*2.3.4.2. Direct cortical stimulation (DCS)*

For this technique, electrodes are applied directly to the cortical/subcortical surface. Direct cortical stimulation (DCS) to identify the primary motor cortex (PMC) is accomplished using paired electrodes. Stimulation is performed using 4–6 pulse trains at 500 Hz (the reason we denote this paradigm high frequency; this technique is also known as multipulse, which is misleading), with biphasic pulses of 150–200 μs in the duration/phase. Motor evoked poten‐ tials are assessed using pairs of subdermal needles spaced approximately 2 cm apart that are inserted into the contralateral muscles, but surface electrodes attached to the skin can also be used. Depending on the location of the tumour, it is customary to use the following muscles: the orbicularis oculi, orbicularis oris, deltoid, brachial biceps, extensor digitorum carpal flexor, abductor pollicis brevis, abductor digiti minimi, quadriceps, tibialis anterior and abductor hallucis.

Stimulation is initiated at 4 mA and increased continuously in increments of 1–2 mA until a stable compound muscle action potential (CMAP) is recorded, at a minimum amplitude of 30 μV or until an upper limit of 30 mA is achieved without eliciting a CMAP [5,6].

An alternative strategy entails the use of Ojemann's stimulation or low-frequency stimula‐ tion, which consists of a 50–60 Hz train that is 3–5 seconds in length and has a pulse width as high as 0.5 ms [37,38].

Although a systematic comparison between both strategies remains to be performed, it is important to be aware that neither electrical thresholds nor muscle response or electrical safety are equivalent.

In this sense, it is important at this point to consider some electrophysiological variables concerning patient safety. The effect of any type of electrical stimulation over the neural tissue is mediated by the total amount of charge applied to the system and the duration of applica‐ tion [39]. The electric current (*i*, in mA) is defined as follows:

$$i\left(t\right) = \frac{dq}{dt}\tag{8}$$

where *q* is the charge (in μC) and *t* is time (in ms). Thus, the total charge applied during time

*tpw* (time of pulse width) can be calculated from Eq. (8) as *<sup>q</sup>*(*t*)= *∫* 0 *t pw i*(*t*)*dt*. For square pulses (which are the most common), the integral equals the amplitude × duration, e.g.:

$$q\left(t\right) = \mathbf{i} \times \mathbf{t}\_{\text{pw}}\tag{9}$$

However, this expression only provides information about the charge/pulse. Therefore, to elucidate the total charge administered to the tissue (*q*total), we must multiply by the number of pulses (*N*) as follows:

$$q\_{\text{total}} = \mathbf{i} \times \mathbf{t}\_{pw} \times \mathbf{N} \tag{10}$$

Another relevant feature concerning safety is the maximum charge density (*ρ*max), which is defined as *q*max/*A* (μC/cm2 /phase), where *A* is the area (usually in cm2 ). This parameter directly depends on the size and shape of the stimulation electrode.

A comparison of these magnitudes is provided in **Table 1** for three different stimulation paradigms.


HF, high frequency.

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

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

For this technique, electrodes are applied directly to the cortical/subcortical surface. Direct cortical stimulation (DCS) to identify the primary motor cortex (PMC) is accomplished using paired electrodes. Stimulation is performed using 4–6 pulse trains at 500 Hz (the reason we denote this paradigm high frequency; this technique is also known as multipulse, which is misleading), with biphasic pulses of 150–200 μs in the duration/phase. Motor evoked poten‐ tials are assessed using pairs of subdermal needles spaced approximately 2 cm apart that are inserted into the contralateral muscles, but surface electrodes attached to the skin can also be used. Depending on the location of the tumour, it is customary to use the following muscles: the orbicularis oculi, orbicularis oris, deltoid, brachial biceps, extensor digitorum carpal flexor, abductor pollicis brevis, abductor digiti minimi, quadriceps, tibialis anterior and abductor

Stimulation is initiated at 4 mA and increased continuously in increments of 1–2 mA until a stable compound muscle action potential (CMAP) is recorded, at a minimum amplitude of 30

An alternative strategy entails the use of Ojemann's stimulation or low-frequency stimula‐ tion, which consists of a 50–60 Hz train that is 3–5 seconds in length and has a pulse width as

Although a systematic comparison between both strategies remains to be performed, it is important to be aware that neither electrical thresholds nor muscle response or electrical safety

In this sense, it is important at this point to consider some electrophysiological variables concerning patient safety. The effect of any type of electrical stimulation over the neural tissue is mediated by the total amount of charge applied to the system and the duration of applica‐

μV or until an upper limit of 30 mA is achieved without eliciting a CMAP [5,6].

tion [39]. The electric current (*i*, in mA) is defined as follows:

located near the anterior horns.

218 Neurooncology - Newer Developments

the total charge supplied with longer pulses.

*2.3.4.2. Direct cortical stimulation (DCS)*

hallucis.

high as 0.5 ms [37,38].

are equivalent.

\*The surface is calculated from a 1.3 mm diameter disk electrode.

\*\*The surface is calculated from a 1 mm diameter spherical electrode, assuming that only 1/4th of the surface is in contact with the cortex.

**Table 1.** Comparison of magnitudes.

Although there are no well-defined limits forthe above-mentioned magnitudes, from the table we can observe that Ojemann's technique is the paradigm with the highest *q*total/train, and the stimulation for awake craniotomy has the highest *ρ*max.
