Venous Anatomy and Embryology of the Tentorium.

The venous system within the tentorium cerebelli has diverse communications with the cerebrum, cerebellum, and brainstem, and a thorough knowledge of the anatomy of this venous system is necessary for the safe and successful conduct of neurosurgical procedures and neuroendovascular treatments. This review outlines the embryologic development of the tentorium cerebelli and its adjacent venous structures, and the fundamental structure and classifications of the tentorial venous sinuses based on the embryologic background. The term tentorial sinus refers to any venous channel running within the tentorium cerebelli. These channels collect blood via bridging veins from the supratentorial and infratentorial compartments and drain into major dural sinuses such as the transverse-sigmoid sinus, straight sinus, torcular Herophili, and in some cases, the superior petrosal sinus. In addition, there are small intratentorial veins that drain blood directly from the tentorial tissue into a dural sinus without a bridging vein. The courses and drainage patterns of these tentorial sinuses are variable, and understanding these variations can influence the success and safety of neurosurgical procedures or neuroendovascular treatments. Embryologically, the tentorium cerebelli begins to form as the meningeal layer of the dura folds inward with the expansion of the cerebrum, and in the process, incorporates surrounding dural venous plexuses and multiple primitive brain veins to form the dural venous sinuses. Because the tentorial sinuses develop during this complex embryonic reorganization, varying degrees of regression or persistence of the fetal venous channels lead to diverse adult venous drainage patterns and anatomic variations. Various classification schemes for the tentorial sinuses have been proposed based on the morphological features or sources of inflow and routes of outflow, but in essence, they can be divided into (1) a type that receives cerebral or cerebellar venous blood via bridging veins and drains into the major sinuses, and (2) a type consisting of small veins that arise within the tentorial leaf and drain directly into a major sinus. Each classification highlights the characteristic intratentorial venous pathways. Clinically, recognizing the tentorial sinuses and the surrounding venous system is important, and an understanding of their embryologic background is useful for this purpose.