Spinal Cord Procedures

Tethered cord syndrome results from abnormal fixation of the spinal cord to surrounding structures, producing progressive neurological, urological, and orthopedic dysfunction from chronic cord traction. Surgical release requires precise identification of the tethering structure — whether a thickened filum terminale, fibrous band, lipoma, or adhesion — while preserving all functional nerve roots in the operative field.

IONM for tethered cord release centers on the lower sacral nerve roots and pelvic floor musculature, where the consequences of inadvertent injury are most clinically significant. The monitoring approach includes:

• Free-run EMG of bilateral lower extremity muscle groups and anal and urethral sphincter muscles provides continuous detection of nerve root irritation throughout dissection.
• Triggered EMG via direct nerve stimulation allows the surgeon to distinguish functional nerve roots from non-neural tethering structures before any tissue is divided — a critical step when anatomy is distorted by prior surgery, lipomatous infiltration, or congenital anomaly.
• Bulbocavernosus reflex (BCR) monitoring provides an objective measure of sacral arc integrity.
• Pudendal SSEPs assess the afferent limb of the sacral reflex arc and provide additional documentation of pelvic floor sensory pathway status. Lower extremity SSEPs and tcMEPs are included to monitor cord integrity throughout the procedure.

Selective dorsal rhizotomy is a neurosurgical procedure performed primarily in pediatric patients with spastic diplegia secondary to cerebral palsy, aimed at reducing lower extremity spasticity by selectively sectioning abnormally firing dorsal rootlets at the L1–S1 levels. The success of the procedure — and the avoidance of complications including hypotonia, sensory loss, and bladder dysfunction — depends entirely on accurate rootlet identification and selective sectioning guided by intraoperative neurophysiology.

Each dorsal rootlet is individually stimulated and its EMG response pattern evaluated across a comprehensive lower extremity and pelvic floor muscle recording montage. Rootlets producing abnormal, sustained, or spreading responses — indicative of pathological hyperexcitability — are selected for sectioning, while those producing normal, well-contained responses are preserved. Free-run EMG monitors for unintended motor root involvement throughout the procedure. BCR and anal sphincter EMG are monitored continuously to protect sacral roots governing bladder, bowel, and sexual function. Train of four confirms the complete absence of neuromuscular blockade, which is mandatory for valid rootlet grading throughout the procedure.

Spinal cord and spinal cord-tethering procedures

Spinal cord and spinal cord-tethering procedures represent some of the highest-stakes cases in neurological surgery. The proximity of surgical manipulation to the cord itself — and in many cases, within it — demands continuous, multimodal neuromonitoring throughout every phase of the procedure. IONM in these cases is not a supplementary precaution; it is an integral part of the surgical plan, providing real-time feedback that directly guides surgical decision-making from positioning through closure.

Spinal Cord Tumor Resection

Spinal cord tumors are classified by their relationship to the cord and its coverings. Intramedullary tumors arise within the cord parenchyma itself — most commonly ependymomas and astrocytomas — and their resection carries the highest neurological risk of any spinal procedure. Intradural extramedullary tumors, including schwannomas, meningiomas, and myxopapillary ependymomas of the conus and filum, are located within the dural sac but outside the cord and generally carry a more favorable monitoring profile, though nerve root preservation remains critical.

For intramedullary tumor resection, the full monitoring stack is employed:

• SSEPs provide continuous assessment of the dorsal columns throughout approach, myelotomy, and resection. tcMEPs monitor corticospinal tract integrity and are particularly sensitive to the vascular and mechanical insults that accompany intramedullary dissection. D-wave monitoring — where available and applicable — provides an additional, cortically independent measure of corticospinal tract integrity that is particularly valuable when tcMEP amplitudes are reduced by anesthetic or physiological factors. Free-run EMG monitors for nerve root irritation at the level of the tumor and adjacent segments.

For intradural extramedullary tumors, SSEPs and tcMEPs monitor cord integrity during retraction and manipulation, while free-run and triggered EMG guide nerve root identification and preservation — critical when tumor capsule and nerve root are adherent or anatomically distorted.

Tethered Cord Release

Selective Dorsal Rhizotomy

Lipomyelomeningocele Repair

Lipomyelomeningocele is a form of occult spinal dysraphism in which a lipomatous mass is continuous with the spinal cord, tethering it and producing progressive neurological deterioration if untreated. Surgical repair involves detethering the cord by resecting the lipomatous attachment while preserving all functional neural tissue — a distinction that is not reliably made on the basis of visual appearance alone.

Triggered EMG is the primary tool for distinguishing neural from non-neural tissue within the lipoma-cord interface prior to resection. Free-run EMG monitors the lower extremity and pelvic floor muscle groups continuously throughout dissection. BCR and pudendal SSEPs provide ongoing assessment of sacral arc integrity. Lower extremity SSEPs and tcMEPs monitor cord integrity throughout the procedure, with particular attention during the detethering maneuver itself.

Dermoid and Epidermoid Cyst Excision

Spinal dermoid and epidermoid cysts most commonly occur at the conus medullaris or within the cauda equina and frequently present with tethering of the cord or nerve roots. Their excision carries risk of injury to adherent neural structures, particularly when inflammation or prior rupture has obscured normal tissue planes.

SSEPs and tcMEPs monitor cord integrity throughout approach and resection. Free-run EMG provides continuous surveillance of nerve root irritation, and triggered EMG is used for nerve root identification when anatomy is distorted by cyst wall adhesion or inflammatory change.

Myelomeningocele Closure

Myelomeningocele closure — whether performed in the neonatal period or as a revision procedure — requires careful handling of the neural placode and preservation of any functional nerve roots incorporated within or adjacent to the malformation. IONM assists in identifying residual functional tissue within the placode prior to closure and monitors cord and nerve root integrity throughout the repair.



Free-run EMG of lower extremity and sphincter muscle groups is maintained continuously. Triggered EMG helps identify and preserve functional rootlets. BCR and pudendal SSEPs document sacral arc integrity at baseline and throughout the procedure.

Advanced Modality Quick Reference

• D-Waves: cortically independent corticospinal tract monitoring; particularly valuable during intramedullary resection

• Bulbocavernosus Reflex (BCR): Monitors sacral arc integrity; bladder, bowel, and sexual function protection

• Pudendal SSEPs: Afferent (sensory) sacral pathway assessment; pelvic floor sensory monitoring