Article Title: Utility of Motor and Somatosensory Evoked Potentials for Neural Thermoprotection in Ablations of Musculoskeletal Tumors

The Big Question:

First, let me apologize up front for the quote-heavy content of this write-up. Though I have been in the field of IONM for a while I have not had experience with ablations of musculoskeletal tumors so immediately this articles title intrigued me.

Essentially there are two techniques of ablation outlined in this article: cryoablation (where the tumor cells and adjacent tissue is frozen) and radio frequency ablation (which has the opposite effect by essentially cooking the tumor and adjacent tissue).

Though there is a history of studying the risks of these procedures, the published data shows a varying degree of risk and deficit percentages for each ablation modality. Although the authors (Yoon, et al.) cite many of these this publication seeks to further determine the utility of SSEPs and MEPs for these surgeries where the tumor ablation can potentially put a nearby neural structure at risk (thus the title).

Background:

As with all IONM, though it goes without saying, I liked the statement: “The inclusion of IONM was determined by the performing interventional radiologist based on qualitative risk assess-ment for nerve injury, ie, proximity to the spinal cord or spinal and/or peripheral nerves” (p. 2).

For the sake of the readers’ understanding and the fact that the results section included a lot of information I’ve included the specifics of both the methods and the results directly rather than paraphrase, with some mild revisions.

Method:

As reported in the study,

Warning criteria for abnormal SSEP changes were defined as a 60% reduction in baseline amplitude and/or 10% increase in latency per the institutional standards for spinal surgeries. Similarly, for TCeMEP monitoring, an abnormal change was defined as a 100-V increase above baseline threshold activation for a given myotome. When TCeMEP or SSEP warning criteria were met, the ablations were immediately terminated… (p.5)

Results:

As also reported in the study:

Warning criteria for TCeMEP and/or SSEP monitoring were met in 12 of 30 procedures (40%). Seven of 30 (23%) met warning criteria for TCeMEPs, 3 (10%) met warning criteria for SSEPs, and 2 (7%) met warning criteria for both. Eleven of these 12 procedures (92%) were cryoablations, and only 1 (8%) was an RF ablation. Nine of these 12 procedures (75%) targeted tumors involving the spine, and the remaining 2 (25%) involved the scapula.

[During the surgical period] five of the 12 abnormal TCeMEP/SSEP changes (42%) did not recover, with the remaining 7 (58%) being transient.

Three of 5 procedures with unrecovered abnormal changes (60%) and 2 of 7 procedures with transient abnormal changes (29%) had new charted motor (n = 1) and/or sensory (n = 4) symptoms.

As a whole, any abnormal TCeMEP or SSEP change was 100% sensitive… and 72% specific.,.. for neurologic sequelae, whereas any unrecovered change was 60% sensitive.. and 92% specific ….

Any abnormal TCeMEP change was 100% sensitive… and 72% specific… for new motor deficits; unrecovered TCeMEP changes had the same sensitivity, but a specificity of 93% …. Any abnormal SSEP activity change was 75% sensitive… and 92% specific… for new sensory deficits or radicular pain; unrecovered SSEP activity changes were 50% sensitive… and 100% specific. (p. 5)

Discussion:

In a nutshell, the authors’ acknowledge the sample size was small and the numbers related to risk in this study varied from other studies (though there are a number of contributing factors for this). In the end a total of 16% of patients done at this facility had reported IONM changes conveyed (based off the facility’s reporting criteria) that emerged from surgery with notable deficits. All of these patients with identified deficits were a result of cryoablation versus radio frequency ablation. Based off the sensitivity and specificity it seems that Neuromonitoring assisted in accurately identifying which patients could expect to have neurological deficits post-operatively. Unfortunately, as the authors also acknowledge, this is predictive versus preventative, the most important goal of IONM.

In conclusion, and on a brighter note, in the authors’ words:

Despite [the] limitations, the present study shows a correlation between neurologic sequelae and increased latency and/or decreased amplitude of SSEPs or an increase in TCeMEP threshold stimulation during percutaneous ablation procedures of musculoskeletal tumors. Monitoring of SSEPs and TCeMEPs should be considered in ablations in which there is concern for neural thermal injury

References:

• J Vasc Interv Radiol. 2020 Apr 24. pii: S1051-0443(19)31079-6. doi: 10.1016/j.jvir.2019.12.015. Utility of Motor and Somatosensory Evoked Potentials for Neural Thermoprotection in Ablations of Musculoskeletal Tumors. Yoon JT, Nesbitt J, Raynor BL, Roth M, Zertan CC, Jennings JW.

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