Electrophysiologically guided untethering of secondary tethered spinal cord syndrome
Authors: Pouratian Nader , Elias W. Jeff , Jane John A. , Phillips Lawrence H. , Jane John A. 
Published: July 2010

Object
Many patients develop neurological symptoms related to spinal cord tethering after perinatal repair of myelomeningocele. This is referred to as secondary tethered cord syndrome (STCS). The authors describe their methodology and evaluate the intraoperative utility and postoperative outcomes of electrophysiologically guided untethering for STCS. In addition, the authors describe the use of electrophysiological guidance to identify an “autonomous placode” in the untethering of the cord in STCS.

Methods
The authors retrospectively identified 46 untethering procedures in 38 patients who had undergone perinatal myelomeningocele repair and in whom the index surgery was for tethered cord release at the site of the repair. In all cases, both passive (electromyography) and active (detection of compound muscle action potentials) electrophysiological monitoring was used. The proximity to neural elements was determined based on the current used; eliciting compound muscle action potentials with a ≤ 10-mA stimulation was assumed to represent direct neural stimulation. Clinical records were reviewed to evaluate the utility of electrophysiological guidance and patient outcomes.

Results
The median age at the time of untethering was 9.5 years (range 0.5–54 years). The median follow-up time was 42 months (range 3–172 months). Progressive bowel and bladder dysfunction, diagnosed either clinically or by cystometrogram, and low-back pain were the most common presenting symptoms. Intraoperative findings indicated that the most common causes of tethering were dense scar (76%) and a tethered placode (39%). Electrophysiological monitoring identified functional neural tissue near tethered elements and provided intraoperative guidance in all cases. In 41% of cases (19 cases), the untethering plan was noted to have been significantly influenced by intraoperative neurophysiological findings. Moreover, an autonomous placode was identified in 6 patients who were nonambulatory preoperatively and had presented with increasing pain and spasticity. In electrophysiologically silent areas, more aggressive dissection and untethering were possible. Symptoms of low-back pain, lower-extremity paresthesia, and lower-extremity spasticity were most likely to improve after untethering surgery (91, 88, and 82%, respectively). Sectioning above the electrophysiologically defined autonomous placode resulted in significant improvement in back pain and lower-extremity spasticity in 5 of 6 patients. There was 1 case of immediate postoperative neurological deterioration (fecal incontinence). All patients remained clinically stable or improved on long-term follow-up, except for 6 (16% of patients) who required a total of 7 additional procedures for recurrent symptoms (median time to repeat surgery 36 months). Complications were noted in 8 cases, including infections and CSF leaks.

Conclusions
Surgical untethering of STCS halts progression and often improves preoperative symptoms. Electrophysiological monitoring, using both a threshold-based interpretation system and continuous electromyography monitoring, provides an efficient, effective, and reliable method for intraoperative guidance, thereby limiting iatrogenic injury and providing a means to identify and untether autonomous placodes. Electrophysiological monitoring also allows for more aggressive dissection and untethering in functionally silent regions, possibly decreasing retethering rates.