**2. Neurophysiological techniques**

The possibility of using neurophysiological techniques to study the physiology of the NS is based on the way of function of the NS. Therefore, before introducing the techniques, we must understand, at least in a general manner, the function of this system for recording signals because all of the information that can be obtained and used clinically ultimately depends on both features.

#### **2.1. Some insight regarding the function and recordings of the nervous system**

expectancy [1,2]; second, a main goal of every surgery is to avoid introducing new iatrogen‐ ic lesions. The relative weight of every one of these principles can be changed based on individual considerations of the type of tumour, the structures affected, the life expectancy and even the social considerations of each patient. These features are particularly relevant to patients suffering from high-grade gliomas, for whom survival is directly related to the degree of tumour removal. Therefore, to maintain an adequate quality of life, the primary goal of

Central nervous system tumours are relatively common in adults; they are the second most common form of cancer and the most common type of solid tumour in children. Although more than half of these tumours are benign, they can cause substantial morbidity. The most common tumours in adolescents and adults aged 15–34 years are gliomas and meningiomas [3]. Glioblastoma multiforme (GBM) is the most common type of glioma. Meningiomas derive from meningothelial cells and comprise approximately 20 percent of primary brain tumours. GBM are more commonly located in the supratentorial region, with the frontal lobe being the

Advances in surgical techniques, such as intraoperative neurophysiological monitoring (IONM), intraoperative magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), stereotactic guidance and fluorescent-guided resection (FGR), have facilitated the delinea‐

Neurosurgery can be considered a radical method to treat some illnesses and can seriously damage the nervous system (NS). These injuries may not be apparent by visual inspection by the surgeon in the operating room but subsequently evolve into a definite lesion [7]. To avoid deleterious effects, such injuries can be detected during their initial development by IONM. Therefore, IONM is a powerful set of techniques that provide increased functional knowl‐ edge during a surgical operation, resulting in the safer removal of a radical tumour [8,9].

The operating room is an aggressive environment to perform recordings due to the presence of several sources of noise. Therefore, it is very important to identify the source of electro‐ magnetic noise and to determine how to manage it. Unfortunately, this subject is beyond the scope of this chapter, but we refer the reader to Pastor J, 2014, [10] for a detailed discussion of

In this chapter, we review the most relevant and frequently performed IONM techniques. We are especially interested in the clear and concise exposition of the methodological peculiari‐ ties, the fields of application and the flaws associated with the different techniques dis‐ cussed, with a focus on practical applications. Therefore, we show examples of real operations

The possibility of using neurophysiological techniques to study the physiology of the NS is based on the way of function of the NS. Therefore, before introducing the techniques, we must

tion of tumour borders and can aid in optimizing safe surgical resection [4–6].

surgery is to achieve GTR without compromising neurological function.

most common site [3].

208 Neurooncology - Newer Developments

these topics.

performed at our institution.

**2. Neurophysiological techniques**

The basic functional unit ofthe NS is the action potential (AP), which is the stereotypical change in the transmembrane voltage of a neuron [11–13]. In general, the AP originates in the neural soma or axon hillock and is transmitted by the axon to the synapse. All potentials originate from closed circuits of current [14], and the extracellular component can be recorded using the appropriate electrodes. In general, small metal electrodes are used to detect these currents.

Bioelectrical signals coming from the brain originate from synaptic currents in the cortex [13,14], or in deep nuclei of the thalamus or brainstem. These currents (clearly together with their intracellular components) form closed circuits that spread via volume conduction. The relationship between the current density (mA/cm2 ) and the electric field (*E* <sup>→</sup> , in V/cm) is given by the generalized Ohm's law:

$$
\vec{J} = \sigma \vec{E} \tag{1}
$$

where *σ* is the tensor of conductivity (mS/cm). Considering that conductivity is the inverse of resistivity, ρ (kΩ cm). In the electrostatic approach, the electric field can be expressed in terms of the electrostatic potential (∅, in V) by the following expression:

$$
\vec{E} = -\nabla \mathcal{Q} \tag{2}
$$

where ∇ is the symbol for the gradient operator.1 Substituting this expression into the first equation, we obtain the following:

$$
\vec{J} = -\sigma \nabla \mathcal{Q} \tag{3}
$$

which provides the current in terms of the potential. We want to highlight the presence of symbol σ. In the real head, conductivity depends on position (inhomogeneity) and direction (anisotropy). Therefore, it is not possible avoid the vectorial approach in Eq. (3) that can be written in the three spatial dimensions (*x, y, z*) as follows:

