*2.3.5. Stimulated electromyography (sEMG)*

For this technique, EMG is induced by an electrical shock that is delivered consciously, in contrast to the application described for fEMG. The pulses are usually delivered through bipolar (either concentric or parallel) or monopolar probes. In the latter case, it is important to use a cathodal current to stimulate the nerve and place the anode at a non-stimulated tissue.

The parameters must include pulses of a short length to avoid diffusion of the current. Normally, 50–100 μs should be adequate. The current must be maintained as low as possible to avoid injury to the neural tissue and diffusion. The latter effect must be considered in particular when a functional block is apparent because a high current or a long pulse (or both) can stimulate a region of the nerve distal in the damaged or blocked area and result in misleading information.

It is common to use sEMG in the next situation: (i) to explore a region to identify the proxim‐ ity of nerves that are not easily visible (e.g., when the CN runs inside a schwannoma); (ii) to determine the identity of a nerve; (iii) to evaluate the functionality of the nerve. We usually measure the threshold to elicit a muscularresponse at two different points (proximal and distal to the region at risk or more damaged by the surgery) and calculate the ratio of *i*distal/*i*proximal. The closer this ratio is to 1, the better the function of the nerve.

The settings used to record sEMG depend on the muscle being recorded, but it is quite common to use a 50–3000 Hz bandwidth, with the notch off, a gain consistently greater than 30 μV/div (typically 50 μV/div), and a time base between 1.5 and 4.5 ms/div.

#### **2.4. Cortical and subcortical surgery in awake patients**

In recent years, there has been a renewed interest in surgery in awake patients [38,40,41]. This procedure uses the asleep–awake–sleep anaesthetic technique, which consists of induction with propofol + sevoflurane and topical blocking with svedocain + lidocaine around the skin incision. During exploration, the patient must be awaken slowly by removing the sedation. In recent years, a new anaesthetic, dexmedetomidine, has been introduced forthis type of surgery and is considered the most effective [18].

Cortical stimulation is usually performed through a bipolar probe with ball-tips that are separated by 0.5 cm. It is common to use 60 Hz trains over 1–4 s. The pulse width is usually 1 ms, with a current intensity of 2.5–10 mA [42,43].

A low rate of intraoperative seizures has been reported (approximately 3–3.4%) [44], and some authors have concluded that control by ECoG is not mandatory [42]. However, this conclu‐ sion has been debated and remains to be validated.

In addition to this prevention, some authors have focussed on possible secondary effects derived from this technique. A normal human response to such an exceptional situation as awake craniotomy can, for instance, result in the delayed appearance of unintentional distressing recollections of the event or some type of post-traumatic stress disorder (as yet undescribed), despite the satisfaction of the patient concerning the procedure [45].

The limitations of awake surgery must be considered seriously. During such surgeries, the patient is awake with the head fixed and covered with cloth; and the patient may be kept awake for up to 2 h. Hence, patients must have both adequate cognitive function and the emotional maturity necessary to withstand such an environment. In fact, the Japan Society for Awake Surgery Guidelines limits the target patient population to those ranging from 15 to 65 years of age. Although with some limitations, awake craniotomy can be used in the paediatric population [50]. Nevertheless, use in mentally handicapped patients remains problematic or impossible.

However, no differences in the immediate postoperative motor status, extent of resection, or threshold intensity were found between IONM in anaesthetized patients and stimulation during awake craniotomy [46], although a detailed evaluation has not been performed for the different techniques or surgeries.

For selected patients, an awake craniotomy presents an option to reduce the risk of surgeryrelated neurological deficits, especially forlanguage mapping. However, the benefits and risks of this type of procedure should be carefully considered, and the decision should serve the interests of the patient.
