The transoccipital parasigmoid canal of the vertebral artery through a vestige of the occipital vertebra.

Purpose: The vertebral artery (VA) typically courses through the foramen magnum to enter the posterior cranial fossa. A transosseous course of the VA through the lateral part of the occipital bone is an unexpected finding. Such an anomaly of the VA is reported here.

Methods: The archived angioCT file of an 81 y.o. female was studied on planar sections and by three-dimensional volume renderings.

Results: The right VA looped above the transverse process of the atlas, and then it descended to enter a canal through the lateral part of the occipital bone. That canal was medial to the posterior condylar canal and the sigmoid sinus. It was termed the parasigmoid canal. The right VA entered the posterior fossa higher than the left VA, immediately posterior to the hypoglossal canal. A distinctive osseous plate was added on the inner side of the right occipital condyle and fused to the posterior arch of the atlas, narrowing the vertebral canal. A left osseous plate was fused medially to the occipital condyle. Both these plates may be remnants of an occipital vertebra. Thus, the right VA's anomalous course may have resulted from an aberrant course of a hypoglossal artery near but not within the hypoglossal canal.

Conclusion: The anomalous transoccipital course of the VA is a scarce but possible anatomical variation that may alter the neurosurgical landmarks, and it should be equally known by surgeons and interventionists.