Intraoperative Monitoring For Tethered Cord Surgery: An Update
Object: Intraoperative neurophysiological recording techniques have found increasing use in neurosurgical practice. The development of new recording techniques feasible while the patient receives a general anesthetic have improved their practical use in a similar way to the use of digital recording, documentation, and video technology. This review intends to provide an update on the techniques used and their validity.
Methods: Two principal methods are used for intraoperative neurophysiological testing during tethered cord release. Mapping identifies functional neural structures, namely nerve roots, and monitoring provides continuous information on the functional integrity of motor and sensory pathways as well as reflex circuitry. Mapping is performed mostly by using direct electrical stimulation of a structure within the surgical field and recording at a distant site, usually a muscle. Sensory mapping can also be performed with peripheral stimulation and recording within the surgical site. Monitoring of the motor system is achieved with motor evoked potentials. These are evoked by transcranial electrical stimulation and recorded from limb muscles and the external anal sphincter. The presence or absence of muscle responses are the parameters monitored. Sensory potentials evoked by tibial or pudendal nerve stimulation and recorded from the dorsal columns via an epidurally inserted electrode and/or from the scalp as cortical responses are used to access the integrity of sensory pathways. Amplitudes and latencies of these responses are then interpreted. The bulbocavernosus reflex, with stimulation of the pudendal nerve and recording of muscle responses in the external anal sphincter, is used for continuous monitoring of the reflex circuitry. Presence or absence of this response is the pertinent parameter that is monitored.
Conclusions: Intraoperative neurophysiology provides a wide and reliable set of techniques for intraoperative identification of neural structures and continuous monitoring of their functional integrity.

Intraoperative neurophysiological monitoring has long been performed by neurosurgeons during procedures to release the tethered spinal cord. Urological and electrophysiological techniques have been applied to monitor procedures involving the conus and the cauda equina: continuous recording of EMG activity in sphincter muscles, recording of CMAPs from leg and sphincter muscles to electrical stimulation in the operating field, direct SSEP recording from roots to electrical stimulation in the periphery, and measurement of sphincter tone and bladder pressure. The use of these techniques in the context of surgery for dysraphism has been presented in various formats in the past in updates by several authors.

During surgery for release of a tethered cord it may be necessary to cut the terminal filum, to dissect broad-based scar tissue that envelops the conus medullaris and/or the cauda equina nerve roots, to cut numerous fibrous bands that hold the conus rigidly in its position, or it may be necessary to resect partially a lipoma located at or within the conus. Thus, correct distinction between functional nervous tissue and fibrous bands is essential in these situations to avoid postoperative sensorimotor deficits and sphincter dysfunction. Direct stimulation of these structures in the surgical field or direct recording from them has improved this distinction beyond morphological recognition under the surgical microscope and reliance on the surgeon's experience. By using the mapping concept all functional neural structures of the lumbosacral region can be correctly identified and thus preserved.

During untethering procedures the conus or individual nerve roots may be inadvertently damaged by traction, compression, or coagulation. Many times this damage is reversible if detected early and if its cause is corrected. To detect such potentially reversible damage the functional integrity of the involved pathways has to be assessed continuously with monitoring.

Monitoring and mapping of the cauda equina and conus medullaris includes the simultaneous application of a number of modalities. It therefore requires the availability of a powerful multichannel recording system. All recordings in our institution are obtained with the Axon Sentinel-4 EP analyzer (Axon Systems Inc., Hauppauge, NY), which is equipped with dedicated software for controlling transcranial stimulation paradigms.