Operative Planning

• Intracarotid sodium amytal testing (Wada test) may be indicated to determine the hemisphere of language dominance
  
• Preoperative object naming, at 4 sec per image, must be better than 75%
  
• Motor mapping requires normal, or near normal, power preoperatively
  
• Somatosensory mapping requires normal, or near normal, sensation preoperatively
  
• Anticonvulsants should be administered, with therapeutic levels achieved

Equipment

• Major craniotomy tray
  
• Grass CE-1 electrode holder (Grass Technologies, West Warwick, RI) and cortical electrodes, for language mapping
  
• Electroencephalography machine, for language mapping
  
• Eight-contact strip electrode, with cable and connector for somatosensory evoked potentials (SSEPs)
  
• SSEP machine, for SSEP monitoring
  
• Ojemann Cortical Stimulator
  
• Fifteen to 30 small (3 to 5 mm) paper numbered tickets
  
• Brain diagram for drawing electrode montage on the brain
  
• Slide projector (or computer slide show) with 50 to 100 object drawings, presented at a rate of one object every 3 or 4 seconds, depending on the patient’s verbal ability
  
• Iced irrigation fluid

Operating Room Set-up

• As for major craniotomy

Anesthetic Issues

Positioning

• Awake patients must have the head in lateral, or near lateral, position
  
• Mayfield pin head holder is applied using local anesthesia for awake cases

Electrode Placement

• Three electrodes are placed on the neck as the reference electrode for electroencephalogram EEG monitoring (averaged neck reference)
  
• Median nerve or tibial nerve stimulating electrodes are placed contralateral to the hemisphere where SSEP testing is to be performed

Craniotomy

• As per standard temporal or frontal craniotomies, but the opening must provide access to all areas to be mapped

Somatosensory Evoked Potentials Mapping

• Can be performed in awake or asleep patients
  
• Place 8–contact strip electrode in transverse (axial) orientation, traversing the presumed central sulcus
  
• For bipolar montage, note phase reversal to identify somatosensory cortex
  
• For median nerve referential montage, note N20 (somatosensory gyrus) and P22 (motor gyrus)
  
• Move strip electrode and repeat procedure to verify accuracy
  
• Because the brain is not being directly stimulated, seizures cannot be evoked with SSEP mapping

Cortical Stimulation Motor Mapping

Cortical Stimulation Sensory Mapping

• Requires an awake patient
  
• Can be performed with or without concomitant electrocorticography (ECoG, see later)
  
• When using ECoG, stimulate with increasingly higher currents until ADs are noted. Use a current 1 to 2 mA below the AD threshold for mapping (see later).
  
• When not using ecog, begin stimulation mapping at 2 mA and increase the current by 1 to 2 mA until sensation is noted by the patient.
  
• Stimulate the cortex with bipolar stimulator for 2 to 3 seconds, having the patient report after each stimulation epoch any sensation in the contralateral body.
  
• Avoid stimulating the same area repeatedly without a pause to prevent seizures.
  
• If a seizure occurs, irrigate the brain with iced irrigation solution; if the seizure continues, administer midazolam intravenously in 2 mg doses.

Language Mapping

Electrocorticography

• The patient must be awake and cooperative for language mapping.
  
• ECoG is performed by attaching the CE-1 electrode holder to the skull, using an epidural skull clamp or a skull clamp that screws directly into the skull.
  
• Cortical electrodes are placed ~2 to 3 cm apart, covering the area to be mapped.
  
• Electrode positions are drawn on the brain diagram, which is passed off to the EEG team for EEG montage creation (typically arranged anterior to posterior and superior to inferior).
  
• Numbered tickets are placed ~1 cm apart, covering the area to be mapped.

After Discharge Threshold Determination

• Using the bipolar stimulator, beginning at 2 mA current, stimulate 3 to 5 areas of the brain region to be mapped, calling out the nearest cortical electrode to the EEG team to record.
  
• Watch the EEG for ADs: if none, increase the current by 2 mA increments until ADs are elicited (this is the AD threshold); if persistent ADs occur, irrigate the brain with cold irrigation fluid.
  
• A current 1 to 2 mA below the AD threshold is used for mapping.

• While the patient is naming objects presented for a 3- to 4-second duration, current is applied to the cortex prior to display of the object, continuing until the object is named correctly or the next object appears.
  
• The numbered ticket closest to the area stimulated is called out following stimulation and recorded.
  
• Object naming errors define the areas of essential language cortex; this may be evidenced by complete speech arrest, significantly slowed speech, or para-phasic errors.
  
• Avoid stimulating the same area repeatedly without a pause to prevent seizures.
  
• If a seizure occurs, irrigate the brain with iced irrigation solution; if the seizure continues, administer midazolam intravenously in 2 mg doses.