Thoracic & Lumbar Monitoring | Signal IONM

While thoracic and lumbar spine procedures share many monitoring fundamentals, the termination of the spinal cord at approximately L1 represents a critical anatomical boundary for monitoring strategy. Caudal to L1, the cauda equina — composed of individual nerve roots rather than spinal cord — responds differently to injury and does not benefit meaningfully from tcMEP monitoring under standard conditions. For lumbar procedures at L2 and below, the monitoring emphasis shifts from spinal cord surveillance to lumbosacral nerve root protection via EMG.

Exceptions to this framework include patients with preoperative neurological deficits affecting baseline waveforms (paraplegia, significant peripheral neuropathy), those with concurrent thoracic or cervical pathology, and patients with intradural tumors or high-grade trauma where spinal cord involvement cannot be excluded. In these cases, tcMEPs remain part of the monitoring montage regardless of operative level, with pre-positioning and post-positioning baselines documented to distinguish positional effects from surgical injury.

In posterior and lateral approaches to the thoracic and lumbar spine, the brachial plexus is often positioned in a mechanically vulnerable configuration. Upper extremity SSEPs are used to monitor for positional brachial plexus compression, peripheral ischemia, or radial, ulnar, or median nerve compromise — risks that increase with case duration. tcMEPs may provide additional corroborating evidence of upper extremity motor pathway status.
Anterior approaches introduce the additional risk of retractor-related iliac artery compression, which can produce lower extremity ischemia detectable via rapid loss of lower limb SSEPs and tcMEPs. Prompt identification and pressure relief are essential to preventing permanent injury.

Free-run EMG provides continuous, sensitive detection of nerve root irritation throughout decompression. Spontaneous neurotonic discharges triggered by retractor placement, forceps manipulation, or other mechanical contact provide immediate feedback, allowing the surgeon to adjust technique before injury progresses. Triggered EMG using a flush-tipped stimulation probe (0.3–3 mA) is used for nerve root identification during discectomies and other procedures where anatomical visualization is limited.

Triggered EMG is used to evaluate pedicle screw position by stimulating the screw head via a ball-tip probe. A stimulation threshold of 10 mA or greater is used as the industry standard for acceptable screw placement, providing a margin of safety that accounts for variability in bone density. Thresholds below 10 mA suggest inadequate cortical insulation around the screw, and thresholds below 7 mA may indicate a breach of the pedicle cortex toward the spinal canal — at which point SSEP and tcMEP changes would be expected if the cord is compromised. Screw holes can be probed prior to screw placement to verify trajectory and assess bone integrity.

All waveform data must remain comparable to established baselines prior to wound closure

IONM plays a dual role during spinal cord stimulator (SCS) implant procedures: protecting the spinal cord during lead placement and confirming therapeutic lead position intraoperatively. SSEPs and tcMEPs monitor cord integrity throughout the procedure. Free-run EMG provides continuous detection of nerve root irritation during lead advancement. Once leads are positioned and prior to finalizing battery placement, EMG-guided stimulation mapping documents the specific muscle groups activated by stimulation — providing objective verification of target coverage that becomes part of the permanent implant record.

Thoracic and Lumbar Surgical Procedures