Effects of opening peritumoral syrinxes on motor evoked potentials during resection of an intramedullary spinal cord tumor--case report

A 29-year-old woman presented with mild tetraparesis caused by an intramedullary cervical cord tumor extending from the C-5 to C-6 levels associated with rostral and caudal syrinxes extending to the C-1 and C-7 levels. Transcranial motor evoked potential (MEP) monitoring was carried out during tumor resection using high-frequency repetitive electrical stimulation. When the rostral syrinx was opened, the MEP amplitude in the right upper limb suddenly decreased by about 50%. When the caudal syrinx was opened, the amplitude immediately recovered to the baseline. During dissection of the attachment of the tumor, the amplitude decreased again by 70%, and did not recover. The tumor was totally removed. The histological diagnosis was ependymoma. Postoperatively, motor weakness in the right arm deteriorated but gradually recovered. The initial loss of amplitude on opening of the rostral syrinx may have resulted from distortion of the intramedullary pressure that compromised function of the motor tract nearby. Opening of the caudal syrinx normalized the intramedullary pressure and allowed the MEP to recover. Intraoperative monitoring of MEPs is valuable in minimizing injury to corticospinal pathways, but care must be taken in evaluating changes in MEP. Intramedullary spinal tumors with associated syrinxes are not rare, so surgeons should realize that opening of a syrinx can lead to MEP changes.