Intrathecal Morphine on Transcranial Electric Motor-Evoked Potentials

Overview

Patients undergoing posterior spinal fusion (PSF) procedures for scoliosis are at risk for iatrogenic neurologic injury of the spinal cord and/or spinal nerve roots during surgical correction of the abnormal spinal curvature. The degree of neurologic injury can range from minor sensory deficits to complete paraplegia. Surgeons at CHOP utilize neurophysiologists to identify impending neurologic injury. These consultants monitor spinal cord pathways by recording and analyzing evoked potentials during the operation. Evoked potentials are low voltage electrical signals generated in response to transcranial or transcutaneous electrical stimulation of motor and sensory neural pathways. Some patients undergoing PSF receive an injection of morphine into the cerebrospinal fluid during the operation. This intrathecal (IT) morphine has potent analgesic effects. While most commonly used anesthetic agents have well-characterized effects on evoked potentials, little data exists on the effects of IT morphine on transcranial electric motor-evoked potentials (TceMEPs). This is a prospective observational study to characterize the effects of IT morphine on TceMEPs.

Posterior spinal fusion surgery (PSF) is associated with significant postoperative pain. Numerous postoperative pain management strategies have been employed for patients undergoing these procedures, including continuous narcotic infusions, intravenous patient-controlled opioid analgesia, epidural analgesia, intrathecally administered narcotics, and combinations of these regimens. Most patients at CHOP have their pain managed with intravenous patient-controlled opioid analgesia alone or in combination with a single dose of intra-operatively administered intrathecal (IT) morphine. Patients undergoing spinal fusion procedures are also at risk for perioperative neurologic injury. Surgeons at CHOP and other institutions routinely utilize neurophysiologists to evaluate at-risk neural pathways to identify impending spinal cord and spinal nerve root injury. These consultants monitor motor and sensory pathways by recording and analyzing evoked potentials and electromyography during the operation. Evoked potentials are low voltage electrical signals generated in response to transcranial or transcutaneous electrical stimulation of motor and sensory nerves. At CHOP, IT morphine is injected by the surgeon after the scoliotic curvature has been corrected and spinal instrumentation is complete. This injection therefore occurs after the critical period for neurophysiologic monitoring and risk of spinal cord injury. It is injected after the neurophysiologist has given a reassuring assessment of neural integrity. This injection is given to patients who have spinal fusions extending below the second lumbar vertebral body. Injections are limited to this group of patients because the appropriate intervertebral spaces for intrathecal injection are not exposed in procedures that do not extend below this level. Most commonly used anesthetic agents have well-characterized effects on evoked potentials. The effects of IT morphine on sensory-evoked potentials have been studied. However, little data exist on its effects on transcranial electric motor-evoked potentials. This study aims to characterize these effects; we hypothesize that intrathecal morphine has no effect on transcranial electric motor-evoked potentials in the doses used at our institution.