*2.3.4. Motor evoked potentials (MEPs)*

BAEPs are indicated when tumours affect the auditory system (mainly schwannomas of CN

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

These potentials were initially thought to be generated in the primary auditory cortex (PAC), located deeply in the white matter of the lateral fissure of the transverse gyrus of Heschl. However, other different areas, including the second auditory cortex (SAC) and the insula are capable of eliciting cAEPs. There is considerable inter-subject and inter-hemispheric variabil‐

cAEPs are characterized by a series of waves, which can be systematized as follows: (a) short latency waves: N13/P17/N30. These waves are typically recorded from the PAC. This com‐ plex is absent in the SAC and (b) intermediate latency waves: peak between 60 and 100 ms.

A stimulation frequency of 2.18 would be adequate. The bandwidth filter is 0.5–1500 Hz, with the notch filter off, and a minimum intensity of 70 dB (sensation level) is applied to the

These waves exhibit the characteristics of near-field potentials generated from the primary visual cortex. In the surgery room, the most widely (and probably the only) technique used for stimulation is the application of flashes of light. A normal VEP in response to a patternreversal checkerboard is a positive midoccipital peak that occurs at a mean latency of 100 ms with three separate phases: an initial negative deflection (Nl or N75), a prominent positive

Although we think that VEP has undeniable utility, there have been some questions about its efficacy. However, more recent results have demonstrated stable recordings and a strong correlation with the postoperative visual function [28,29]. Therefore, intraoperative VEP monitoring will be mandatory for surgeries harbouring a risk of visual impairment [30].

Stimulation is performed by flashing light-emitting diodes at 2.18 Hz, with 10 μs pulse width, and a bandwidth of 10–1000 Hz. We considered an increase in latency of 10% or a reduction in amplitude greater than 50% amplitude compared with baseline as alarm criteria [6].

In some cases, VEP can be directly recorded from the cortical surface. In these cases, poten‐ tials are much more stable, require fewer stimuli (in fact, a very small number of stimuli can induce the response) and are 2–3 orders of magnitude higher than the scalp recording [31].

These waves, which are always present in the SAC, can also be present in the PAC.

deflection (P1 or P100) and a later negative deflection (N2 or N145).

VIII) and tumours affecting the brainstem.

*2.3.2.2. Cortical auditory evoked potentials (cAEPs)*

ity [27], and the whole structure remains to be elucidated.

stable waveforms.

216 Neurooncology - Newer Developments

contralateral ear.

*2.3.3. Visual evoked potentials (VEPs)*

Motor evoked potentials are the recordings that are obtained from muscles in response to stimulation of the motor system at different levels (cortex, inner capsule, corticospinal/ corticobulbartracts or spinal cord) [22]. Considering the amplitude of the response, these types of evoked potentials do not need to be averaged.

## *2.3.4.1. Transcranial electrical stimulation (TES)*

This technique consists of the stimulation of the motor pathway by an electrical current delivered through electrodes placed outside the cranium, usually in the scalp [32]. The introduction of TES revolutionized the field of IONM [33].

It is commonly believed that TES excites the white matter of the inner capsule (IC) rather than cortical neurons. In fact, an increase in the magnitude of the current provides stimulation at the level of the brainstem [34]. This possibility must be recognized and kept in mind by the neurophysiologist, especially in the case of surgery at the supratentorial level.

Electrodes can be subdermal needles or cork-screws and are placed at different sites, depend‐ ing on the region to be stimulated. The most common sites are as follows [35].


The parameters used to elicit MEP through TES are variable, but we use trains of 4–6 pulses, with a 50–75 μs pulse width, an inter-stimulus interval (ISI) of 2 ms (i.e., 500 Hz) and a voltage ranging from 120 to 450 V. It is extremely unusual to use a higher voltage. The recording should be performed with at least 50 μV/div, although it is quite common to use up to 500 μV/div for some muscles.

In some tumours located in the brainstem, cervical or upper thoracic cord, it can be useful to use a D wave recording rather than MEP. This wave reflects the travel wave of CNAP from the lateral cortico-spinal tract and must be recorded through electrodes placed in the imme‐ diacy of the spinal cord [32]. This technique is very useful because it can be performed under total neuromuscular relaxation and elicited by only one pulse. However, it cannot be used for vascular pathology of the spinal cord and must be cautiously considered when tumours are located near the anterior horns.

There is a false dispute, in our opinion, regarding the superiority of constant-current over constant-voltage stimulation. Of course, there are benefits and flaws of both techniques, but safe and reliable monitoring can be performed using voltage stimulation. Moreover, using a current of 61 mA with a pulse width of 500 μs [35], the total charge applied is 30.5 μC. However, a current of 340 mA over 50 μs [36] supplies a charge of 17 μC, which is approximately half of the total charge supplied with longer pulses.
