"Filtering effect" of cerebrospinal fluid on spinal somatosensory evoked potentials: case report

A 31-year-old man presented complaining of radicular (L4) pain. MRI and myelo-TC of the thoraco-lumbar spine showed the presence of an extradural CSF cyst extending from Th6 to L2, posterior to the cord. The patient underwent a Th12 laminectomy, the cyst was opened and drained. A recording electrode was placed epidurally, dorsal to the cyst, at the level of the Th12 vertebral body, and the potentials evoked by stimulation of the tibial and saphenous nerves were monitored during surgery. After cyst drainage the epidural potentials were significantly larger (x5) in amplitude and more defined in shape than those recorded in the presence of the cyst. Our case demonstrates the "filtering effect" of CSF on spinal somatosensory evoked potentials.