Mapping of the Language Cortex

Abstract

Awake craniotomy with intraoperative neurophysiological language mapping (INLM) is an established procedure for patients undergoing surgery to resection tumors in the language cortex area. INLM and continuous neurophysiological monitoring allow assessment of the language function, which is not possible under general anesthesia. INLM of the brain areas provides a helpful tool to the operating surgeon in reducing the risks associated with tumor resection in the motor and language cortex. We present a literature review and the technical method used for INLM by utilizing direct electrical cortical stimulation. We also report the usefulness of INLM for evaluation of the language function during resection of cortical tumors, epilepsy foci, and arteriovenous malformations (AVMs) located near language areas. First, the central sulcus is identified by sensory mapping, followed by the motor cortex's identification by direct electrical cortical stimulation (DECS). Neurological assessment of the patient is done by auditory and visual feedback. The patient is asked to repeat numbers, days, words, sentences, read words, and name pictures during cortical stimulation. DECS may cause a slurring or speech arrest. Electrocorticography (ECoG) is also performed during cortical stimulation to identify any after-discharges. Examination of the patient occurs immediately after surgery, and then 24 hours, one week, six months, and 12 months postoperatively. Bipolar DECS for motor mapping with ECoG can safely and reliably be utilized to identify essential language areas with minimizing permanent language deficits and maximizing the extent of tumor resection.

Keywords: broca’s; cortical mapping; eloquent area; ionm; language mapping; neuromonitoring; neurophysiology; tumors; wernicke’s.

Figure 1. Cortical grid placement over the exposed brain

Placement of a cortical grid (six contact 1 x 6) over the exposed area for sensory mapping phase reversal.

Figure 2. Cortical sensory mapping

Sensory mapping phase reversal. On-axis median nerve sensory mapping by a 2 x 4 grid with a triphasic phase reversal, including a P25 response. The phase reversal is between G1/2 and G5/6. The responses from G1 and G5 are pre-central, and G2, G3, G4, G6, G7, and G8 are post-central.

Figure 3. Taniguchi method

High-frequency motor mapping. Top left: motor mapping responses from right flexor carpi ulnaris and abductor pollicis brevis muscles. Top right: spontaneous electromyogram (EMG) recording from the contralateral muscles. Bottom: Electrocorticography (ECoG) recordings from a 1 x 8 grid.

Figure 4. Penfield method

Slow frequency motor mapping. Motor mapping responses from deltoid-biceps, triceps, and forearm (flexor carpi ulnaris and brachioradialis) and hand (abductor pollicis brevis-abductor digiti minimi muscles (blue arrows).

Figure 5. Motor cortex

After localization and verification of the exact location of the motor areas, the tumor was resected. The patient underwent continuous awake testing of the right upper and lower extremity motor function. Papers marked with 1, 2, 3, 4, and 5 represent various cortical functional areas.

Figure 6. Intraoperative neurophysiological language mapping (INLM)

Intraoperative recordings during a language mapping awake craniotomy. (A) Video input from the microscope showing the cortex. (B) Spontaneous electromyogram (EMG). (C) DECS - Direct electrical cortical stimulation. (D) Median nerve phase reversal (PR) window. (E) Electrocorticography - ECoG.

Figure 7. Intraoperative neurophysiological language mapping (INLM)

Language mapping. Intraoperative recordings during a language mapping awake craniotomy. (Left upper) Direct electrical cortical stimulation (DECS) with responses from orbicularis oris and tongue muscles. (Right upper) Spontaneous electromyogram (EMG). (Bottom) Electrocorticography (ECoG) with after discharges (ADs) present after direct cortical motor stimulation (orange arrows).