Clinical Anatomy and Diagnostic Visualization of Emissary Veins

Objective: Emissary veins are often neglected in their clinical relevance. However, at the retrosigmoid approach especially during semisitting positions these transosseous venous connections become potentially live threatening. Therefore, knowledge of presence and the configuration of emissary veins is essentially in the preoperative planning. Methods: A total of 200 consecutive patients planned for the retrosigmoid approach, due to cerebellopontine angle pathology, were evaluated by computed tomography in standard technique (5-mm slices) and spiral technique for the posterior fossa (1-mm slices). The presence and size of emissary veins, entry point in the sigmoid sinus, the predilection side, and the visibility in either CT technique were evaluated. Results: Emissary veins were present in 80% of the patients, varying from 0.3 to 4 mm in diameter, whereas 1% (right)and 2% (left) of the patients have more than one emissary vein, respectively. The mean length of the intraosseous course of the vein to the entry point in the sigmoid sinus was 7.6 mm (right side) and 5.1 mm (left side). The study shows that with standard CT technique only emissary veins larger than 1.0mm can be identified and that either entry point in the skull base or sigmoid sinus cannot be securely visualized. On the other hand, spiral technique CT scans visualized emissary veins of 0.3mm with their exact course from the skull to the entry point in the sigmoid sinus. Conclusions: Thin sliced CT scans are inevitable in the preoperative planning of the retrosigmoid approach to identify the presence and the course of emissary veins, reducing unnecessary morbidity during surgery.