F94. Predicted stimulus thresholds of thoracic pedicle screws in the pediatric population

Abstract

Introduction Previous studies conducted on pedicle screws in lumbar spine surgeries have determined that electrophysiological monitoring using stimulus evoked electromyography (EMG) can reduce the risk of nerve root trauma based on specific stimulation threshold criteria. However, the validity and efficacy of traditional pedicle screw testing (PST) techniques applied to thoracic pedicle screws has been questioned. Furthermore, there is limited data regarding PST in the pediatric population. Data from pediatric scoliosis operations performed at C.S. Mott Children’s Hospital allows for a comprehensive analysis of thoracic pedicle screw stimulation values and corresponding post-operative status. Methods A retrospective review of thoracic PST in the pediatric population was performed. 167 pediatric orthopedic spinal corrective procedures utilizing thoracic pedicle screw implantation and electrical testing techniques were performed at C.S. Mott Children’s Hospital between June 2010 and August 2015. 1447 screws placed at the T8 - L1 levels were electrically tested using cathodal, monopolar stimulation with a pulse width of 200 μs and current intensity up to 20 mA. Alarm criteria thresholds were set to an observable compound muscle action potential (CMAP) from a corresponding muscle (rectus abdominus or iliopsoas) in response to a stimulating current of 6 mA or less. A retrospective review of stimulus thresholds, specific to each level and side was performed. For each spinal level the average stimulation threshold was calculated. Results Stimulation of 22 screws in 19 patients evoked a CMAP using current levels less than 2 standard deviations below the average stimulation threshold for that vertebral level. Eight of these screws tested above the established alert criteria of 6 mA. Of the remaining screws, 3 remained in place, 3 were removed and replaced, and 7 were removed and left out. None of the 19 patients had post-operative deficits. Conclusion These findings are inconclusive regarding the efficacy of current thoracic pedicle screw testing methods. It is possible that PST at these spinal levels is unreliable due to the wide variability in bone density and cortical and trabecular bone cell maturity in pediatric patients. It is also possible there were electrophysiological breaches of the pedicle wall though these were not clinically significant due to a safe margin between the pedicle screw and nerve root. ( Prospective ) Longitudinal studies should be conducted to provide data on the long-term outcomes of these patients. Additionally, future research should explore the use of multi-pulse stimulation in pediatrics as well as the sensitivity of EMG responses in the abdominals and iliopsoas to detect nerve root trauma.