*2.2.2.1. EEG*

Recording electrodes are placed onto the skin of the scalp. The most common types are subdermal stainless steel needles that are placed inside the skin or cork-screw electrodes, which are more easily fixed to the patient but are clearly also more aggressive. Subdermal electrodes allow low impedance (less than 5 kΩ is fine), which is important for acquiring a good recording. We can use the International System 10–20 [16] to position the electrodes, but it is more common to use a reduced version of this system.

EEG can be used to monitor the state of the cerebral cortex, and the main indications in IONM are blood flow alterations and epileptic activity. Bioelectrical activity directly depends on blood flow, and a reduction of this variable will be observed as a slowing of the brain activity denoted by the appearance of theta/delta activity [17]. Epileptic activity can appear after a perfusion alteration or as a consequence of an insult to the cortex (mechanical, chemical or electrical). Considering that high-voltage electrical stimulation is common during IONM, EEG should be used in all patients with an increased risk of epileptic seizure. Similarly, we must be aware that general anaesthetics can increase the likelihood of seizures [18]. Customarily, some degree of quantification should be useful.

The bandwidth filter should be at least 0.5–30 Hz, with the notch on. For this type of record‐ ing, higher frequencies are uncommon in the presence of anaesthesia. The gain must be set between 7 and 15 μV/div and the time base at approximately 15–30 mm/s.
