Cloud–like Thalamic Edema after Redo MR-guided Focused Ultrasound VIM Thalamotomy—A Case Report

INTRODUCTION: Cavitation-associated adverse events in MR-guided focused ultrasound (MRgFUS) thalamotomy are uncommon but potentially serious. We report a case of acute "electric-shock" sensation during an alignment/verification sonication followed by thalamic T2 hyperintensity/edema after a redo ventral intermediate nucleus (VIM) thalamotomy. We discuss plausible biophysical mechanisms and risk-reduction strategies. CASE PRESENTATION: A 53-year-old woman with essential tremor underwent repeat MRgFUS five months after her initial treatment. During the 4th alignment/verification sonication (planned 200 W, 20 s; measured power approximately 192 W), cavitation monitoring triggered a halt at 19.2 seconds after 2037 J was delivered. Despite a maximum target temperature <50 °C, the patient immediately reported an "electric-shock" sensation. Post-procedure, she developed right-sided sensory symptoms, mild dysarthria, and hypogeusia. Early MRI showed irregular thalamic hyperintensity consistent with edema, but without confirmed off-target thermal rise on intra-operative thermometry. CONCLUSION: The most plausible pathogenesis is mechanically mediated injury from cavitation-related acoustic activity. We hypothesize that substantial overlap with the prior ablation zone was the primary predisposing factor, potentially augmented by calcification-related scattering, No-Pass Region (NPR)-driven aperture distortion, occult skull marrow changes, and standing-wave formation. We discuss biophysical mechanisms and propose physics-guided safeguards for redo procedures.