Venous inversion flows is when blood goes in the opposite direction that is was meant to, that is, back into the brain instead of out of it. The part of the skull that contains the brain is called the cranial vault. Skull shape and the design of the cranial vault affects the layout, length, pitch and attachment angles of the veins of the brain called dural sinuses. It is the layout of the posterior fossa portion of the cranial vault that affects the impact of venous inversion flows, called backjets, into the brain.
The outer white lines in the brain scan to the left represent the borders of the posterior fossa which contain the brainstem and cerebellum – two key structures in the brain. The brain scan is from a paper published in Cerebrospinal Fluid Research on 12/18/05 by Raymond F. Sekula, Jr. et al. titled “Dimensions of the posterior fossa in patients symptomatic for Chiari 1 malformamtion but without cerebellar tonsilar descent.”
The dural sinuses are the large drainage veins of the brain. In the brain scan above several dural sinues are located along the two rear white lines that form the back side of the posterior fossa. The top white rear line, which is the top of the posterior fossa, is also the location of the tentorium cerebelli (see the word tentorium). The tentorium cerebelli is made of connective tissue and covers the posterior fossa separating it from the compartments above. An important dural sinus, called the straight sinus follows the course of the tentorium cerebelli. The pitch and angle between the top and bottom lines that represent the rear border of the posterior fossa and the course of the dural sinuses may play a role in the formation of the lesions seen in multiple sclerosis due to venous inversion flows.
In contrast to the image above which shows the borders of the posterior fossa, the image below is an MRI of the veins of the brain injected with dye. It’s called a venogram. The S-like structure near the base of the skull on the right side is called the sigmoid sinus. The long section going to the top of the head is called the superior sagittal sinus. In between these two sections is a shorter section called the transverse section which starts at the top of the S shape and ends at the beginning of the superior sagittal sinus. The sigmoid (S-shaped) and transverse sinus make up the bottom portion of the posterior fossa represented by lines in the first image.
At the junction of the transverse sinus and superior sagittal sinus is a small sinus that runs upward and forward at about forty-five degrees in this individual. It’s called the straight sinus and was mentioned above. In some people the straight sinus’s attachment varies and can be close to a ninety degree angle. The length of the transverse sinus varies as well. As mentioned above the straight sinus follows the cover over the top of the posterior fossa called the tentorium cerebelli represented by the top line in the first brain scan above.