Thoracic and Lumbar

While there are many similarities involving spine surgery at T1-L1 vs L2-S1 there is one critical factor to consider from a monitoring perspective. L1 marks the termination of the spinal cord and the beginning of the cauda equina. This is significant because it is universally accepted that Trans Cranial Motor Evoked Potentials (TcMEPs) do not add any significant value post L1. Therefore a typical monitoring montage does not include “motors” on an L3-5 PLIF. It is however always available for use. Examples of exceptions to this rule are:

The patient has preoperative symptoms including total paralysis, quadriplegia, paraplegia, significant preexisting peripheral nerve damage or any other significant preexisting conditions that will result in lack of waveforms. These are important to notate pre incision (or preposition) to establish a baseline reading that reflects these preexisting conditions.

Patients with heavy trauma, intradural tumors or patients with additional thoracic/cervical spine issues. These should be monitored in an effort to minimize potential damage to the spinal cord intraoperatively and to avoid positional issues resulting in damage. Preposition and post position baselines are utilized to assure spinal cord integrity.

For spinal surgeries involving T1-L1 TcMEPs are part of the standard of care. It is imperative that the integrity of both ascending/sensory/posterior and descending/motor/anterior spinal pathways be closely monitored for preservation.

Historically, peripheral issues resulting from improper positioning are uncommon. Nonetheless the potential exists. During thoracic/lumbar spinal surgeries with posterior or lateral approaches the brachial plexus is in a uniquely awkward position. This increases the potential of brachial plexus damage and upper extremity peripheral ischemia. Depending on the case length this damage can vary from short lasting to permanent. It is therefore imperative to guard against these dangers. The easiest and most efficient way of doing this is utilizing Somato Sensory Evoked Potentials (SSEPs). This modality is typically utilized to monitor spinal cord integrity, however it is equally useful guarding against both peripheral ischemia and possible radial, ulnar and median nerve damage at the brachial plexus. TcMEPs can at times be utilized to corroborate peripheral insufficiency.

This same danger is present with anterior approaches, however the reason in this case is tension or traction from taped shoulders. Additionally during the anterior approach there have been instances where retraction has placed too much pressure on the iliac artery resulting in ischemia in the lower peripherals. If the pressure is not alleviated this could lead to permanent nerve damage. Utilizing SSEPs and TcMEPs is crucial in quickly identifying and resolving the issue.

During decompression, it is crucial to monitor against any nerve root damage. The utilization of electromyography (EMG) helps us detect any irritation caused by retractor, forceps or any other surgical tool. Special attention is placed on emg monitoring during these crucial periods and irritation is easily and quickly detected.

Another use for emg is nerve isolation. Utilizing a flush tipped stimulation probe with a stim value of 0.3 to 3 mA we can aid the surgeon in identifying nerve roots. This is especially useful for discectomies.

For this stage of surgery we utilize a combination of SSEP, TcMEP, Triggered and Free run Emg. The goal is to verify proper screw placement for the preservation of both the cord and the nerves themselves. Triggered Emg is utilized to stimulated screw heads via a ball tip probe. The American Board of Registration of Encephalography and Evoke Potential Technologists (ABRET) recommends that stim values of 8mA or more indicate proper screw placement. However new studies suggest that 10 mA should be the industry standard. The idea is that a threshold of 10 mA takes into account abnormal bone density. bone density. Soft bone can lead to false negatives conversely bone that’s more dense than average can result in false negatives. A good stim value represents a screw fully insulated by the vertebral body surrounding it. Essentially the closer the screw is to the edge of the vertebral body the less insulation it’s getting thus the lower the stim value. At the point of a breach in the body the compound muscle action potential (cmap) would be seen at a value of 0.3 to 7 mA. If the screws breaches the body even further it runs the risk of penetrating through the dura into the spinal canal. At this point SSEPs and TcMEPs (spinal cord monitoring) will be affected.

Prior to screw placement screw holes can be tested utilizing a similar methodology. This can aid in verifying hole location and in verification of adequate twitches.

Its Imperative that all signal remain comparable to baseline reading prior to closing.

Thoracic and Lumbar Surgical Procedures