**3.8 Invasive EEG studies**

In general, the indications to consider invasive EEG monitoring are as the followings:



**Figure 4.** *Clinical photo showing the setup for WADA test.*

#### **Figure 5.**

*Types of invasive EEG studies: (a) subdural strips and grids, (b) intracerebral depth electrodes, (c) insular depth electrodes insertion with neuro-navigation guidance, (d) foramen ovale electrodes, and (e) stereoelectroencephalography (SEEG).*


Traditionally, modalities of invasive EEG monitoring include subdural electrodes, intracerebral depth electrodes, epidural peg electrodes and foramen ovale electrodes. A comprehensive review on risks and benefits in using subdural and depth electrodes showed that the related complications include epidural or subdural haemorrhage, intracerebral haemorrhage or contusion, meningitis, oedema around electrode, cerebral oedema, increased intracranial pressure etc. The overall complication rate ranges from 0.4% to 6.6%.

Stereoelectroencephalography (SEEG) is getting its popularity to enable precise recordings from deep cortical areas in bilateral and multiple lobes without subjecting the patients to have bilateral large craniotomies. The key and most important concept in considering SEEG is to test individualized *anatomo-electro-clinical hypothesis*. Based on clinical history, semiology, preoperative imaging and vEEG data, the findings of SEEG help the clinicians to understand the spatial and temporal dynamics of seizure i.e. where it starts, when and when it spreads. Study from Italian group showed that SEEG is a useful and relatively safe tool to localize the epileptogenic zone with procedure-related morbidity 5.6%. Other centres incorporate the neuro-robotic system in performing SEEG and showed comparable results. In general, SEEG had equivalent efficiency in determination of epileptogenic zone with lower operative morbidities and complications including CSF leak and intracranial haemorrhage, and better tolerance to patients. Current application of EEG recordings is not only limited to scalp EEG and intracranial EEG with subdural electrodes and depth electrodes (**Figure 5**) [46–49].
