Hypoxic-ischemic brain injury most often results from insults such as cardiac arrest, vascular catastrophe, poisoning (such as carbon monoxide intoxication or drug overdose), or head trauma. While many patients expire without recovering awareness, improved techniques in resuscitation and artificial life support have resulted in greater numbers of patients surviving with variable degrees of brain injury. The evolution of hypothermic treatment for comatose survivors of cardiac arrest has furthered the potential to improve neurologic morbidity and lessen mortality following anoxic brain injury [1-3].
While progress has also been made in the early identification of patients at greatest risk of poor neurologic outcome after cardiac arrest, reliable prediction of good outcomes, with intact memory and independence, has lagged. The evaluation and prognosis of patients with non-traumatic hypoxic-ischemic brain injury are reviewed here.
Coma is defined as a state of pathologic unconsciousness; patients are unaware of their environment and are unarousable. It is caused by either dysfunction of the reticular activating system above the level of the mid-pons, or dysfunction of both cerebral hemispheres. Physical examination permits localization of the level of central nervous system dysfunction (see 'Clinical assessment' below) [4,5].
Coma must be distinguished from the persistent vegetative state, which is also characterized by unawareness, but in which patients have normal sleep-wake cycles and are arousable. Patients in a coma may progress to a vegetative state, but this may not be associated with an improvement in their overall functional outcome. Both coma and persistent vegetative states must be distinguished from brain death, locked-in syndrome (a condition in which the patient is awake and aware but cannot move or communicate due to muscle paralysis), akinetic mutism (a condition resulting from frontal lobe injury in which the patient does not initiate speech or movements), and dementia (table 1) [4,6]. (See "Stupor and coma in adults", section on 'Conditions mistaken for coma'.)
The vegetative and minimally conscious states are clinically defined syndromes. Importantly, prognosis for recovery can vary depending on the underlying etiology and differs between patients who have hypoxic ischemic versus traumatic brain injury .