Brain death

Abstract

Declaration of brain death requires demonstration of irreversible injury to the whole brain including the brainstem. Current guidelines rely on bedside clinical examination to determine that the patient has irreversible coma, absent cranial nerve reflexes, and apnea. Neurophysiologic testing to support the clinical diagnosis of brain death has primarily consisted of EEG and evoked potentials—typically a combination of somatosensory evoked potential and brainstem auditory evoked potential. The diagnostic accuracy of these ancillary tests has been studied for the last few decades but the role of ancillary neurophysiologic testing in brain death continues to be a source of controversy. This chapter reviews the relevant studies and guidelines about EEG and evoked potentials in ancillary testing for brain death. Clinical scenarios in which neurophysiologic testing may aid the declaration of brain death include equivocal results of clinical examination findings, inability to perform some aspects of the neurologic examination, concern for residual sedative effects, suspected spinal cord or neuromuscular injury, and posterior fossa lesions with brainstem involvement. In these scenarios, EEG and evoked potentials may offer supportive evidence for irreversible injury to the whole brain. This chapter also discusses differences between current adult and pediatric guidelines for the role of ancillary testing in brain death.

Introduction

The concept of brain death has evolved in parallel with advances in intensive care medicine and ventilator support since the 1950s. Criteria had to be established for the determination of death in patients with irreversible cessation of brain function. Developing standardized procedures for the declaration of brain death was also crucial to advances in solid organ transplant. Formal efforts at defining brain death began in earnest with the Harvard Criteria in 1968, followed by the US Uniform Determination of Death Act (UDDA) in 1981. The UDDA stated that “an individual who has sustained either 1) irreversible cessation of circulatory and respiratory functions, or 2) irreversible cessation of all functions of the entire brain, including the brainstem, is dead. A determination of death must be made with accepted medical standards” (UDDA, 1980). In 1995, the American Academy of Neurology (AAN) published a practice parameter codifying the process for declaration of brain death (Quality Standards Subcommittee of the AAN, 1995), which was subsequently updated in 2010 (Wijdicks et al., 2010). In most countries, including the United States, brain death is defined as irreversible cessation of all functions of the entire brain including the brainstem—referred to as whole brain death. The clinical definition of whole brain death includes irreversible coma, absence of all brainstem reflexes, and apnea in the setting of a known brain injury that is sufficient to cause brain death. Some countries—England and Canada among them—have defined brain death as “brainstem death.” In addition to national laws and professional guidelines, the process for determination of brain death—how many examinations are required, the qualifications of examiners, mandatory observation period, and use of ancillary testing—is also dictated by state and local laws as well as institutional policies. The American Academy of Pediatrics (AAP) has published separate guidelines for brain death in children. Inconsistent policies have contributed to significant variability in clinical practice, particularly with regard to the application of ancillary tests.

The most recent AAN practice parameter provides practical guidance on the clinical examination necessary for brain death declaration including exclusion of drug intoxication, severe metabolic derangements, shock, hypothermia, and other brain death mimics (Wijdicks et al., 2010). The neurologic examination requires demonstration of coma, absence of all cranial nerve reflexes, and apnea. The current AAN practice parameter does not recommend any ancillary testing beyond a single bedside neurologic examination and apnea test unless a complete examination cannot be reliably performed or the results are equivocal (Wijdicks et al., 2010). Common reasons for inability to perform a complete neurologic examination include residual sedative drug effects, metabolic derangements, cervical spinal cord or peripheral nervous system injuries that preclude reliable examination, and facial injuries with inability to examine all cranial nerves. Common reasons for inability to perform apnea testing include hypoxia, hemodynamic instability, cardiopulmonary bypass, or patients with baseline hypercarbia due to chronic lung diseases.

Ancillary testing for brain death is broadly divided into two categories: tests of cerebral perfusion and neurophysiologic tests. Perfusion tests that have been described in the medical literature include conventional cerebral angiography, CT angiography (CTA) and perfusion, MR angiography (MRA) and perfusion, transcranial Doppler ultrasonography (TCD), and nuclear medicine brain perfusion scans (most commonly SPECT). Complete absence of blood flow to the brain including the brainstem is considered evidence of brain death. In the case of TCD, absent cerebral perfusion is demonstrated by recording systolic spikes oroscillating flow within the bilateral intracranial internal carotid and middle cerebral arteries (Monteiro et al., 2006). Although catheter angiography is often described as the gold standard for demonstrating absence of cerebral perfusion, this test is rarely performed in routine clinical practice. CTA and TCD are commonly included as ancillary tests in Europe, but the 2010 AAN practice parameter found insufficient evidence to recommend these tests (Wijdicks et al., 2010). The nuclear medicine brain perfusion scan remains a preferred ancillary test in the AAN guidelines and is commonly employed in the United States (Wijdicks et al., 2010). The remainder of this chapter focuses on ancillary testing for brain death with EEG and multimodality evoked potentials.