1 The gradient operator is the partial derivative for the space and is given by <sup>∇</sup> <sup>=</sup> <sup>∂</sup> <sup>∂</sup> *<sup>x</sup> i* <sup>→</sup> <sup>+</sup> <sup>∂</sup> <sup>∂</sup> *<sup>y</sup> j* <sup>→</sup> <sup>+</sup> <sup>∂</sup> <sup>∂</sup> *<sup>z</sup> k* →

$$
\begin{pmatrix} J\_x \\ J\_y \\ J\_z \end{pmatrix} = - \begin{bmatrix} \sigma\_{xx} & \sigma\_{xy} & \sigma\_{xz} \\ \sigma\_{yx} & \sigma\_{yy} & \sigma\_{yz} \\ \sigma\_{zx} & \sigma\_{zy} & \sigma\_{zz} \end{bmatrix} \begin{pmatrix} \frac{\partial \mathcal{Q}}{\partial x} \\ \frac{\partial \mathcal{Q}}{\partial y} \\ \frac{\partial \mathcal{Q}}{\partial z} \\ \frac{\partial \mathcal{Q}}{\partial z} \end{pmatrix} \tag{4}$$

This tensorial equation can be represented for every spatial dimension *x, y, z* as follows:

$$\vec{J}\_{l} = -\sigma\_{l\text{x}} \frac{\partial \mathcal{Q}}{\partial \mathbf{x}} \vec{i} - \sigma\_{l\text{y}} \frac{\partial \mathcal{Q}}{\partial \mathbf{y}} \vec{j} - \sigma\_{l\text{z}} \frac{\partial \mathcal{Q}}{\partial \mathbf{z}} \vec{k}; \mathbf{i} = \mathbf{x}, \mathbf{y}, \mathbf{z} \tag{5}$$

Hence, for the same voltage source, the current obtained depends on the conductivity of the different structures. Consequently, structures with higher resistivity, such as the skull and the skin, will only allow a lower current [14]. Similarly, if we recall that higher frequency oscillations of the cortex imply smaller synchronized regions, we can understand why frequencies above the beta band (13–30 Hz) are extremely difficult to record from the scalp. The necessity to record very small currents is the main reason why a low impedance is needed at the patient–electrode interface.

In general, two types of recordings can be distinguished in neurophysiology [15]: near-field and far-field potentials. These concepts are completely different from the same words used in electromagnetic theory. The generators of near-fieldpotentials are locatedin the cerebral cortex with limited spreading on the scalp. That is, we assume that the neurons responsible for the potential are in the immediate proximity of the region in which this potential is observed. However, far-field potentials originate from the deepest structures (white matter, basal ganglia or brainstem nuclei), and their distribution throughout the scalp is more extensive.

We can divide the neural response recorded by electrodes into three types according to the type of stimuli that induces the response in the neural tissue [13]: (i) electrically induced responses. We apply a controlled stimulation to activate different structures. Among these, we have all the types of evoked potentials or the response of a muscle after electrical stimula‐ tion of its innervating nerve (stimulated electromyography or sEMG); (ii) neural response by involuntary stimulation. Those responses appearing in the neural tissue after surgical aggression, e.g., mechanical compression or torsion, ischemia or heating induced by electro‐ coagulation, must be included in this group. A typical example of this is the neurotonic discharge of a muscle (free electromyography or fEMG) induced by stretching or overheat‐ ing of a cranial nerve (CN); (iii) physiological response of the NS. These are spontaneously induced responses that are intrinsically generated by the neural activity, either as a physio‐ logical or pathological expression of the activity, and can include electrocorticography (ECoG) or electrocardiography (EKG).

Neurophysiological techniques can be classified according to the manner by which the signals can be obtained, by means of a previous stimulation (a) or without that stimulus (b and c).
