Click here to read this beautiful comprehensive document on somatosensory evoked potentials (SSEPs) by the International Society of Intraoperative Neurophysiology (ISIN). Whether you have been doing SSEPs for years and want to refresh your memory or you want to learn about them for the first time, this document has everything about SSEPs you need to know. It is concise and each sentence has valuable information. Where possible, they also present the original data behind their recommendations. Being so rich in information, it may be your preference to read it in short bits over a period of time to take it all in.
The following article will highlight 5 ways in which the ISIN SSEP recommendations may differ from prior guidelines you have followed.
1. Do not stick to a set number of averages
There is a real emphasis in these recommendations on minimizing the time it takes to get feedback to the surgeon. As such they recommend that IONM clinicians and technologists tailor the number of SSEP averages for every case. That means doing everything you can to minimize the variability or noise in your traces so you can in turn minimize the number of averages you need to run. For high quality consistent SSEPs, averages should be between 2-200.
2. Omit frontal and subcortical Recording
One way to reduce the number of averages you need is to reduce the noise in your waveforms. MacDonald has already demonstrated over a series of papers that certain montages are noisier than others. By using quieter montages, we can reduce our averages and the time it takes to provide feedback to the surgeon.
The brain generates frontal dominant EEG noise the contaminates our SSEP signal. So, while recording montages that use Fpz and Fz often result in the highest amplitude SSEP responses, they are nosier than the montages that rely exclusively on the centro-parietal (CP) electrodes. Research has shown us that montages with Fpz and Fz require a greater number of averages to produce a highly stable waveform, thus prolonging the time it takes to acquire monitorable SSEPs. So counterintuitively, the montages with the frontal derivations, which give high amplitude SSEP responses, should be avoided. ISIN recommends:
For upper limb SSEPs: CP3-CPz for right stimulation and CP4-CPz for left stimulation
For lower limb SSEPs: CPz-CP3 for the right stimulation and Cpz-CP4 for the left stimulation or alternatively CP3-CP4 for the left stimulation and CP4-Cp3 for right stimulation (or 4 other close variations on those).
In addition to the above montages they recommend always having peripheral montages, They recommend recording from the popliteal fossa and cubital fossa.
Breaking with tradition, they do not recommend subcortical or cervical monitoring be routinely included. Again, this is because these montages often include a high level of noise and take more averages to collect a stable waveform. They recommend that only if the noise level for these montages are low enough that they do not need more averages than your cortical + peripheral montages should you decide to include them. The montages they recommend if you opt to include them are Fpz-Mastoid (M) for lower limb subcortical and for the upper limb subcorticals use the montages CP3-M for left and CP4-M for right stimulation. The cervical montage they recommend if you want to monitor the upper extremity segmental SSEP (N13) is C5S-M.
3. Alarm – It’s all about Amplitude
ISIN believes historically there has been an unwarranted emphasis on SSEP latency shifts and have based their alarm criteria solely on amplitude. First, they stipulate it is an error to set an early baseline and not to reset it later to reflect the changing systemic states and natural baseline shifts. Second, they stipulate that in order to call a surgical change (after other factors have been ruled out) three questions must be considered: 1) Does the amplitude decrease exceed trial-to-trial variability? 2) Is it abrupt? And 3) is it Focal? If the answer to all three of these is yes, then the alarm criteria is met. If the decrease in amplitude is ≤ 50% or “borderline”, then it may be verified by a concordant MEP decrease and/or further SSEP deterioration on subsequent trials.
It is possible to have pathologic slower deterioration of signals that is neither abrupt or focal. But ISIN posits that in these cases you can still discriminate between a surgical versus systemic changes based on the surgical, anesthetic and systemic context.
4. Who can monitor?
ISIN represents the intraoperative monitoring community globally. As such, its members come from regions with contrasting models of medicine and economies. To be inclusive they have made the general recommendation that people who do intraoperative SSEP monitoring “should have the relevant training, experience and qualifications”. They do not define qualifications or training. They do state there should be a Professional who assumes clinical responsibility whose required qualifications are decided by the appropriate jurisdiction. They cite MDs and PhDs as examples. And ISIN recommends these professionals delegate, under their supervision, technical aspects of the monitoring to qualified technologists.
Have you ever had a discussion with a surgeon that SSEPs should be monitored on this case? This paper does a great job of reviewing and summing up the surgeries that SSEP monitoring should be conducted on. They explain why and cite the literature to support their reasoning. It’s a handy reference guide to what surgeries you should be doing SSEPs on why.
Some great little SSEP tidbits included in the article:
- Spinal grey matter receives 4 times the spinal blood flow of white mater. This additional perfusion makes it more sensitive to ischemia. So acute ischemia is often first seen in MEPs due to anterior horn cell (grey matter) failure and may be delayed or not seen in SSEPs because they are conducted through the white matter dorsal columns. So abrupt loss of SSEPs is more indicative of pathophysiology like compression.
- Lower the stimulation rep rates generate higher amplitude responses.
- Strong Tibialis Anterior contractions can cause anterior tibial compartment syndrome if stimulating near the knee (e.g. popliteal fossa or peroneal) you want to use a lower stimulation intensity.
- They recommend you display “self contained averages” to detect abrupt decreases in amplitude. This is not available on all machines so the next best alternative is to show the traces as they are averaging.
- Erbs-M is a good montage for detecting shoulder malpositioning.
- Use the CPc-M and CPi-M montage to verify there is a somatosensory pathway decussation (i.e. signal from right limb crosses over to the left brain).
- When the patient is in a sitting position for posterior fossa cases you can get air over the hemispheric convexities, which reduce your SSEPs. They recommend in these cases for lower limb cortical signals to record from CPz-Fpz (the bridging veins along the midline preventing air buildup) and for upper limb cortical signals to place your recording electrdoes more lateral and use CP5/Cp6 or T3/T4.
If you were writing SSEP recommendations what would you include?