Dynamic Mapping of the Corticospinal Tract for Surgeries Beyond Tumor Resection
Key findings
- This paper reports the use of continuous corticospinal tract (CST) mapping during thoracic cordotomy (two patients) and myelomeningocele repair with cord untethering (one patient)
- There was a good correlation between transcranially evoked muscle motor evoked potentials monitoring, CST mapping and postoperative motor outcome
- The mapping thresholds varied, likely with the distance between the stimulating probe and CST fibers and also in direct relationship to the patient's motor function at baseline
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Resection of intramedullary spinal cord tumors carries an inherent risk of postoperative neurologic complications. Intraoperative neurophysiologic monitoring techniques help the surgeon locate the sensory and motor tracts and mitigate the risk of injury.
At Massachusetts General Hospital, mapping of the corticospinal tract (CST) has been used in other spinal cord procedures associated with a high risk of postoperative motor deficits. Neurologist Spyridoula Tsetsou, MD, and senior author Mirela V. Simon, MD, MSc, of the Department of Neurology and an expert in intraoperative neurophysiologic mapping techniques; neurosurgeon William Butler, MD, of the Department of Neurosurgery at Mass General, Mass General Cancer Center and MassGeneral for Children; and colleagues describe their techniques in the Journal of Clinical Neuroscience.
Three Cases
The journal article presents brief case reports on:
- Thoracic cordotomy to interrupt nociceptive pathways in a 49-year-old woman with intractable right leg pain due to metastatic disease of the sacrum and a 67-year-old woman with intractable right leg pain due to a thoraco-lumbar chordoma
- Cord untethering and surgical repair of a large thoracic myelomeningocele (T3–T7) in a 74-year-old man with progressive leg weakness
In all three cases, standard multimodality neurophysiologic testing was used intraoperatively: somatosensory evoked potentials, and transcranially evoked muscle motor evoked potentials (tcmMEPs) in addition to the newly described dynamic mapping of the CST.
Results
The article summarizes the specifics of motor mapping and monitoring and reports a good correlation between tcmMEP monitoring, CST mapping and postoperative motor outcome.
Given the absence of infiltrative pathology, and thus unlike the case of intramedullary tumor resection, the surgical approach during cordotomy and myelomeningocele repair allowed reliable appreciation of the location of the stimulation in relation to the location of the CST and/or dorsal columns. Stimulation at threshold further contributed to the selective mapping.
The mapping thresholds varied, with the differences seeming to depend on the distance between the stimulating probe and CST fibers and also on the patient's motor function at baseline.
Commentary
A cathodal monopolar multipulse train technique was used because:
- The radial current spread from the tip of a cathodal monopolar probe allows optimal depolarization of differently oriented motor nerves at the lowest thresholds, which increases the specificity of stimulation
- Multiple pulses trigger time-locked muscle motor evoked potentials (MEPs) that are easier to quantify and suitable for continuous mapping under general anesthesia
Whenever direct muscle MEPs were triggered, the stimulus intensity was decreased to an on/off response. Muscle MEPs were consistently triggered at very small currents, often isolated to one myotome distal to the spinal level where stimulation was taking place. This experience confirms reports by other groups that stimulation of isolated CST fascicles is possible.
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