Evoked potentials in the diagnosis of brain death

Abstract

In conclusion, the wealth of data available on ancillary tests, the greater knowledge of the process of dying and the evolution of the concept of BD suggest a need to redefine the role of EEG and MEPs in the confirmation of BD. The main facts are the following: 3.1. The Concept of Death: The concept of death is the kernel of the whole matter: differences and apparent discrepancies in the adopted criteria by different countries cannot be overcome until a definitive, unique, and universally accepted definition of BD is adopted. 3.2. The Brain Stem is the Kernel: Whatever the adopted definition of BD, death of the brain stem is the kernel, and the conditio sine qua non, of the diagnosis. This is why the evaluation of all clinical signs of brain-stem function should be mandatory; since the death of the entire brain stem is to be diagnosed, the arbitrary exclusion of any of these signs from diagnostic criteria would be conceptually inconceivable. 3.3. Diagnostic Tools Capable of Exploring the Brain Stem: As a consequence, all diagnostic tools capable of exploring the brain stem should be deemed relevant for the diagnosis; there is no reason to regard brain-stem reflexes as essential, yet not even mention ABRs and SEPs. The absence of evoked potentials from published criteria reflects the historical approach of BD, since before the 1980's they were not widely available in hospitals; in the past two decades they have proved to be very relevant and able to provide objective data on brain-stem structures, which cannot be clinically explored in BD. Moreover, they remain effective even when both clinical examination and EEG are no longer able to provide information. It is now time to discuss and define their role in diagnostic criteria; any other stance would be manipulative, rather than scientific. 3.4. Visual Evoked Potentials: VEPs can provide evidence of absent activity in deep hemispheric structures and occipital cortex, thus improving the evaluation in comparison to the use of EEG in isolation. 3.5. The Electroencephalogram: The EEG, despite the overestimation of its sensitivity and specificity, remains a valuable tool in the diagnosis of whole brain death. In brain-dead patients following supratentorial primary lesions and severe intracranial hypertension, the viability of cortex seems very unlikely. Conversely, in patients with primary brain-stem insults and a clinical picture of BD, even short latency evoked potentials may be abolished, while the EEG may transiently show well preserved cortical activity. In these cases EEG and VEPs may be the most relevant tests, related to the concept of whole brain death. 3.5. Multimodality Evoked Potentials: MEPs may detect residual function in the brain stem in a few hopeless cases with the clinical and EEG picture of BD (despite the absence of any reversible cause of coma), showing that their use may improve diagnosis safety. In this regard, we must be aware that the essence of diagnosis is the determination that the process of dying has concluded. It must never be a prognosis, which could turn the diagnosis of death into euthanasia. The latter is a very relevant and ticklish but completely different philosophical debate. Hence, we propose the use of MEPs brain-dead patients to increase diagnostic reliability.