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Intraoperative neuromonitoring (IONM) is considered a standard-of-care practice in many surgical procedures for the treatment of spinal conditions, including tethered cord syndrome (TCS). Due to a failure during spinal cord development, tissue attachments, commonly a fatty filum terminale or lipoma, cause the spinal cord to be bound to the spinal column and stretched, which puts oxidative stress on neural structures in the conus medullaris and cauda equina region. Often presenting in children, TCS is a progressive condition where the stretching caused by tethering increases as the patient grows and manifests as neurological symptoms such as sensory and motor impairments in the lower limbs and bladder and bowel incontinence.
Typical treatment for TCS involves removing the tissue attachments, either by cutting the filum terminale or resecting the lipoma causing tethering. IONM plays a critical role in both mappings surrounding neural structures and ensuring that nerve integrity of the conus and cauda equina is not compromised while untethering the cord. A growing source of literature has shown that multimodality IONM with a combination of somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), electromyography (EMG), and bulbocavernosus reflex (BCR) is the most effective in maximizing sensitivity and specificity[1] [2] [3] [4].
SSEPs from pudendal nerve stimulation monitor the ascending sensory pathways from S2-3-4 roots in the spinal cord. Due to variable rates in sensitivity, it is highly recommended to only use SSEPs in conjunction with other modalities such as MEPs which report high sensitivity that can accommodate for SSEPs. MEPs recorded from the quadriceps, gastrocnemius, tibialis anterior, abductor hallucis, external anal sphincter, and external urinary sphincter help to confirm the integrity of the parapyramidal motor fibers, which is crucial for preserving motor control in the lower extremities and bladder and bowel control.
Spontaneous EMG can be added as an additional free-running indicator of nerve irritation that may precede potential injury using the same muscle set-up as MEPs. Triggered EMG is always used to confirm the integrity of the external anal and urinary sphincters. While monitoring from the anal sphincter may be sufficient to read the functional status of the pudendal nerve, the urinary sphincter is innervated by a different branch of the pudendal nerve than the anal sphincter. Thus its functional status cannot be confirmed solely by an intact anal sphincter response [5]. A technique growing in relevance for lower cord procedures is the recording of EMG responses from the urinary sphincter using electrodes embedded on Foley catheters which can offer more accuracy in preserving the motor status of the urinary sphincter. Triggered EMG can also map out the neural structures as fatty filum and lipomas will not record a neural response at supramaximal stimulation intensities. This guides the surgeon during resection and tethered cord release to avoid permanent postoperative deficits caused by the sectioning of neural structures.
Bladder and bowel incontinence is one of the condition’s deficits that significantly impact the quality of life, and that, without proper neuromonitoring, can also worsen after surgery. Hence, BCR monitoring is one of the most important IONM applications for TCS procedures as it monitors the functional integrity of both motor and sensory roots of S2-S4. BCR monitoring is based on the reflex response of the spinal muscles around the anal sphincter to dorsal penile or clitoral nerve stimulation. One of its strengths is that the BCR response does not habituate. Therefore, BCR testing can run continuously and without interference, allowing more consistent monitoring during surgical maneuvers. BCR responses are sensitive, allowing for better protection; however, as a polysynaptic reflex, this also makes it susceptible to anesthetic changes, one of the important considerations for BCR monitoring. Overall, BCR monitoring is a highly recommended inclusion for a multimodality IONM protocol as it is a sensitive and relatively stable response that can be run continuously and safely to monitor both motor and sensory function of the sacral nerves that are most at-risk during TCS procedures.
Variable spinal cord anatomy and fatty tissue obstruction are common issues navigating neural structures during tethered cord release procedures. To maximize resection for cord release while minimizing postoperative deficits, it is imperative to utilize IONM in a multimodality approach using pudendal SSEPs, MEPs, EMG, and BCR monitoring.
References:
- Sala, F., Squintani, G., Tramontano, V. et al. Intraoperative neurophysiology in tethered cord surgery: techniques and results. Childs Nerv Syst 29, 1611–1624 (2013). https://doi.org/10.1007/s00381-013-2188-3.
- Hoving EW, Haitsma E, Oude Ophuis CM, Journée HL. The value of intraoperative neurophysiological monitoring in tethered cord surgery. Childs Nerv Syst. 2011;27(9):1445-1452. doi:10.1007/s00381-011-1471-4.
- Dulfer, S.E., Drost, G., Lange, F. et al. Long-term evaluation of intraoperative neurophysiological monitoring-assisted tethered cord surgery. Childs Nerv Syst 33, 1985–1995 (2017). https://doi.org/10.1007/s00381-017-3478-y
- Udayakumaran S, Nair N, S, George M: Intraoperative Neuromonitoring for Tethered Cord Surgery in Infants: Challenges and Outcome. Pediatr Neurosurg 2021;56:501-510. doi: 10.1159/000518123.
- Jahangiri FR, Silverstein JW, Trausch C, Al Eissa S, George ZM, DeWal H, Tarasiewicz I. Motor Evoked Potential Recordings from the Urethral Sphincter Muscles (USMEPs) during Spine Surgeries. Neurodiagn J. 2019;59(1):34-44. doi: 10.1080/21646821.2019.1572375.
