Brain Death: Ancillary Tests
Advanced, Special Problems or Issues in Anesthesiology
Ancillary tests, such as Transcranial Doppler, Cerebral Angiography, CTA, MRA, Nuclear Med, Somatosensory Evoked Potentials and EEG, are available if confounding factors are present and in some countries they are required in addition to the clinical exam. Ancillary tests can only support or not support a clinical diagnosis of brain death. They cannot be used alone to diagnose brain death. These tests are designed to either demonstrate the absence of cerebral electrical activity or the presence of cerebral circulatory arrest.
MRA and CTA are also tests to verify cerebral circulatory arrest. The utility of CTA has been questioned with systematic reviews finding variable sensitivities and specifies when used to determine brain death.
Transcranial Doppler : Use of ultrasound to investigate cerebral blood flow is safe, non-invasive and can be performed at the bedside but does require expertise to perform. Temporal bone density can also limit the study of many of the necessary vessels. Small systolic peaks without diastolic flow or reverberating flow patterns would demonstrate circulatory arrest. Specificity and Sensitivities have been demonstrated to be about 97 and 70 percent respectively.
Nuclear Medicine: Injected radionuclide tracer is taken up by parenchyma proportional to blood flow and remains in place for several hours. The absence of signal, “Hollow Skull Phenomenon,” supports a loss of cerebral blood flow and a diagnosis of brain death.
Electroencephalography: EEG records summated synaptic electric potentials transmitted across the neocortex. It does not record subcortical structures such as the brainstem. Flat EEG tracings, electro-cerebral silence, is consistent with brain death but can be present in confounding clinical scenarios such as hypothermia, sedation, toxin or metabolic disarray. It is also possible that electrical “noise” in the surrounding environment, especially in an ICU, can cause false signals mistaken for cortical activity.
Evoked Potentials: Somatosensory Evoke Potentials (SSEPs) and Brainstem Auditory Evoked Potentials (BAEPs) can be used to demonstrate either the lack of cortical or brainstem response to peripheral stimuli. Each test is dependent on a particular pathway in the nervous system and does not test other aspects of the CNS. If a lesion is present within the tested pathways, it could result in the lack of expected response and possible false positive. Signals are often monitored at multiple locations along the pathway to reduce this risk, such as confirming the presence of Wave 1, the cochlea signal, during BAEPs. SSEPs and BAEPs are less affected by sedatives but are still sensitive to hypothermia, toxins and metabolic derangements.