Intraoperative neurophysiology of the conus medullaris and cauda equina

Intraoperative neurophysiological techniques are becoming routine tools for neurosurgical practice. Procedures affecting the lumbosacral nervous system are frequent in adult and pediatric neurosurgery. This review provides an overview of the techniques utilized in cauda and conus operations. Two basic methodologies of intraoperative neurophysiological testing are utilized during surgery in the lumbosacral spinal canal. Mapping techniques help identify functional neural structures, namely, nerve roots and their respective spinal levels. Monitoring is referred to as the technology to continuously assess the functional integrity of pathways and reflex circuits. For mapping direct electrical stimulation of a structure within the surgical field and recording at a distant site, usually a muscle is the most commonly used setup. Sensory nerve roots or spinal cord areas can be mapped by stimulation of a distant sensory nerve or skin area and recording from a structure within the surgical field. Continuous monitoring of the motor system is done with motor evoked potentials. These are evoked by transcranial electrical stimulation and recorded from lower extremity and sphincter muscles. Presence or absence of muscle responses are the monitored parameters. To monitor the sensory pathways, sensory potentials evoked by tibial, peroneal, or pudendal nerve stimulation and recorded from the dorsal columns with a spinal electrode or as cortical responses from scalp electrodes are used. Amplitudes and latencies of these responses are measured for interpretation. The bulbocavernosus reflex, with stimulation of the pudendal nerve and recording from the external anal sphincter, is used for continuous monitoring of the reflex circuitry. The presence of absence of this response is the pertinent parameter monitored. Stimulation of individual dorsal nerve roots is used to identify those segments that generate spastic activity and which may be cut during selective dorsal rhizotomy. Electromyographic activity can be continuously observed during surgery, and monitoring concepts developed in cranial nerve surgery may be used in the cauda equina as well. A range of intraoperative neurophysiological techniques are available for neurophysiological testing of the neural structures of conus medullaris and cauda equina.