What is the minimum vertebral level below which D-waves cannot be recorded?

D-waves, or direct waves, are a type of Corticospinal Tract (CST) response that can be recorded from the spinal cord during intraoperative monitoring. They originate from the cortex and travel down the spinal cord. The capability to record these waves is contingent upon the anatomical positioning of the spinal cord and the associated pathways.

D-waves can be recorded from the cervical and upper thoracic regions, as these are the regions where the corticospinal tracts are still intact and functional. Below the level of L1, the spinal cord transitions into the cauda equina, where the direct paths for these waves are absent. As such, recording D-waves cannot be achieved below this vertebral level because the spinal cord proper has ended, and the nerve roots that make up the cauda equina do not allow for the direct transmission of the D-waves.

Thus, the minimum vertebral level below which D-waves cannot be recorded is indeed L1. This level marks the end of the functioning spinal cord in terms of direct wave transmission, which is critical knowledge in neurophysiological monitoring during surgical procedures involving the spine or related structures.