Split Cord Malformation and Neuromonitoring

Split cord malformation (SCM, or diastematomyelia) is a rare congenital defect in…

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Split cord malformation (SCM, or diastematomyelia) is a rare congenital defect in which the neural tube, often in the lower thoracic or lumbar segments, improperly develops to form two separate “hemicords” either in two different spinal canals (Type I SCM) or co-existing in one single spinal canal (Type II SCM) [1]. SCM can present with other forms of spinal dysraphism such as tethered cord syndrome, myelomeningoceles, fatty filum, and lipomas and similarly produces stretch-induced traction on the spinal cord. Symptoms and clinical presentations of SCM also mirror that of other spinal dysraphisms such as lower extremity pain and weakness, incontinence, sexual dysfunction, and lesions. Most surgical treatments for SCM symptom management focus on untethering the cord by filum, bony septum, or tissue resection. Reviews of SCM cases have found significant improvement in patient symptoms postoperatively after surgical untethering treatment [2].
While a rare form of spinal deformity, the presence of SCM can complicate spinal anatomy and physiology, with literature findings noting it as one of two categories with the most common occurrence of irreversible postoperative deficits[3]  after untethering procedures [3]. As a high-risk group, SCM cases can greatly benefit from the addition of intraoperative neurophysiological monitoring (IONM). Typical IONM protocols [4,5] involve a multimodality approach consisting of somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), electromyography (EMG), train of four (TOF), and bulbocavernosus reflex (BCR). As in standard tetheredcord surgeries, evoked potential modalities help to confirm neural integrity of the motor and sensory spinal columns, while EMG provides an instantaneous feedback measure of nerve irritation. Whereas BCR provides feedback regarding the pudendal nerve integrity. Additional urinary bladder EMG and MEP have recently been reported as additional modalities to minimize postoperative neurogenic bladder [4,5].
Specifically, in SCM untethering, accurate identification of bony or fibrous septa is critical for maximal resection while sparing surrounding neural tissue. Vascular anatomy in the surrounding area creates another challenge for surgeons. While vessels supplying the septum must also be resected, any disruption in essential vasculature can temporarily or permanently affect the functional nervous tissue. Changes in evoked potentials can be used as indicators of either damage or ischemia to important neural structures, allowing for safe resection of tethering structures to minimize interruption of neural function postoperatively.
Triggered EMG is a valuable tool in mapping the surgical site and distinguishing between functional neural tissue and non-neural structures. SCMs often accompany other spinal pathologies such as scoliosis, spinal bifida, and cysts, further complicating spinal anatomy. A common source of tethering, fatty filum terminale can be differentiated from nerve roots by using triggered EMG stimulation when the conus medullaris is implicated, and filum resection is necessary after septum removal. EMG stimulation can also be used to determine neural functional status and identify non-functional and aberrant nerves, which may also contribute to tethering, allowing the surgeon to mark which roots are safe for rhizotomy if necessary.
As outlined in this particular case report, a loss in motor evoked potential led a surgeon to halt the procedure and avoid potential consequences of irreversible paraplegia. Postoperative MRIs after the cancellation to investigate the cause of MEP loss resulted in new findings that shaped further treatment plans. After following surgeries, the patient had a successful outcome with no postoperative deficits [6]. As SCM cases are complicated due to comorbidities and rarity, IONM aids the surgeon in making better-informed decisions about the course of surgical treatments for the maximal benefit of the patient.
References:
  1. Bo Xiu, Fuyun Liu, Aijia Shang, Rui Zhang. Chinese expert consensus on diagnosis and management of split cord malformation. Journal of Neurorestoratology, 8(2), 2020, Pages 83-92. doi.org/10.26599/JNR.2020.9040010.
  1. D'Agostino EN, Calnan DR, Makler VI, Khan I, Kanter JH, Bauer DF. Type I split cord malformation and tethered cord syndrome in an adult patient: A case report and literature review. Surg Neurol Int. 2019 Jun 7;10:90. doi: 10.25259/SNI-66-2019.
  1. Hoving, E.W., Haitsma, E., Oude Ophuis, C.M.C. et al. The value of intraoperative neurophysiological monitoring in tethered cord surgery. Childs Nerv Syst 27, 1445–1452 (2011). https://doi.org/10.1007/s00381-011-1471-4
  1. 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.
  1. Jahangiri FR, Asdi RA, Tarasiewicz I, Azzubi M. Intraoperative Triggered Electromyography Recordings from the External Urethral Sphincter Muscles During Spine Surgeries. Cureus. 2019 Jun 10;11(6):e4867. doi: 10.7759/cureus.4867.
  1. Jahangiri FR, Sayegh SA, Azzubi M, Alrajhi AM, Annaim MM, Al Sharif SA, Aziz T, Al Eissa S. Benefit of Intraoperative Neurophysiological Monitoring in a Pediatric Patient with Spinal Dysmorphism, Split Cord Malformation, and Scoliosis. Neurodiagn J. 2017;57(4):295-307. doi: 10.1080/21646821.2017.1396780.