Intraoperative neurophysiological monitoring for intradural extramedullary tumors: Why not?

BACKGROUND: While intraoperative neurophysiological monitoring (IOM) for intramedullary tumors has become a standard in neurosurgical practice, IOM for intradural extramedullary tumors (IDEMs) is still under debate. The aim of this study is to evaluate the role of IOM during surgery for IDEMs. METHODS: From March 2008 to March 2013, 68 patients had microsurgery with IOM for IDEMs (31 schwannomas, 25 meningiomas, 6 ependymomas of the cauda/filum terminalis, 4 dermoid cysts and 2 other lesions). The IOM included somatosensory evoked potentials (SEPs), motor evoked potentials (MEPs), and - in selected cases - D-waves. Also preoperative and postoperative neurophysiological assessment was performed with SEPs and MEPs. All patients were evaluated at admission and at follow up (minimum 6 months) with the Modified McCormick Scale (mMCs). RESULTS: Three different IOM patterns were observed during surgery: no change in evoked potentials (63 cases), transitory evoked potentials change (3 cases) and loss of evoked potentials (2 cases). In the first setting surgery was never stopped and a radical tumor removal was achieved (no stop surgery group). In 3 cases of transitory evoked potentials change, surgery was temporarily halted but the tumors were at the end completely removed (stop and go surgery group). In 2 more patients the loss of evoked potentials led to an incomplete resection (stop surgery group). No patients presented a worsening of the pre-operative clinical conditions (at admission 47 patients presented mMCs 1-2 and 21 patients mMCs 3-5, while at follow up 62 patients are mMCS 1-2 and 6 patients mMCs 3-5). CONCLUSIONS: In our series significant IOM changes occurred in 5 out of 68 patients with IDEMs (7.35%), and it is conceivable that the modification of the surgical strategy - induced by IOM - prevented or mitigated neurological injury in these cases. Vice versa, in 63 patients (92.65%) IOM invariably predicted a good neurological outcome. Furthermore this technique allowed a safer tumor removal in IDEMs placed in difficult locations as cranio-vertebral junction or in antero/antero-lateral position (where rotation of spinal cord can be monitored) and even in case of tumor adherent to the spinal cord without a clear cleavage plane.