Anterior spinal cord injury with preserved neurogenic ‘motor’ evoked potentials
Introduction
Intraoperative spinal cord monitoring via evoked potentials is widely used as a means to reduce surgical injury and to provide reassurance to the surgeon and patient (Nuwer, 1999). Survey results of the Scoliosis Research Society are helpful in demonstrating a positive impact of somatosensory evoked potential (SEP) monitoring (Dawson et al., 1991, Nuwer et al., 1995). Despite this positive impact, major neurological deficits due to spinal cord injury may occur in the face of intact somatosensory signals (Lesser et al., 1986, Zornow et al., 1990, Mustain and Kendig, 1991, Ben-David et al., 1987b, Galla et al., 1999, Ginsburg et al., 1985). In the 1995 Scoliosis Research Society report by Nuwer et al., 51 263 patients undergoing major spinal cord surgeries were identified, 184 suffered definite and immediate postoperative deficits, and 150 of these injuries were predicted with intraoperative SEP monitoring (Nuwer et al., 1995). This study reflects a very low absolute incidence of false negative results (0.063%, 34 of 51 263), however, in the group that did suffer injuries, the rate of false negative monitoring was high (18.5%, 34 of 184). Therefore, SEP monitoring can detect most, but not all intraoperative mishaps, and there is a need to improve the sensitivity of intraoperative monitoring.
Some false negative results may be due to selective injury to motor pathways when only SEPs are used for monitoring. As a result, attempts have been made to introduce motor evoked potentials (MEPs) into the monitoring regimen. The most easily performed purported test of motor function has been termed ‘neurogenic motor evoked potentials (NMEPs)’ (Owen, 1993) or, more recently, ‘neurogenic mixed evoked potentials’ (Wilson-Holden et al., 2000). The use of these potentials in combination with SEPs has been advocated as a reliable standard of spinal cord monitoring (Wilson-Holden et al., 2000, Padberg et al., 1998, Péréon et al., 1998, Lubitz et al., 1999). However, there is controversy regarding whether sensory, motor, or a combination of these pathways are monitored (Piatt et al., 1984, Toleikis et al., 2000, Leppanen et al., 1998, Owen, 1999, Péréon et al., 1998). Few would still claim that these signals are exclusively motor, but it is implied that these signals hold an advantage over SEPs in gaining insight into anterior spinal cord function. Given the controversy as to the neural pathways assessed by what others call NMEPs, we use the neutral term coined by Toleikis et al: spinally-elicited peripheral nerve responses (SEPNRs) in its place (Toleikis et al., 2000). We report two cases of preserved SEPNR and SEP signals in patients who demonstrated an immediate postoperative deficit, preserved posterior column function on clinical exam, and persistence of both of these evoked potential modalities after discovery of paraplegia.
Section snippets
Methods
Stimulation and signal averaging were accomplished with the Nicolet Viking III. All recording and peripheral nerve stimulating electrodes were subdermal needle EEG electrodes (Xomed, 12 mm stainless steel). A reference ground electrode was placed on one shoulder. Nerves and the spinal cord were individually stimulated using the parameters listed below.
Patient #1
This 40-year-old woman had a long history of rheumatoid arthritis and kyphoscoliosis. Prior surgical history included an uneventful C1–2 fusion and a T2–L1 spinal fusion complicated by recurrent infection and requiring removal of instrumentation. Subsequently, she noted progressive spinal deformity, multiple compression fractures, and back pain, prompting the current surgery. A preoperative neurologic exam showed marked weakness of the right extensor hallucis longus and sensory loss in the
Discussion
SEPs have traditionally been the mainstay of intraoperative neurophysiological monitoring. This is despite the fact that SEPs only evaluate dorsal column sensory function (Greenberg et al., 1987) and selective injuries to the posterior or anterior spinal cord may occur (Ben-David et al., 1987a, Ben-David et al., 1987b, Kalkman et al., 1994). In addition, the anterior spinal cord may be preferentially vulnerable, especially in the setting of ischemic injury (Zornow et al., 1990, Bennett, 1983,
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