The BAEP has five peaks or waves that are monitored in the intraoperative setting (Fig. 9.2) [12–14]. The peaks are named peak I through to peak V (wave is often substituted for peak). Peak I is generated in the distal auditory nerve, and there is very little controversy about the origin of peak I. However, for the rest of the peaks, the situation is a little more complicated as each peak has more than one potential generator. We will initially consider the primary generators or at least the generators that are most commonly considered the primary generators. Peak II is generated from the auditory nerve, but in this case the intracranial portion is also called the proximal portion of the auditory nerve. The third peak, peak III, is the first that originates from the secondary neurons, meaning that they are the first peaks for which a synapse is involved. The caudal pontine tegmentum is the principal generator of peak III, as well as the negative peak between peak III and peak IV, sometimes known as IIIa. There is some evidence however that there is a contribution from the cochlear nucleus to peak III as well as a contribution from the ascending activity within the lateral lemniscus. Peak III is usually not altered in individuals with lesions in the upper or middle pons, or even the mesencephalon, which is evidence that the generator lies caudal to this point. The most likely generator for peak IV is the SOC, but there is no conclusive evidence to date for a precise origin to be determined. The lateral lemniscus remains a candidate for the generator of peak IV. Peak V, the last of the BAEP peaks, is generated predominantly by the contralateral mesencephalon, specifically the inferior colliculus. The lesions of the pons and mesencephalon frequently affect peak V first in the time course of the disease, and as such, this peak may be abnormal in patients undergoing surgery even when the tumor is considered to be relatively small [13]. Later peaks are not considered as part of the BAEP for the purposes of IOM at present.

Preoperatively, the neuromonitorist must determine the baseline hearing of the patient. Often audiologists assess this formally before the surgical procedure is planned. An appropriate stimulation level for intraoperative BAEPs can then be determined. Preoperative BAEPs may be helpful if there is time to obtain one. This is one of the easier evoked potentials to perform on an awake patient. Very few people find the process uncomfortable [3, 15]. Gathering these data before the surgery may give insight into any apparent abnormalities in the operative baseline and help distinguish between preexisting pathology and a technical issue.

The size of the ear canal and whether it is occluded with earwax should also be determined prior to surgery. The presence of wax in the ear canal results in a conductive hearing deficit and will impact the monitoring data. If determined during the preoperative visit, the patient can be asked to clean their ears prior to surgery. If not discovered until the patient is seen in holding, then an ENT consult should be considered for wax removal prior to surgery.

The anesthetic regiment has little to no effect on the potential, and so any concerns or discussions with the anesthesia members of the team are more likely to focus on modalities other than the BAEP [8].

In the operating room, stimulation is usually provided through ear inserts, placed into the ear canal and connected to the electromechanical stimulator through relatively rigid tubing of a known length. The neuromonitorist in the operating room must therefore determine an acceptable location to place the stimulators that will allow them to move with the patient but out of the way of the surgeons. In practice I find affixing them to the Mayfield clamp the most reliable way of performing this important step of setup. Replacing the ear inserts if they fall out during a case can be difficult and is not going to win you much appreciation from the rest of the team.