Fenestrated giant mastoid emissary vein, a novel finding

To document an extracranial fenestration of the mastoid emissary vein (MEV) and to clarify its drainage pattern and potential procedural relevance. A 46-year-old male underwent multidetector CT angiographic evaluation of the cervical carotid system. DICOM data were post-processed in Horos using multiplanar reconstructions and three-dimensional volume-rendered images; morphometric measurements were obtained on the reconstructions. A large left MEV (6.6 mm) exited through a single mastoid foramen situated 2.95 cm postero-supero-medially to the mastoid tip. The vein divided 2.2 mm distal to the foramen into an anterior limb (5.0 mm) and a posterior limb (1.1 mm) that rejoined 2.33 cm inferiorly, forming a long fenestration. Three deep communicating veins connected the fenestrated segment to the suboccipital venous plexus, and the distal MEV continued as the deep cervical vein. Extracranial fenestration is a plausible variant of a prominent MEV. Recognition on CT may prevent misinterpretation as vascular duplication or pathology and may influence mastoid, retrosigmoid, and endovascular planning in the mastoid–suboccipital region.