Electrocorticography

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Marc R. Nuwer and Inna Keselman

Electrocorticography (ECoG) is electroencephalography recorded in the operating room directly from exposed cerebral cortex. The goal for ECoG and direct cortical stimulation is separating regions recommended for resection and those that should be left intact. ECoG techniques used in the operating room resemble recording methods used in the epilepsy monitoring unit with implanted the grid and strip electrodes. In clinical practice ECoG is frequently used to identify cortical regions with epileptiform discharges during tumor and epilepsy surgery. ECoG recording during cortical stimulation searches for stimulus-evoked epileptiform activity termed after-discharges. Inhalation anesthesia agents such as desflurane, isoflurane, and sevoflurane, used during some ECoG recordings, may cause a rhythmic frontocentral 10- to 15-Hz activity. Low doses of the short-acting barbiturate methohexital may be used to activate epileptiform spike activity during ECoG and also can produce seizures.

anesthesia agents, barbiturate methohexital, direct cortical stimulation, electrocorticography, epilepsy monitoring unit, epilepsy surgery, operating room, seizures, stimulus-evoked epileptiform

Anesthetics, Barbiturates, Electrocorticography, Epilepsy, General Surgery, Methohexital, Operating Rooms, Seizures

Electrocorticography (ECoG) is EEG recorded in the operating room directly from exposed cerebral cortex. ECoG locates epileptic discharge sources prior to resection in surgery for epilepsy. This recording method also is used during surgical direct cortical stimulation studies for identifying eloquent cortex, that is, motor, language, and sensory cortex during tumor resection. The goal for ECoG and direct cortical stimulation is separating regions recommended for resection and those that should be left intact.

ECoG techniques used in the operating room resemble recording methods used in the epilepsy monitoring unit (EMU) with implanted grid and strip electrodes. In the EMU the electrodes are left in place for days or weeks awaiting epileptic seizure onsets. This chapter addresses ECoG as it is used in surgery.

364ECoG METHODS

Many different sizes of strips and grids include 1 × 8 strips and 4 × 5 grids (Figure 14.1), which are commonly used in operating room recordings. Larger 8 × 8 grids are used in the EMU for subdural recordings of epileptic seizures. In surgery, strips and grids can be moved around to different areas of exposed cortex. Sometimes more than one is used, such as a 4 × 5 grid plus a 1 × 8 strip (Figure 14.2). ECoG is recorded most often using strip and grid electrodes.

Some clinicians add acute surgically implanted depth electrodes to the ECoG, a technique referred to as stereo-EEG. This allows sampling of potential seizure activity from deeper structures of the brain including hippocampus and parahippocampal gyrus. When used acutely during surgery, these depth electrodes are removed at the end of recording, providing a much shorter recording time than for patients with chronically implanted depth electrodes in the EMU.

**FIGURE 14.1.** Grid electrodes are rows of 3 to 5 mm metal discs set in a silicon rubber sheet. Wires leading from each electrode are tethered together to form a ribbon cable leading to a multi-pin connector.

**FIGURE 14.2.** A 4 × 5 grid **(A)** is used here together with a 1 × 8 strip electrode **(B)**. Note the numbering on the grid and strip. An acute depth electrode also is implanted **(C)**.

*Source:* From Nuwer MR, Eliashiv E, Stern J. Intracranial EEG monitoring. In: Wasade V, Spanaki M, eds. *Understand Epilepsy*. Cambridge University Press; 2019:304–325. https://doi.org/10.1017/9781108754200.017

**FIGURE 14.3A.** The same electrocorticography (ECoG) recording shown in two different montages. A referential recording to a muscle reference. Twenty electrodes are shown at the top of the figure, as recorded from a 4 × 5 grid. The electrodes at the bottom of the figure are from a 1 × 8 strip.

**FIGURE 14.3B.** The same electrocorticography (ECoG) recording shown in two different montages. The same data as shown in **(A)**, redisplayed in a bipolar montage. The grid electrodes are arranged in the montage so that the five electrodes in a row are recorded in adjacent pairs.

Montages can be bipolar or referential. A bipolar montage connects each electrode to one or two of its immediate neighboring electrodes in the same row or column. A referential montage uses a neutral electrode applied to a muscle or scalp. ECoG amplitude is several-fold higher amplitude than scalp EEG, so referential recordings often are used. The voltage contribution to the recording obtained from a muscle or scalp reference electrode is substantially less than the voltage recorded from the grid or strip electrode on the brain’s surface (Figure 14.3).

366ECoG Interpretation

Intraoperative ECoG is used to localize epileptic cortex during tumor or surgical resection for drug-resistant epilepsy. Frontocentral fast activity, low-amplitude generalized slowing, and frontopolar triangular slow waves are seen during many types of anesthesia. Recordings above the Sylvian fissure would show more fast activity which is less evident when recording ECoG below the fissure (Figure 14.4).