Mapping of Transverse and Sigmoid Sinus Junction: Application in Vertical Extension of Suboccipital Craniotomy

Introduction: There is paucity of data regarding the surface landmarks and the vertical course of the sigmoid sinus. Moreover, the junction between transverse and sigmoid sinus is gradual, with transitional dural zone seen over superior and inferior limb and not at a focal point. Earlier studies have addressed the distances between various bone landmarks and the junction but not the “angle.” We have used the technique of mapping the sinus and defining the “GENU” of the transverse sigmoid junction so that it can aid the vertical extension of suboccipital craniotomy.

Methods: Eight sides of cadaver skulls were studied. The asterion and the zygoma root (ZR) were exposed. These two points were connected with a straight line. A line was drawn orthogonal to this, passing the asterion. Distances between several surface landmarks were measured, like asterion and ZR, asterion to digastrics point (DG), asterion to spine of Henle, and asterion to mastoid tip. The entire transverse and sigmoid sinus was exposed .The thinning of dura was taken as the junction point, and it was marked on both superior and inferior limbs. The entire sinus complex was mapped using the x-y coordinate system. The reference point was asterion, which was set as zero.

Results: Distances between the various surface landmarks were measured. There was no significant difference between left and right side. The GENU of the transverse sigmoid region was defined where the bend angle starts increasing from 30 degrees and end where the sigmoid vertical limb takes a straight trajectory. The mean angle between the transverse and the sigmoid sinus was 128.42° (SD, 8.7°; range, 115°–138°); the mean angle that the vertical sigmoid limb made with the y axis (orthogonal to ZR-A line) was 46.5° on the right side and 50.1° on the left side. Average thickness of the GENU was 8.5 mm on left side and 8.4 mm on right side. In 85.7% cases, asterion overlay the sinus.

Conclusions: Our study focuses on the importance of the GENU of the transverse sigmoid sinus, as it is paramount to the operating surgeon to know the orientation of the vertical limb of the sigmoid sinus while performing the suboccipital craniotomy. A strategic burr hole can be placed on the asterion. From the lower part of the burr hole, at an angle of not more than 45 degrees to the y axis, the vertical extension can be safely made without injuring the sigmoid sinus.