The usefulness of electrical stimulation for assessing pedicle screw placements.

The purpose of this study was to further establish the efficacy of pedicle screw stimulation as a monitoring technique to avoid nerve root injury during screw placement. The study population consisted of 662 patients in whom 3,409 pedicle screws were placed and tested by electrical stimulation. If stimulation resulted in a myogenic response at a stimulation intensity of 10 mA or less, the placement of the screw was inspected. Inspection was necessary for 3.9% of the screw placements in 15.4% of the study population. None of the patients in the study experienced any new postoperative neurologic deficits. These findings provide guidelines for the interpretation of stimulation data and support the use of this technique as an easy, inexpensive, and quick method to reliably assess screw placements and protecting neurological function.

Spinally elicited peripheral nerve responses are sensory rather than motor.

OBJECTIVES: Spinally elicited peripheral nerve responses, commonly called neurogenic motor evoked potentials (NMEPs), are widely used to monitor spinal cord motor function during surgery. However, numerous evidence suggests that these responses are primarily sensory rather than motor. The collision technique was utilized to address this issue. METHODS: Collision studies were performed in 7 patients during surgery. An ascending volley of sensory (AS) and motor activity (AM) was elicited by posterior tibial nerve stimulation at the popliteal fossa. After a short time delay, high cervical spinal stimulation produced a descending volley of sensory (DS) and motor (DM) activity. The AM volley ascended only to the anterior horn cells whereas the AS and DS volleys collided in the spinal cord. The inter-stimulus delays were varied so as to affect the degree of spinal cord collision. The DS and DM activity which remained after collision was recorded from the posterior tibial nerves at the ankle. RESULTS: Inter-stimulus delays of 18 ms or less resulted in no apparent peripheral descending volleys. These findings were consistent for all the patients studied. CONCLUSIONS: Spinally elicited peripheral nerve responses are primarily sensory rather than motor and are mediated by the same neural pathways as SEPs.

The use of dermatomal evoked responses during surgical procedures that use intrapedicular fixation of the lumbosacral spine.

Monitoring of electrophysiologic function during intrapedicular fixation of the lumbosacral spine can be useful because this fixation technique has been associated with a significant number of postoperative radicular complications. Somatosensory evoked potentials (SEPs) traditionally have been used to monitor neurologic function during spinal instrumentation procedures. A case is presented of an intrapedicular fixation procedure that was monitored with SEPs and that resulted in false-negative SEP findings. This result suggests that SEPs may not be a sensitive enough monitoring tool for detecting compromise of single root function, and as a result, other monitoring techniques should be used. Dermatomal somatosensory evoked potentials (DSEPs) have been reported to be useful in this regard. To test their usefulness, 81 lumbosacral intrapedicular fixation procedures were monitored with DSEPs. Repeatable responses were obtained from all but one of the patients. The responses were sensitive to the compromise of root function. Predictions of postoperative outcome were dependent only on the responses at closing and not on changes that occurred during surgery.