Dorsal column mapping is a technique the neurophysiologist can use to help identify structures in the spinal cord during intramedullary tumor resection. Intramedullary tumors arise from cells within the spinal cord and their location can cause disruption of the anatomy of the spine. The surgeon will perform a laminectomy, which removes the lamina or back portion of the vertebra that covers the spinal cord. The surgeon will also open up the dura, which is the outermost membrane that covers the spinal cord. Resection of the tumor requires a myelotomy, meaning the surgeon will have to cut into the spinal cord to reach the tumor.
Removal of these tumors while preserving function of the sensory pathways in the dorsal portion of the spinal cord can be difficult. Once the surgeon has accessed the spinal cord, the ideal place to make an incision is along the physiologically inert midline. Incision made away from this midline puts the sensory pathways in the dorsal spinal cord at risk. Several methods have been developed to assist the neurophysiologist with the mapping of the dorsal spinal cord.
Use a Grid Electrode
The first method of dorsal column mapping involves the use of a grid electrode, which is placed on the dorsal spinal cord. The tibial nerves are individually stimulated and the somatosensory evoked potentials (SSEP) are recorded at the grid electrode, creating an amplitude gradient. The largest amplitude responses obtained will be located lateral to the physiologically inert midline, creating the ideal location for the myelotomy to be performed. As these responses are recorded closer to the midline, amplitude differentiation can become more difficult.
Anterograde Stimulation of the Spinal Cord
The next method of dorsal column mapping involves anterograde stimulation of the spinal cord that is recorded at the scalp. The SSEP scalp electrodes, particularly C3’ and C4’, are necessary for this method’s success. The surgeon begins approximately 5mm lateral to the suspected midline and stimulates the dorsal columns on this side. As the surgeon moves closer to the midline, there will be an area that does not respond to either left or right dorsal column stimulation. Once the surgeon passes this area, the polarity of the response will flip. The area of the spinal cord that is not activated by stimulation and located between the polarity flip will represent the physiological midline and safest place to perform the myelotomy.
Retrograde Dorsal Column Mapping
Dorsal column mapping can also be completed retrograde, where the dorsal columns are stimulated and recordings are made at a peripheral nerve such as the median or tibial nerve. Similar to the anterograde stimulation method, the surgeon will begin approximately 5mm lateral to the likely midline and stimulate the dorsal column on this side. The surgeon will move incrementally towards the midline until there is no response recorded in the peripheral nerve. This process is completed on the other side as well, marking the area that does not respond to stimulation as the midline.
A study of 91 patients who had intramedullary tumors showed that dorsal column mapping was not performed for 80 of the patients while it was performed for the remaining 11. Postoperative dorsal column dysfunction was found in 50% of the patients where dorsal column mapping was performed compared to 9% of patients with dysfunction when the mapping was performed (Mehta et al., 2012). The use of dorsal column mapping, particularly when used in conjunction with somatosensory evoked potentials (SSEP), motor evoked potentials (MEP) and D-wave recordings, can lead to improved patient outcomes with less post-operative dorsal column insult and sensory deficits.
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