Neurophysiology of Complex Spinal Cord Untethering

Intraoperative Neurophysiological Monitoring in Tethered Cord Syndrome Surgery: Predictive Values and Clinical Outcome

INTRODUCTION

"Tethered cord syndrome" (TCS) refers to a congenital abnormality associated with neurological signs and symptoms. The aim of surgery is to prevent or arrest their progression. This study reports a retrospective case series of tethered cord syndrome surgeries, supported by intraoperative neurophysiological monitoring.

METHODS

The case series comprises 50 surgeries for tethered cord syndrome in which multimodal intraoperative neurophysiological monitoring was performed using motor evoked potentials (transcranial motor evoked potentials [TcMEPs]), tibial nerve somatosensory evoked potentials (TNSEPs), and pudendal-anal reflex (PAR). The intraoperative neurophysiological monitoring results are reported and correlated with clinical outcomes.

RESULTS

Sensitivity, specificity, and negative predictive value were high for TcMEPs and TNSEPs, while PAR exhibited low sensitivity and positive predictive value but high specificity and negative predictive value. Fisher's exact test revealed a significant correlation between changes in TcMEPs, TNSEPs, and clinical outcome ( P < 0.000 and P = 0.049 respectively), but no correlation was detected between PAR and urinary/anal function ( P = 0.497).

CONCLUSIONS

While TcMEPs and TNSEPs were found to be reliable intraoperative neurophysiological monitoring parameters during tethered cord syndrome surgery, PAR had low sensitivity and positive predictive value probably because the reflex is not directly related to bladder function and because its multisynaptic pathway may be sensitive to anesthetics. New onset muscle weakness and sensory deficits were related to postoperative changes in TcMEPs and TNSEPs, whereas changes in PAR did not predict bladder/urinary impairment. Urinary deficits may be predicted and prevented with other neurophysiological techniques, such as the bladder-anal reflex.

Updates on Intraoperative Neurophysiology During Surgery for Spinal Dysraphism

Spinal dysraphism is a group of disorders resulting from an embryologic failure of spinal cord development which can lead to a radicular-medullary mechanical stretch that generates vascular compromise and hypoxic-ischemic damage to the nervous structures of the conus-cauda region.Thus, the clinical relevance of the different types of spinal dysraphism is related to the possible neurologic deficits resulting from spinal cord tethering. The clinical presentation is heterogenous: from asymptomatic to very compromised patients. The indications and the time of a detethering surgery are still subject of debate, although there is an agreement on the high standards of treatment that have to be offered by the surgery. Intraoperative neurophysiology (ION) contributes to the safety of tethered cord surgery in reducing the risks of iatrogenic neurological damages.

Not Just an Anchor: The Human Filum Terminale Contains Stretch Sensitive and Nociceptive Nerve Endings and Responds to Electrical Stimulation With Paraspinal Muscle Activation

BACKGROUND

Neural components of the fibrous filum terminale (FT) are well known but are considered as embryonic remnants without functionality.

OBJECTIVE

To investigate the ultrastructure of human FT specimens for sensory nerve endings and record paraspinal muscle activity on electrostimulation of the FT.

METHODS

We prospectively investigated a cohort of 53 patients who underwent excision of the FT for the treatment of tethered cord syndrome. Surgical FT specimens were investigated by light and transmission electron microscopy. Intraoperative electrophysiological routine monitoring was extended by recording paraspinal muscles above and below the laminotomy level.

RESULTS

Light microscopy revealed tiny peripheral nerves piercing the pia mater of the FT and entering its fibrous core. Transmission electron microscopy unveiled within the fibrous core of the FT myelinated nerve structures in 8 of the 53 patients and unmyelinated ones in 10 of the 53 patients. Both nerve endings encapsulated in fibrous tissue or unencapsulated nonmyelinated Schwann cell nerve bundles, that is, Remak cells, were found. Those nerve endings resembled mechanoreceptor and nociceptive receptor structures found in human skin, muscle tendons, and skeletal ligaments. Specifically, we found Ruffini mechanoreceptors and in addition nerve endings which resembled nociceptive glioneural structures of the skin. Bipolar electrostimulation of the FT was associated with paraspinal muscle activity above and below the spinal segment at which the FT was stimulated.

CONCLUSION

Morphological and electrophysiological results indicate the presence of functional sensory nerve endings in the FT. Like other spine ligaments, the FT may serve as a proprioceptive element but may also contribute to back pain in spine disorders.

Intraoperative neurophysiological monitoring in paediatric neurosurgery

Intraoperative neurophysiological monitoring (IONM) is commonly used in various surgical procedures in adults, but with technological and anaesthetic advancements, its use has extended to the paediatric population. The use of IONM in children poses a unique set of challenges considering the anatomical and physiological differences in this group of patients. The use of IONM aids in the localization of neural structures and enables surgeons to preserve the functional neural structures leading to decreased incidence of postoperative neurological deficits and better patient outcomes. In this article, we review the use of IONM in paediatric patients undergoing various spinal and cranial neurosurgical procedures. We discuss the patient characteristics, type of surgeries, and technical and anaesthetic considerations about IONM in this population.

Double neurophysiological certification of the filum terminale during sectioning surgery in pediatric population

Background:

Surgery of thickened-fibrolipoma filum terminale (FT) is performed routinely and without conflict but is not a risk-free surgical procedure. Intraoperative neurophysiological monitoring with mapping techniques can help to certify the FT before sectioning. However, a tailored surgical approach to cauda equina and a low threshold of surrounding nerve roots can confuse the final surgical decision. The aim is to demonstrate the usefulness of this double methodology for FT certification.

