Pearls
In the retrosigmoid approach, the transverse sinus, sigmoid sinus, and transverse-sigmoid junction are identified to prevent venous injury.
A burr hole that leaves 5mm of clearance superiorly to a line connecting the inion to the root of zygoma at the level of the asterion is unlikely to encroach on the transverse sinus.
The asterion is the confluence of the temporal, occipital, and parietal bones.
Details
I have yet to be in an operating room where puns are considered to be funny (excepting when the attending makes the joke, in which case the sub-I chuckles politely but not loudly while everybody else shifts around pretending they did not hear). BUT if they were funny, "What are you asterion at?" would be the way to go in a retro-sigmoidal approach.
For better or worse, I have yet to be pimped on surface anatomy for the retrosigmoid approach, but I'll give you the drawing anyway just in case. See below:
In this approach, it is important to identify and expose the transverse and sigmoid sinuses in order to maximize the operative corridor and prevent venous injury. Three structures that can be used to identify the location of these structures are the inion, asterion, and root of zygoma. The transverse sinus has been described to follow the inion-zygoma line (IZ). The transverse sigmoid junction is said to sit beneath the asterion - the point at which the parietal, occipital, and temporal bones meet. It also classically the case that an emissary vein emerges from the transverse sigmoid junction.
Ultimately, the goal is to place a burr hole such that it does not plunge straight into the sinus. For that reason, a landmark that does not perfectly overly the the sinuses is helpful to prevent accidental injury. Based on this paper reporting the relationship of these structures based on CT, the IZ line is on average 5-6mm of caudal to the TS (more on the left side as the right sinus tends to be larger). Moreover, none of the TS were found more than 5mm below IZ. The authors also report that the the asterion was at the TSF in 74% of cases and below it in 8% of cases on the left and 6% of cases on the right. An emissary vein was present 29% of the time.
Summary
The TS is unlikely to be found more than 5mm below the IZ line. It is important to remember, however, that the drill has a diameter (about a centimeter), so a hole that is centered 5mm below the the IZ will reduce this margin. Back in my engineering days, we would call this material lost due to the width of the cutting tool 'kerf.' I always liked this word. #BringBackKerf
One last tidbit: the sinuses may have formed an impression in the bone of the skull, so the bone overlying it may be thinner than the surrounding bone. Take care when removing bone not to plunge too deep without knowing what is beneath!
Comentarios