<p>Anatomic variations in the posterior fossa are influenced by mastoid pneumatization and sigmoid sinus development. Accurate identification of the sigmoid sinus location is crucial for precise craniotomy during microvascular decompression (MVD) for hemifacial spasm (HFS).We investigated individual variations in the bony eminence between the occipitomastoid suture and the digastric groove (the inferior retrosigmoid eminence, IRE) and its relationship to the sigmoid sinus.A retrospective review was conducted on 104 patients who underwent MVD for HFS. Radiological findings, demographic data, and operative videos were used to examine the anatomical characteristics of the IRE.The IRE was identified in 99 patients (95.2%) using three-dimensional computed tomography (3DCT). It has an average width of 5.5 mm and a height of 6.25 mm. The mean distance from the medial margin of the sigmoid sinus was 0 mm. Craniotomy involving the IRE was performed in 44.2% of cases, and mastoid air cells were opened in 26.9%. Postoperative resolution of HFS was immediately observed in 87 patients (83.7%) and 103 patients (99%) at the one-year follow-up.Recognizing anatomical variations of the IRE is valuable for estimating the location of the inferior sigmoid sinus. The IRE aligns with the medial margin of the lower sigmoid sinus, which could serve as a reliable bony landmark.</p>