P495: Selective loss of the D-wave evoked by contralateral hemispheric transcranial electric stimulation (TES)

Abstract

Methods: Description of a case series. Eight NF1 patients underwent interventions in order to remove spinal roots neurofibromas causing cervical spinal cord compression. A second intervention at the lumbosacral spine was necessary to remove neurofibromas located at the cauda equina in 2 cases. In all cases, IONM by means of motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs, epidural SSEPs), D-Wave monitoring and Bulbocavernosus Reflex (BCR) was performed. Additionally, in order to identify motor nerve roots and to preserve their function during surgery, neural tissues mapping was performed by direct electrical stimulation of the structures with a bipolar probe. Results: A total of 54 neurofibromas at the cervical and lumbosacral spine were removed. Motor nerve roots were identified and subtotally resected, sparing rootlets with small tumour nodules. Tumours on sensitive nerve roots were fully resected. When tumours were located at the cauda equina, nerve roots were also monitored using a train of stimulus to determine if these sensitive roots were critically involved in BCR. BCR was successfully elicited in both patients with tumours involving cauda equina and were preserved after tumour removing. No significant changes occurred in MEPs, SSEPs or D-Wave monitoring during surgeries. Conclusions: IONM provides continuous, real-time information about the function of neural pathways at risk during surgery and helps the surgeon to identify motor neural structures. The importance of BCR for the clinical outcome of the patients requires the use of special techniques intended to intraoperatively maintain the reflex. As opposed to schwannomas, neurofibromas grow encasing neural fibres from motor and sensory nerve roots. Subtotal resection should be performed in motor roots in order to preserve motor function.