Combined monitoring of motor and somatosensory evoked potentials in orthopaedic spinal surgery.

OBJECTIVES: To demonstrate possible advantages of combined (motor and sensory) versus single modality (either motor or sensory) intra-operative spinal cord monitoring and to investigate risk factors for post-operative neurological sequelae.

METHODS: Recordings of lower limb motor evoked potentials (MEPs) to multi-pulse transcranial electrical stimulation (TES), and tibial nerve somatosensory evoked potentials (SEPs), were attempted during 126 operations in 97 patients (79 with spinal deformity and 18 with miscellaneous spinal disorders).

RESULTS: Combined motor and sensory monitoring was successfully achieved in 104 of 126 (82%) operations. No response to either modality could be recorded in two patients with Friedreich' s ataxia. In 18 patients monitoring was possible in only one modality: SEPs could not be recorded in two patients and MEPs in 16. Significant intra-operative EP changes occurred in one or both modalities in 16 patients; in association with instrumentation in 10 cases, and with systemic changes in 6. After appropriate remedial measures, SEPs recovered either fully or partially in 8/8 patients and MEPs in only 67% (10/15 patients). New deficits were present post-operatively in 6 of the 16 patients with abnormal intra-operative EPs. Normal MEPs at the end of the operation correctly predicted the absence of new motor deficits in all cases. SEPs either remained unchanged or recovered fully after remedial measures in 3 patients with new post-operative motor deficits. Neurological complications were more frequent in patients with miscellaneous spinal disorders and/or pre-existing neurological deficits. No complications occurred in patients with idiopathic scoliosis.

CONCLUSIONS: Combined SEPs and multi-pulse TES-MEPs provide a safe, reliable and sensitive method of monitoring spinal cord function in orthopaedic surgery. This method is superior to single modality techniques, both for increasing the number of patients in whom satisfactory monitoring of spinal cord function can be achieved and, for improving the sensitivity and predictivity of monitoring. Combined SEP/MEP methods may enhance the impact of neuromonitoring on the intra-operative management of the patient and favourably influence neurological outcome.