Objectives: Postoperative weakness following frontal lobe resection can result from supplementary motor area (SMA) syndrome or corticospinal tract (CST) damage. While intraoperative motor evoked potentials (MEPs) are specific to CST lesions, technical challenges can reduce certainty. This study describes a case of postoperative SMA syndrome examined using transcranial magnetic stimulation (TMS) MEPs, underscoring its potential as a powerful and reliable diagnostic tool.
Methods: A 29-year-old male underwent left frontal craniotomy for an IDH-mutant astrocytoma. Intraoperative transcranial electrical stimulation (TES), direct cortical stimulation (DCS), and subcortical (SC) MEPs were performed. Postoperative right hemiplegia prompted TMS-MEP testing.
Results: Contralateral TES-MEPs were recorded and remained stable throughout the procedure. DCS-MEPs were challenging; despite multiple attempts to place the electrode strip, only small amplitude hand potentials were recorded. Furthermore, the DCS-MEPs were lost twice during resection, presumably due to strip movement. SC-MEPs triggered eight warnings at the 10 mA threshold. Given the postoperative weakness, TMS-MEPs were performed, confirming CST integrity. The patient fully recovered motor function at 3-month follow-up, consistent with SMA syndrome.
Discussion: This case highlights the diagnostic utility of post-operative TMS-MEPs in resolving clinical uncertainty following suboptimal intraoperative monitoring. The transient right hemiplegia observed aligns with classic SMA syndrome features: preserved muscle tone, intact CST integrity, and delayed recovery of volitional movement despite intraoperative MEP stability. However, intraoperative MEPs can be suboptimal for various reasons. As observed in this case, DCS-MEPs may be compromised due to incorrect or difficult strip placement or movement during surgery. While generally reliable, TES-MEPs can be affected by subcortical stimulation, especially when applying high-amplitude stimulation. This can potentially lead to false-negative results, masking CST damage. Thus, post-operative TMS-MEPs offer a valuable non-invasive method to confirm CST integrity, differentiating from SMA syndrome. This technique not only provides critical reassurance to clinicians and patients but also enhances patient management, guiding appropriate expectations for recovery.
Conclusion: Post-operative TMS-MEPs are valuable for confirming SMA syndrome. This non-invasive technique differentiates between SMA syndrome and CST damage, aiding in patient management and prognosis. Future research should explore standardised protocols for post-operative TMS-MEPs in SMA syndrome confirmation. Additionally, investigating the role of pre-operative navigated TMS-MEPs could enhance risk stratification and surgical planning, potentially improving outcomes in SMA region surgeries.