Spinal cord monitoring improves patient safety

Intraoperative monitoring is a technique that is directly aimed at reducing the risk of neurological deficits after operations that involve the nervous system. Using recordings of electrical potentials from the nervous system during surgical operations, this approach is designed to detect injuries before they become so severe that they cause deficits after the operation – including devastating iatrogenic spinal cord injury. In an effort to minimise the prevalence of this complication, multimodal intraoperative monitoring, using a combination of Somatosensory Evoked Potentials (SSEPs) and Transcranial Electrical Motor Evoked Potentials (MEPs) signals, is increasingly being used during corrective procedures such as scoliosis. The SSEP/MEP signals are continuously monitored throughout the procedure, especially during placement of instrumentation and deformity correction. Immediate action by the surgeon is required when damage to the spinal cord is suspected at any time during the procedure, in response to changes of >50% amplitude and >10% latency in the SSEP/MEP signals.

Somatosensory Evoked Potentials

Electrical stimulation of the posterior tibial nerve at the ankle results in ascending volleys (non-synaptically) via the dorsal columns to the thalamus and then (synaptically) to the somatosensory cortex. SSEPs can be recorded either from the spinal cord (epidural electrodes) or from the scalp. Signal averaging techniques are used to extract the small evoked signal (0.5-3 μV) from the EEG and background noise; in the theatre typically 200-300 trials (sweeps) are necessary. The SSEP reflects the functional status of the sensory tracts which are mainly located in the dorsal columns. The major limitation of SSEP is the relative insensitivity to spinal cord ischaemia, especially when confined to the anterior part of the cord (anterior spinal artery territory). Therefore, when only SSEPs are monitored, ischaemia limited to the motor tracts or anterior horn may go undetected, which makes SSEP less suitable for assessment of spinal cord function during procedures where the main mechanism of injury is spinal cord ischaemia (for example, in thoracic aortic procedures).