This document summarizes a case study of a 61-year-old female undergoing thoracic spinal reconstruction surgery. Prior to surgery, long-latency motor evoked potentials (LLMEPs) were observed in addition to standard motor evoked potentials (MEPs) when the recording window was expanded beyond 200ms. During surgery, the LLMEPs disappeared before standard MEPs as the spinal stenosis was decompressed, suggesting LLMEPs may be a sensitive indicator of spinal cord function. Post-surgery, LLMEPs did not fully return even as standard MEPs persisted, demonstrating the potential utility of LLMEPs in monitoring spinal surgeries. The author encourages expanding MEP recording windows and further study of LLME

1. Title: Long-Latency Motor Evoked Potentials (LLMEPs): Changes Observed During Thoracic Spinal Reconstruction
Author: David Barnkow, Au.D., D.ABNM, CCC-A, Medsurant Health, Denver, Colorado, March 28, 2019
The Case Presentation: A 61 year-old female with a history of mid-thoracic spinal cord infarct and C8 incomplete quadriplegia as a complication of spinal cord
stimulator placement. The chief complaints were increasing hand numbness and weakness, low back and lower extremity spasticity, and loss of range of
motion in the lower extremities. Her diagnoses included T10 vertebral collapse and spinal stenosis. The planned surgery was T10 corpectomy and interbody
graft, and T4- S1 revision posterior spine fusion and instrumentation.Table 1. Motor Strength Assessment
Figure 1. Sensory Assessment – Pin Prick
Figure 2. Left TceMEPs prior to incision. Identification of possible repetitive myogenic
activity beyond 200 ms prompts Neurophysiologist to expand recording window to 300
ms. Normal latency MEPs in red. LLMEP extending beyond 200 ms in orange. LLMEPs
greater than 200 ms in blue.
Figure 3. Left TceMEPs during thoracic decompressive laminectomies. LLMEPs (orange)
emerge before TceMEPs. Conventional TceMEPs (red) persist after LLMEP disappear.
Discussion: Ulnar SSEPs remained identifiable and posterior tibial SSEPs
remained absent throughout the surgery. Right side TceMEPs
demonstrated data similar to left side TceMEPs throughout the surgery.
This case study demonstrates that expanding the recording window of
current standard TceMEP protocols beyond 200 ms can reveal another
category of motor evoked potentials, the LLMEP. The disappearance of
the LLMEPs during a surgical period presumed to improve
neurophysiologic function is interesting. While the underlying
mechanisms and clinical utility of LLMEPs have not been described, this
author encourages all clinicians to consider expanding their recording
window to collect possible LLMEPs, and to publish and share their
LLMEP findings for a greater meta-analysis of this interesting, yet
essentially undescribed, phenomenon.
Figure 4. Left TceMEPs during instrumentation, deformity correction, and closing.
Note the persistence of the conventional TceMEPs (red). Also note the absence of
LLMEPs in VM and AbH montages, and pronounced intermittency of LLMEPs (orange)
in TA montage.
Figure 2. Sensory Assessment - Light Touch