*2.3.2.1. Brainstem auditory evoked potentials (BAEPs)*

BAEPs are the set of waveforms that are recorded at the scalp after auditory stimulation of the middle ear. They are formed by the following potentials [26]: (a) wave I. Originates at the distal action potential of cranial nerve (CN) VIII, typically has a latency of 1.5 ms; (b) wave II. This potential is generated at the entry of CN VIII into the brainstem; (c) wave III. Synapse at the ipsilateral superior olivary nucleus. Latency of 3.5 ms; (d) wave IV is produced by activation of the nucleus or axons of the lateral lemniscus; (e) wave V appears to result from activation of the inferior colliculus; (f) waves VI and VII are presumed to be generated by the medial geniculate body and the thalamo-cortical pathways, respectively.

BAEPs are indicated when tumours affect the auditory system (mainly schwannomas of CN VIII) and tumours affecting the brainstem.

A high intensity (greater than 80 dB) is usually used during IONM. The frequency is 21.14 Hz, and a bandwidth of 10/30–1500 Hz is optimal (a broader bandwidth can allow too much noise), with the notch on. A minimum of 1000 stimuli/train are needed to obtain reproducible and stable waveforms.
