False negative intraoperative somatosensory evoked potentials with simultaneous bilateral stimulation.

A 43 year old man underwent spinal cord surgery for removal of filum terminale lipoma. Intraoperative somatosensory evoked potentials (SSEP) monitoring showed a transient loss of response on simultaneous bilateral posterior tibial nerve (PTN) stimulation that recovered within 20 minutes. The patients exhibited paralysis and abnormal proprioceptive perception in the right leg postoperatively, when SSEP recordings revealed abnormal response to right PTN and normal response to bilateral PTN stimulation. In order to avoid false negative intraoperative responses, stimulation of each leg independently in an alternative fashion is recommended.