Taking a Divide and Conquer Approach to SSEPs
Welcome to the NSSA blog! This is a space for members to write a small piece on any neurophysiology topic they like. Could be a new technique, research project, or an article review. We’d love to hear from you, so get in touch if there’s something you’d like to write about! To kick things off, I wanted to write about a new technique that we’ve been looking at.
One of the things that I like most about working in neurophysiology, is that despite the age of the tests, they remain completely applicable and relevant amongst other diagnostic fields. Although the equipment we use is new and computerised, the core principles of neurophysiological testing remain the same as they were decades ago. Our well established tests are finding new applications with new research all the time.
This is certainly true of the SSEP test. Despite being a technical nightmare at times, in principle, SSEP tests are relatively simple. Stimulate a nerve at one site, and record the conduction as it travels up through the sensory pathways to the brain. We localise pathology by dividing the sensory pathway into segments, and we find pathology in a segment when a response is delayed after passing through it, compared with a set of normal values. That being said, you can really only localise to ‘somewhere between these two recording sites.’ Which, physiologically speaking, can be a whole lot of pathway for a lesion to be in.
A study done by a research group in Japan (reference below) looked at whether the sensitivity of the SSEP test could be increased by dividing one particular segment further, so that they could be more precise about where a lesion might be. They did this for patients being investigated for Chronic Inflammatory Demyelinating Polyneuropathy (CIDP), because this disease can present with demyelinating lesions of the more proximal nerve, and nerve roots, which are somewhat difficult to assess with standard nerve conduction testing.
They added an additional recording point at the greater trochanter (referenced to the ipsilateral iliac crest), essentially dividing the segment from the popliteal fossa to the lumbar spine in two. They found that by doing this, they were able to identify proximal dominant slowing in patients with CIDP, and in their group of patients, demonstrated greater sensitivity to the disease than other methods of testing like standard motor nerve conduction.
If you’re interested, it’s worth having a look at the paper, which outlines the test in much more detail. The protocol is relatively easy to set up, and only requires the application of one additional electrode on each side. I should note that there can be a bit of variability between patients in getting a good recording from the greater trochanter, and there’s not a robust set of normal values for these newly divided segments. Overall it’s a simple way to increase the sensitivity of the SSEP test in a specific patient group, and another new way of applying a well established neurophysiology test.
I hope you’re all travelling well in the current lockdown, stay safe.
Callum Hollis - Royal Melbourne Hospital
Tsukamoto, H., Sonoo, M., Shimizu, T. (2010). Segmental evaluation of the peripheral nerve using tibial nerve SEPs for the diagnosis of CIDP, Clinical Neurophysiology, 121, 77-84.
Written by Callum Hollis
Royal Melbourne Hospital