9.1.1 Primary Concepts

Neurosurgical resections are done with two goals: (1) optimization of resection extent and (2) minimization of deficit extent, especially when eloquent cortex is at risk. ¹ For epilepsy surgery, functional mapping allows the surgeon to maximize resection of the epileptogenic zone in order to eliminate seizures while minimizing the risk of functional loss. Surgical tumor resection, however, is done to increase survival. The purpose of functional mapping is to optimize the postoperative quality of life. ²

9.1.2 Justification

Functional zones can be distorted or topographically obscured by a lesion or its associated edema. The epileptic zone or intra-axial tumor may be in eloquent cortex. Congenital abnormalities may obliterate conventional anatomic landmarks and the location, duplication, and anatomic extent of eloquent cortex is variable. ³ Also, lesion location and a patient’s age at lesion onset leads to varied degrees of brain plasticity.

The focus of this chapter is the review of established functional mapping techniques such as ESM and somatosensory evoked potential (SSEP). However, emerging techniques and their clinical relevance will also be discussed.

9.5.1 Medication and Anesthetic Considerations

Twenty mg/kg phenytoin or one gram of levetiracetam for parenteral anticonvulsant prophylaxis is given before surgery. Reduced doses of mannitol (0.5 g/kg of 20% mannitol) are recommended, since higher doses are associated with nausea and vomiting when the patient is awake. The most frequent strategy for awake craniotomy is the “asleep-awake-asleep” method. Rapidly reversible intravenous anesthesia, usually Propofol, is used initially. Dexmedetomidine is also commonly used. Inhalation anesthetics that suppress EEG signal and increase the latency or reduce the amplitude of SSEPs should be avoided.

9.5.2 Electrode and Stimulator Considerations

To perform ESM, electrodes are placed on the cortex and connected via a switch box to a cortical stimulator that delivers biphasic pulsed electrical charge to a pair of electrodes (▶ Fig. 9.1). For chronic mapping, cortical electrode grids composed of platinum-iridium discs (4 mm in diameter with 2–3 mm exposed) that are spaced 10 mm center-to-center and embedded in silicone to form grids and strips of various sizes (4 × 5 or 8 × 8 electrode arrays) are placed. Grids can be trimmed to match the shape of the craniotomy or are preshaped to fit specific areas (for example, 35-contact grid in a dedicated 4 × 3 mesial flap). Strips with four to twelve contacts provide cortical coverage beyond the extent of the craniotomy. Depth electrodes (composed of 2- to 5-mm-long cylindrical platinum-iridium electrodes with a diameter of 1 mm and spaced 5–10 mm apart) may also be used. Intraoperatively, a handheld Ojemann probe, which has a pair of ball electrodes mounted 5 mm apart, is used to perform ESM throughout the awake resection process.

9.5.3 Stimulation Paradigms and Techniques

Extraoperative, chronic ESM is indicated for epilepsy surgery on candidates who have undergone implantation of subdural grid/strip electrodes or stereo-electroencephalographic (SEEG) depth arrays in or near eloquent cortex. This mapping is done in a monitoring unit. With SEEG stimulation mapping of conventionally inaccessible cortical regions such as the insula, ventral, and medial cortex is possible. The disadvantage of SEEG mapping is that it has a limited sampling size compared to subdural grid coverage.

Epilepsy evaluations require electrocorticography (ECoG) to monitor for seizures and stimulus-induced after-discharges that may summate to seizures. Modern stimulator and switching boxes are often integrated into existing EEG video monitoring systems and provide an intuitive graphical user interface. Intraoperative ESM may be done at the time of tumor or epileptic focus resection. Grid, strip, depth or wand electrode interfaces can be used. Differences in cortical physiology, varied disc/sphere electrode configurations and electrode diameters, inter-electrode distance choices and current shunting through cerebrospinal fluid result in a wide variety of stimulation protocols. (▶ Fig. 9.1). ⁷ Intraoperative ESM may be done on an anesthetized patient to measure motor responses, but patients need to be awake for somatosensory, motor, or linguistic study. Cooperation or a subjective report is needed. Intraoperarative ESM provides localization guidance for both cortical grey matter and subcortical fiber tracts, as both may be stimulated. This “hodotopic” view considers both nodes and networks. It is superior to other available techniques in preserving function as has been shown in many large surgical studies, particularly in patient sundergoing glioma resection. ^{8 , 9}

Both positive responses (regional movement, dysesthesia, phosphenes) and negative responses (motor inhibition, speech arrest, anomia) may occur with stimulation. The effect of ESM is a complex amalgam of neuronal excitation and inhibition, interneuron and local fiber tract involvement. ¹⁰ Distinctions have been codified between eloquent cortex that is obtainable and eloquent cortex that is indispensable. ¹¹ Indispensable cortex refers to primary motor cortex/pyramidal tracts, primary sensory cortex, primary visual cortex, Broca and Wernicke’s regions, and the arcuate fasciculus. Indispensable regions are to be preserved in any resection strategy if possible. Basal temporal language or fusiform gyrus face recognition areas may be mapped. However, absolute avoidance is not necessary. These regions can be resected without significant morbidity.