood flow and is demonstrated in TBI, status epilepticus, and meningitis. In adult studies, cerebral blood flow (CBF) of less than 55 mL/100 g/min of brain tissue (at normothermia) led to inhibition of protein synthesis, key to the regeneration of injured tissues. CBF of less than 20 mL/100 g/min resulted in anoxic depolarization. These thresholds are likely higher in injured brain tissue, rendered vulnerable by excitotoxicity, seizures, and impaired oxygen utilization from mitochondrial dysfunction. Either globally or more focally, disturbance to the neurovascular unit that regulates CBF occurs in all forms of severe acute brain injury as a consequence of direct tissue disruption, edema, intracranial hypertension, vasospasm and loss of autoregulation. Much of the secondary injury in severe ...