False negative in spinal cord monitoring using spinal cord-evoked potentials following spinal cord stimulation during surgery for thoracic OPLL and OLF.

Several electrophysiologic methods have been used for intraoperative monitoring of spinal cord functions. Recently, muscle responses following transcranial electric stimulation have also been widely used. However, in patients with severe myelopathy, only the spinal cord responses following spinal cord stimulation can be recorded. We report a false-negative case in monitoring using spinal cord responses following spinal cord stimulation. The patient presented with severe thoracic myelopathy due to ossification of the posterior longitudinal ligament (OPLL) and ligamentum flavum. Spinal cord responses following spinal cord stimulation were recorded during laminectomy and partial removal of the OPLL. Although the amplitude of those responses decreased transiently to within 50% of control data during decompression, it had recovered to the control level by the end of surgery. Just after surgery, the patient showed complete motor paralysis with preserved sensory function. Spinal cord responses following spinal cord stimulation can be recorded in patients with severe myelopathy. However, motor problems may occur when surgery is continued in the false belief that responses are accurate.