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