Cortical Mapping w/ Phase Reversal for Identification of the Central Sulcus
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For craniotomy procedures for resection of tumor or arterior venous malformation near the central sulcus, this structure must be continuously identified before entering the brain to confirm the integrity of sensory pathways and the corticospinal tract during resection. The role of IONM is to identify sensory and motor cortex after dural opening, assist in placement of the cortical strip electrode for dcMEP monitoring, monitor dcMEP and SEP during tumor resection, and estimate the distance to corticospinal tract during subcortical resection with white matter stimulation.
The necessary modalities are SEP, tcMEP, dcMEP, EMG (spontaneous and triggered) and EEG.
Cortical activity may trigger seizure activity, particularly in patients with epilepsy or seizures related to tumor. EEG should be recorded during cortical stimulation, and preparation for cold irrigation and/or pharmacological interventions by anesthesia confirmed before stimulation.
A recorded baseline of SEP, tcMEP and EEG prior to the craniotomy. Electrode placements may have to be adjusted from standard placement to accomodate craniotomy which may result in asymmetrical baselines which should be noted. After dural opening, identify central sulcus with SEP reversal by having the surgeon place electrode over precentral motor area and adjust location, contacts, and parameters to optimize dcMEP. Monitor SEP, dcMEP, and EEG during resection. If needed use electrified suction or monopolar stimulating probe to estimate distance to CST during subcortical resection.
SEP, tcMEP and EEG baselines are established before the craniotomy. After dural opening, connect 4-contact strip electrode to amplifier inputs, have the surgeon place the electrode as close as possible to presumed hand area of central sulcus, oriented perpendicular to sulcus roughly Anterior/Posterior. Check impedance and confirm all contacts are making good contact with the brain (placing a 1 x 5 cm pattie over the electrode may help.) Record both scalp SEP and cortical MEP from the strip (vs common reference) to stimulation of median or ulnar nerve contralateral of craniotomy. Note that the most distal contact of the strip electrode is #1, which may be the most anterior or most posterior depending on exact location oif craniotomy in relation to central sulcus. Look for major deflection at same latency as scalp N20. If all 4 contacts show a negarivitry, the electrode is too posterior. If all 4 show positivity, the electrode is too anterior. (Note that the central sulcus is often not the one initially hypothesized.) If no contacts show a response, ask surgeon to reposition electrode more medially or laterally until the hand area is located. When location is ideal, reposition electrode more medially or laterally until hand area is location. When location is ideal, a polarity reversal will be seen, with posterior contacts showing N20 negativity, and anterior contacts showing a positivity at a similar latency. Often the contact directly over the precentral motor gyrus will show a positivity at a slightly longer (~2ms) latency, due to activation of a second dipole. Once the central sulcus is identified replug the strip electrode into the stimulator box (begin with 1 & 4), and ask the surgeon to rotate electrode if possible so that all 4 contacts are over precentral motor cortex. Starting at 5mA, deliver single pulse trains (biphasic, 300 ms pw, 5 @ 2ms ISI) and adjust intensity until consistent dcMEPs are obtained, monitoring EEG for any seizure activity or after discharges, treat w/ cold irrigation or anesthetic intervention if necessary.
With ideal placement,
The necessary modalities are SEP, tcMEP, dcMEP, EMG (spontaneous and triggered) and EEG.
Cortical activity may trigger seizure activity, particularly in patients with epilepsy or seizures related to tumor. EEG should be recorded during cortical stimulation, and preparation for cold irrigation and/or pharmacological interventions by anesthesia confirmed before stimulation.
A recorded baseline of SEP, tcMEP and EEG prior to the craniotomy. Electrode placements may have to be adjusted from standard placement to accomodate craniotomy which may result in asymmetrical baselines which should be noted. After dural opening, identify central sulcus with SEP reversal by having the surgeon place electrode over precentral motor area and adjust location, contacts, and parameters to optimize dcMEP. Monitor SEP, dcMEP, and EEG during resection. If needed use electrified suction or monopolar stimulating probe to estimate distance to CST during subcortical resection.
SEP, tcMEP and EEG baselines are established before the craniotomy. After dural opening, connect 4-contact strip electrode to amplifier inputs, have the surgeon place the electrode as close as possible to presumed hand area of central sulcus, oriented perpendicular to sulcus roughly Anterior/Posterior. Check impedance and confirm all contacts are making good contact with the brain (placing a 1 x 5 cm pattie over the electrode may help.) Record both scalp SEP and cortical MEP from the strip (vs common reference) to stimulation of median or ulnar nerve contralateral of craniotomy. Note that the most distal contact of the strip electrode is #1, which may be the most anterior or most posterior depending on exact location oif craniotomy in relation to central sulcus. Look for major deflection at same latency as scalp N20. If all 4 contacts show a negarivitry, the electrode is too posterior. If all 4 show positivity, the electrode is too anterior. (Note that the central sulcus is often not the one initially hypothesized.) If no contacts show a response, ask surgeon to reposition electrode more medially or laterally until the hand area is located. When location is ideal, reposition electrode more medially or laterally until hand area is location. When location is ideal, a polarity reversal will be seen, with posterior contacts showing N20 negativity, and anterior contacts showing a positivity at a similar latency. Often the contact directly over the precentral motor gyrus will show a positivity at a slightly longer (~2ms) latency, due to activation of a second dipole. Once the central sulcus is identified replug the strip electrode into the stimulator box (begin with 1 & 4), and ask the surgeon to rotate electrode if possible so that all 4 contacts are over precentral motor cortex. Starting at 5mA, deliver single pulse trains (biphasic, 300 ms pw, 5 @ 2ms ISI) and adjust intensity until consistent dcMEPs are obtained, monitoring EEG for any seizure activity or after discharges, treat w/ cold irrigation or anesthetic intervention if necessary.
With ideal placement,