Neurophysiology

lation or recording. However, if only lower extremity responses were lost in this case, the surgeon would be more suspicious of injury to the thoracic cord.

A large (n = 51,263), multicenter retrospective survey examining the role of SSEP monitoring in scoliosis surgery suggested a 50% reduction in the rate of neurological defects related to the procedure in patients who had intraoperative SSEP monitoring. The rate of false-negatives was remarkably low in this survey (0.06%), and the authors concluded that spinal SSEP was effective for detecting more than 90% of intraoperative neurological deficits [10]. Others have suggested that SSEPs may be useful in the assessment of compromising mechanical factors or decreases in relative blood flow to the spinal cord. A small study (n = 13) performed on patients with syringomyelia, treated with syringo-subarachnoid shunting, demonstrated a rapid improvement in spinal SSEPs following decompression of the syrinx. This improvement correlated with increased local blood flow to these regions, and the patients had postoperative improvement in their symptoms [11].

However, larger trials have not demonstrated similar consistency. Falsely positive SSEP changes are relatively common [10]. In a review of 182 cervical spine procedures, complete loss of evoked potential recordings occurred in 33 subjects and was associated with post-operative deficit in only 50% of these cases [12]. Partial loss of response was even less predictive, providing an overall specificity of only 27%. Further, false-negative recordings have been described. A large retrospective survey of nearly 190 spine surgeons who routinely used intraoperative SSEP monitoring found that nearly 30% of combined post-operative deficits seen in their patients occurred in the absence of observed spinal SSEP changes [13]. Confound-ingly, recordings may show improvement during a case without correlation to post-operative neurological improvement. Positive changes to SSEP waveforms may reassure the surgeon intraoperatively, while several studies have demonstrated that improvement of SSEP amplitude or latency appears to be of little postoperative clinical significance.

It has been suggested that the use of SSEP monitoring in spinal surgery may be augmented with the concurrent use of another monitoring technique (such as motor evoked potentials).

However, at this time there is no consensus as to the efficacy of isolated intraoperative spinal SSEP monitoring.

Monitoring techniques for surgery of the lumbosacral spine have also been reported. In an attempt to reduce the limited morbidity associated with lumbosacral diskectomy or pedicle screw fixation of the lumbosacral spine, some surgeons monitor nerve root function in the lower extremity during the procedure. Again, no clear efficacy has been demonstrated by controlled study.

Spinal Stimulation

Electrical stimulation of the spinal cord, both directly and indirectly, has been well described over the last decade as an additional method for monitoring the integrity of the descending tracts during surgical manipulation of the spine. The evoked motor responses, termed "neurogenic motor evoked potentials" (NMEPs), can be followed by recording from the sciatic nerve at the popliteal fossa bilaterally or by monitoring for myogenic responses in the lower limbs. The electrodes used to evoke NMEPs can be placed in several locations rostral to the region to be manipulated, including the epidural space, the spinous processes, or in a position that allows for percutaneous stimulation. Direct stimulation through pedicle screws has also been attempted as a means of assessing impingement upon, or damage to, nerve roots owing to misalignment of the hardware.

A recent study evaluated the efficacy of each of these electrode positions in 50 patients undergoing posterior thoracic or thoracolum-bar procedures with instrumentation. The findings demonstrated excellent results for each method; however, epidural placement of the stimulating electrodes was found to be most reliable in terms of the acquisition of initial NMEPs and in maintaining those NMEPs throughout the procedure [14]. The use of electrodes placed on the spinous processes or in the epidural space often requires enlargement of the surgical field and placement of the electrodes within the surgical field, which can result in some inconvenience.

A comparison of NMEP and SSEP was performed by Pereon et al. in a consecutive series of 112 patients undergoing surgical correction

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