Evoked Responses

Brainstem auditory evoked potentials (BAEP) and median nerve somatosensory evoked potentials (SSEP) are used for the diagnosis of brain death (see Chapter 38 ). BAEP are signals generated in the auditory nerves and brainstem after an acoustic stimulus. BAEP consist of five identifiable waves generated from specific brainstem structures along the auditory pathways. Wave I represents the eighth nerve compound action potential; wave II, the eight nerve and cochlear nerves; wave III, the lower pons including the superior olive; and waves IV and V, the upper pons and the midbrain, as high as the inferior colliculus.^[99] Brainstem abnormalities can be detected with BAEP. The loss of waves III to V, II to V, or no reproducible BAEP on both sides is usually regarded as brain death, ^{[100] [101]} although wave I sometimes remains.^{[102] [103]} Previous deafness or severe peripheral auditory system damage must be checked, because it may lead to a false-positive diagnosis. Some cases with a loss of waves II to V or III to V despite maintained spontaneous breathing have been reported.^{[102] [104]}

After electrical stimulation of a peripheral nerve, waves of SSEP are generated from the neural structures along the afferent somatosensory pathways: the brachial plexus, the upper cervical cord, the dorsal column nuclei, the ventroposterior thalamus, and the sensory cortex.^[99] "Practice Parameters for Determining Brain Death in Adults," published in Neurology, recommended bilateral absence of N20-P22 responses with median nerve stimulation as a confirmatory laboratory test.^[10] Facco and coworkers^[103] reported that the absence of components later than N13, which represents postsynaptic activity in the central gray matter of the cervical cord, or N13/P13 dissociation was reliable to confirm brain death. These investigators also recommended the combined use of BAEP and SSEP for determining brain death.^[105] A loss of N18 potential, which is supposedly generated at the cuneate nucleus in the medulla oblongata, is also considered reliable for the diagnosis of brain death.^{[106] [107]}