A new anatomical finding: the paramastoid diverticulum of the sigmoid sinus

The sigmoid sinus (SS) is a major surgical landmark. The paramastoid process (PMP) occurs rarely. Inferior diverticula of the SS were not found or reported previously. We aimed to determine the incidence and detailed anatomy of such morphology of the SS.

Archived angioCT files of 25 males and 25 females were used. The morphology of the SS was checked on planar sections and by three-dimensional volume renderings.

In 3 female cases (6%), inferior paramastoid diverticula of the SS (PMDSSs) were found, two on the right and one on the left. They were all protruding on the inferior side of the jugular process of the occipital bone. Their heights and inner diameters were, respectively, 9.94/11.01 mm, 8.21/4.85 mm and 5.97/8.72 mm. A high jugular bulb was also found on that side in each case. Each PMDSS had a thin or dehiscent bottom. They were closely related to condylar veins, the occipital artery, the vertebral artery and its venous plexus.

The PMDSS should not be mistaken as a PMP to avoid surgical lesions of the SS. The PMDSS is an unexpected landmark in the suboccipital region.