To evaluate intraoperative disappearance of MEPs related to patient positioning in neurologically asymptomatic patients with spinal deformity planed for posterior instrumentation with or without fusion.

This is a retrospective review of all neurologically asymptomatic children and adolescents who were planned for posterior instrumentation under MEP control during the last 3 years. A total of 189 patients were included. Age at surgery ranged between 7 and 17 years. There were 159 patients with adolescent idiopathic scoliosis (AIS), 4 neurofibromatosis, 6 connective tissue disease, 6 skeletal dysplasia, 5 metabolic disorder including muccopolysaccharidosis, and 9 early onset scoliosis or kyphoscoliosis of unknown origin. All patients underwent MEP monitoring and general anesthesia according to general guidelines with some personal variations. MEP recording is undertaken 11 times on average between induction of anesthesia and the end of surgery, and in case of abnormal findings, surgery is temporarily discontinued and necessary measures undertaken (increase temperature and blood pressure…), and if needed a wake-up test is performed. In case of permanent signal disappearance and no gross motor function during the wake-up test, surgery is definitively discontinued. Patients with permanent loss of MEP signal requiring definitive surgery discontinuation were thoroughly reviewed and represent the core of this study.

Six patients showed permanent loss of MEP signal during the early stages of surgery, three of which even before incision. There were one AIS (in which the loss of signal was later found to be related to a simple technical problem), one early onset kyphoscoliosis, one neurofibromatosis, one muccopolysaccharidosis, one Ehlers Danlos disease, and one congenital progressive anterior bloc with multilevel thoracic and lumbar stenosis. All of them resumed normal motor function after wake-up, except for the case with progressive anterior bloc who woke-up paraplegic but has regained ambulation with crutches one year later. An extensive work-up was undertaken secondarily in all 6 cases including total spine MRI, lower limb EMG, etc., to identify any etiology that could explain the loss of signal but was negative in all. The hypothesis of positional temporary suffering of the spinal cord was therefore considered.

AIS seems rarely if ever associated with intraoperative neurological risk even the one related to patient position. When MEPs disappear permanently especially at the very early stages of surgery, this may be due to position, and is much more likely to occur in patients with secondary scoliosis, suspected to have a more vulnerable spinal cord.