Subdural hematoma (SDH) at a remote operative site is uncommon following neurosurgical procedures. However, supratentorial SDH is one of the rare complications following microvascular decompression (MVD). We report a case of supratentorial SDH following MVD. The welling of cerebrospinal fluid (CSF) in the cerebellomedullary fissure was observed unexpectedly during the dissection. It is a signal case that can improve our understanding of the occurrence of such rare complications and of possible

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... echanisms. Case presentation: A 54-year-old woman was diagnosed with left hemifacial spasm (HFS) and had been receiving botulinum toxin injections since 10 years before surgery. CSF welling-up in the cerebellopontine cistern was noticed by the surgeon during the arachnoid dissection. MVD was performed completely. A thin supratentorial SDH was found on the right side (contralateral) on immediate postoperative computed tomography (CT). The SDH was asymptomatic, and the size of hematoma did not increase in follow-up CT scans after surgery. During the follow-up period, she was relieved of HFS. CSF welling-up can be considered the time of initiation of the hemorrhage. Conclusion: Limited case reports of supratentorial SDH following MVD have been published, and none of them have reported time of initiation of the hemorrhage. The phenomenon of abnormal CSF welling-up in the cistern could be a sign of remote supratentorial hemorrhage.