Intradural Tumors: Intraoperative Monitoring
Intraoperative Neurophysiological Monitoring
Intraoperative monitoring with motor evoked potentials (MEPs) is a direct motor tract monitoring technique which is becoming the major monitoring tool for spinal cord surgery. The D-waves and muscle MEPs must be interpreted together. Loss of muscle MEPs during a spinal cord tumor resection indicates at least a temporary disruption of motor function of the spinal cord. The more incremental change of the D-wave amplitude then allows further interpretation of the motor outcome. A loss of muscle MEPs is highly associated with a temporary motor deficit in the lower extremities, even if the D-wave amplitude is unchanged. In the majority of cases, this is even side specific. As long as the D-wave amplitude remains above a cutoff value of about 50% of its baseline value, this motor deficit is temporary, with the patient recovering to preoperative strength within hours to days, sometimes weeks. A further decline in D-wave amplitude or its loss is associated with a long-term severe motor deficit.
Somatosensory evoked potential recording is of use to assess the functional integrity of the sensory system. Their correlation with postoperative motor function is poor and particularly in intramedullary surgery SEPs usually disappear when the midline myelotomy is performed and are then of no further use for the critical assessment of the motor system.
Radiotherapy
While surgery is the treatment of choice for both intra- and extramedullary tumors, some authors have recommended biopsy and radiation to manage astrocytomas and ependymomas. The beneficial results of these studies may be related to the effects of the decompressive laminectomy rather than the adjuvant treatment. The recommended radiation dose is hazardous to the spinal cord, especially in children and young adults, where myelopathy has been reported to occur with doses of 30 Gy. There is no role of radiotherapy in treatment of intradural extramedullary tumors other than metastatic disease.