Methods:

A prospective study collected and reviewed retrospectively, from 2015 to 2018, 40 patients undergoing an FT surgery section were included in the study. After opening the dura mater and under the microscope, the cauda equina mapping is performed and the recording of muscles of the lower limbs and the external anal sphincter. In addition, a high-intensity stimulation of constant current of an isolated FT for a short period of time and in a dry surgical field, obtaining a bilateral-polyradicular-symmetrical response of cauda equina nerve roots.

Results:

Traditional motor mapping identified FT in 65% (26/40) of patients. Although, 35% (14/40) of the patients still have low-intensity stimuli response (<1 mA) of a muscle, especially anal sphincter. When this happens, the optimization of the dissection around FT is performed. After that, 25% (10/40) of the patients still having a muscle response in spite of seem isolated FT. Increasing the stimulation intensity up to 20 mA evoked a cauda equina response in all cases. No postoperative neurological impairment was observed in this series.

Conclusion:

This proposed methodology accurately confirms the FT so that it can be safely found and cut. The Double Neurophysiological Certification improves the gap of the traditional mapping techniques of cauda equina and can be used in a variety of more complex surgeries in this area.

Secondary tethered cord syndrome in adult patients: retethering rates, long-term clinical outcome, and the effect of intraoperative neuromonitoring

BACKGROUND

The strategy for surgical treatment of tethered cord syndrome in pediatric patients is well established but still bares challenges for adult patients. This retrospective study was performed to assess the surgical outcome of adult patients with a secondary tethered cord syndrome and to evaluate the benefit of intraoperative neuromonitoring.

METHODS

Clinical charts of 32 consecutive adult patients who underwent in total 38 surgical untethering procedures at our facility between 2008 and 2018 were retrospectively analyzed. Epidemiological data, MRI scans, and postoperative results were evaluated.

RESULTS

The retethering rate in our patient cohort was 16%. Main complaints were maximal pain (82%), bladder dysfunction (79%), paresthesia (68%), and weakness in the lower extremities (68%). Forty-eight months after surgery, patients' symptoms generally improved, with an average level of pain of 19.1% (95% CI, 5.7-32.5%), paresthesia 28.7% (95% CI, 12.6-44.8%), weakness in the lower extremities 27.7% (95% CI, 11.1-44.4%), and bladder dysfunction 60.2% (95% CI, 41.6-78.7%). The use of neuromonitoring appears to have a positive impact on patient weakness (OR = 0.07; 95% CI, 0.01-0.68) and paresthesia (OR = 0.03; 95% CI, 0.00-2.18). This benefit is less clear for the retethering rate (OR = 0.45; 95% CI, 0.06-3.26) or the overall clinical outcome (OR = 0.70; 95% CI, 0.14-3.45). The presence of a preoperative Chiari syndrome, syringomyelia, or scoliosis had no relevant influence on the retethering rate.

CONCLUSIONS

Our data confirms that untethering surgery in adult patients is relatively safe and has a reasonable chance of clinical improvement of pain, paresthesia, and weakness in the lower extremities. The use of intraoperative monitoring has a positive influence on the improvement of preoperative paralysis.

Effect of anesthesia on motor responses evoked by spinal neural prostheses during intraoperative procedures

OBJECTIVE

The overall goal of this study was to investigate the effects of various anesthetic protocols on the intraoperative responses to intraspinal microstimulation (ISMS). ISMS is a neuroprosthetic approach that targets the motor networks in the ventral horns of the spinal cord to restore function after spinal cord injury. In preclinical studies, ISMS in the lumbosacral enlargement produced standing and walking by activating networks controlling the hindlimb muscles. ISMS implants are placed surgically under anesthesia, and refinements in placement are made based on the evoked responses. Anesthesia can have a significant effect on the responses evoked by spinal neuroprostheses; therefore, in preparation for clinical testing of ISMS, we compared the evoked responses under a common clinical neurosurgical anesthetic protocol with those evoked under protocols commonly used in preclinical studies.

APPROACH

Experiments were conducted in seven pigs. An ISMS microelectrode array was implanted in the lumbar enlargement and responses to ISMS were measured under three anesthetic protocols: (1) isoflurane, an agent used pre-clinically and clinically, (2) total intravenous anesthesia (TIVA) with propofol as the main agent commonly used in clinical neurosurgical procedures, (3) TIVA with sodium pentobarbital, an anesthetic agent used mostly preclinically. Responses to ISMS were evaluated based on stimulation thresholds, movement kinematics, and joint torques. Motor evoked potentials (MEP) and plasma concentrations of propofol were also measured.

MAIN RESULTS

ISMS under propofol anesthesia produced large and functional responses that were not statistically different from those produced under pentobarbital anesthesia. Isoflurane, however, significantly suppressed the ISMS-evoked responses.

SIGNIFICANCE

This study demonstrated that the choice of anesthesia is critical for intraoperative assessments of motor responses evoked by spinal neuroprostheses. Propofol and pentobarbital anesthesia did not overly suppress the effects of ISMS; therefore, propofol is expected to be a suitable anesthetic agent for clinical intraoperative testing of an intraspinal neuroprosthetic system.