The enigmatic occipital emissary vein, foramina, and canals: anatomical study with application to skull base surgery

Compared with mastoid emissary veins, occipital emissary veins have been less studied, and published descriptions of their foramina are inconsistent. This anatomical investigation aimed to clarify their prevalence, morphology, and relevance to skull base surgery. Two hundred fifty adult skulls were examined for internal and external occipital foramina. Selected samples were sagittally sectioned through the foramina. Ten latex-injected adult cadaveric heads were dissected, then sagittally cut to visualize intradiploic and intracranial venous courses. Portions of vein and surrounding bone were sent for histology. Four additional dry skulls underwent blue latex injection via internal or external foramina followed by selective removal of the inner or outer table. No foramina were present in 16% of skulls. External foramina were absent in 34% and internal foramina in 22%. External foramina connecting to an internal foramen occurred on the left in 10% and right in 8%, typically forming a diagonal pathway with a descending trajectory. Most external foramina were 1-2 cm from midline; 8% were bilateral. Internal foramina clustered near the internal occipital protuberance. Isolated foramina frequently ended in blind pits. Latex-injected specimens demonstrated internal or external veins in 50% of cases, with two showing an accompanying artery. Injection studies revealed no through-passage but did show diploic spread. External openings of the occipital foramina are unreliable surgical landmarks, as their position does not predict intracranial venous entry. Internal emissary veins may exist without an external foramen and risk injury during dural elevation. Separate internal and external diploic communication systems likely exist.