Betty L. Grundy

Monitoring of Sensory Evoked Potentials during Neurosurgical Operations: Methods and Applications

Monitoring of sensory evoked potentials (SEPs) may help minimize the risk of neurological injury during neurosurgical operations. The author describes the current state of the art, summarizing basic principles and reviewing current clinical applications. Experience with intraoperative monitoring of auditory, somatosensory, and visual evoked potentials is presented. The pitfalls and limitations of presently available methods are discussed, with some speculation regarding future developments. Given adequate quality control in the acquisition, processing, and interpretation of electrophysiological signals, monitoring of SEPs can be a valuable adjunct to the intraoperative care of selected neurosurgical patients.

DELIBERATE HYPOTENSION FOR SPINAL FUSION: PROSPECTIVE RANDOMIZED STUDY WITH EVOKED POTENTIAL MONITORING

Twenty-four patients requiring spinal fusion with Harrington rod instrumentation were studied prospectively to determine the effects of moderate hypotension on blood loss, operating conditions, operating time and spinal cord function. Hypotension reduced blood loss and improved operating conditions but did not shorten operating time. Five patients had alterations in somatosensory cortical evoked potentials after straightening of the spine that prompted us to reverse hypotension (when present) and haemodilution, and then to do wake-up tests. All wake-up tests were normal and all evoked potential alterations resolved during operation. Hypotension seems unlikely to increase the risk of neurological damage if spinal cord function is monitored. Our findings suggest that patients subjected to spinal fusion need not be awakened during operation for testing of cord function provided somatosensory evoked potentials are monitored and remain stable.RésuméL’emploi d’une hypotension contrôlée modérée (maintien de la pression systolique entre 80 et 90 torr) au cours de fusions vertébrales par tige de Harrington a été étudié de façon prospective. Les paramètres étudiés étant le volume des pertes sanguines opératoires, la qualité du champ opératoire, la durée de l’intervention et le maintien de la fonction médullaire. Les vingt quatre patients inclus dans cette étude étaient assignés au hasard au groupe témoin ou au groupe opéré sous hypotension contrôlée. Nous avons observé des pertes sanguines moindres ainsi qu’une qualité supérieure du champ opératoire chez les patients opérés sous hypotension contrôlée. La durée de l’ intervention n’était cependant pas raccourcie. Cinq des patients ont présenté des modifications des potentiels corticaux induits par stimulation somato-sensorielle; dans ces cas la pression était ramenée à la normale lorsque l’hypotension contrôlée était utilisée, l’hémodilution était corrigée et le malade était éveillé en cours de chirurgie pour vérifier si la motricité volontaire des membres inférieurs était conservé. Tous les tests d’éveil se sont avérés normaux et les potentiels corticaux évoqués se sont corrigés en cours d’intervention. L’emploi d’hypotension contrôlée ne semble pas susceptible d’augmenter le risque de dommages neurologiques si le maintien de la fonction de la moelle épinière est surveillé en cours de chirurgie.Nos résultats suggèrent que les malades soumis à une fusion vertébrale n’ont pas à être éveillés en cours d’intervention pour vérification du maintien de leur fonction médullaire en autant que l’on effectue le monitoring per-opératoire des potentiels corticaux induits par Stimuli corticaux sensoriels et que cette fonction est maintenue stable.

Intraoperative hypoxia detected by evoked potential monitoring.

Somatosensory cortical evoked potentials (SCEP) are used to monitor spinal cord function during operations on the spine and spinal cord (1-3). These intermediate latency responses to stimulation of peripheral nerves originate in the cerebral cortex (4). They are subject to alteration by anesthetics (5,6) and by perturbations of physiologic state that may occur during surgery (7, 8). If SCEP monitoring is to reflect reliably surgical trespass on sensory pathways, potentially confounding factors that also affect SCEP must be monitored and kept as constant as possible. SCEP recording may detect not only physical encroachment on conducting pathways at the operative site but also systemic physiologic changes that require correction. We have observed SCEP alterations as the presenting manifestation of intraoperative hypoxia.

Monitoring of cortical somatosensory evoked potentials to determine the safety of sacrificing the anterior cerebral artery

Cortical somatosensory evoked potentials (SSEPs) reflect the functional integrity of somatosensory pathways from the site of stimulation to the primary somatosensory cortex. We used intraoperative monitoring of cortical SSEPs to determine whether the right anterior cerebral artery (RACA), the major feeding vessel of a large arteriovenous malformation (AVM), could be sacrificed without compromising sensorimotor function in the left lower extremity. The SSEPs recorded after test occlusion of the RACA showed preservation of the initial cortical positivity, and the RACA was divided. The AVM was excised completely, and the patient suffered no neurological deficit.

Evoked Potential Changes Produced by Positioning for Retromastoid Craniectomy

Reversible changes in brain stem auditory evoked potentials (BAEPs) occurred in a patient with hemifacial spasm when she was positioned for retromastoid craniectomy. The peak latencies increased and the amplitudes decreased. These alterations persisted for 3 hours, but returned immediately toward normal when the head was removed from pin fixation and the neck was returned to a neutral position. Similar BAEP changes were documented during a second procedure. Audiograms were normal pre- and postoperatively, although minimal BAEP changes were seen after operation. Our observation sheds light on possible mechanisms of intraoperative BAEP alteration and provides new information about the degree and duration of intraoperative BAEP distortion that can be tolerated in at least some cases without permanent neurological sequelae.