Brain shift is a well-known phenomenon, and it is now clear that the amount of subdural air entered in the skull during the procedure has a negative impact on the precision of both navigation and stereotactic systems based on preoperative image data [14, 15, 16]. Long-term efficacy of DBS may rely on even a modest amount of pneumocephalus [11, 12, 15, 17, 18]. Brain shift tends to displace targets in the posterior and superior directions, resulting in aberrant pathway activation from pre-operative predicted plan . Hill et al. calculated a median brain surface shift after incising the dura ranging from 0.3 to 7.4 mm. Other investigators reported a deviation of up to 4 mm shift of subcortical structures [10, 18, 19].
It is a common belief among surgeons that air influx, rather than being a fast process, takes place over time. As a consequence, longer operative lengths are thought to come at a cost of greater amounts of pneumocephalus. To counteract further air accumulation over time, many centers adopted different strategies like the use of burr hole sealants (like fibrin glue or bone wax), reducing burr hole diameter, and direct dural puncture to reduce CSF egress [20, 21, 22]. Yet, although some studies reported a decrease in air inflow, such approaches did not prevent pneumocephalus from forming [14, 22]. This can be explained by the fact that most of the cerebrospinal fluid loss (which is subsequently replaced by subdural gas) is likely to occur immediately after incising the dura. Indeed, considering fluid dynamics, after burr hole trepanation and dural piercing, the volume of CSF above this point will flow out, pushed by gravity.
Considering that most DBS are performed with the patient lying flat in a supine position (or with slight back upward inclination), the amount of fluid that is lost during surgery according to this theory coincides with that filling subdural spaces from the most posterior point of the burr holes to the frontal poles.
In line with this concept, we found no evidence of association between operation time and volume of pneumocephalus at Institute 1, when using a bivariate correlation. Similar findings have been reported by Ko et al . However, these results have the important flaw of considering time from anesthetic records which, by definition, does not automatically mirror solely the amount of time the dura is open.
To address this problem, we adopted two different approaches. By means of the first approach, we compared patients of Institute 1 who underwent DBS procedures that included intraoperative recordings with those who were implanted by direct image-based targeting. Because the use of recordings inevitably prolongs the operative time while the dura is opened (which was confirmed by the significant difference in mean operative time between the two groups), such strategy is robust to this bias. However, the group of patients who underwent recordings had only a modestly higher amount of intracranial air, which was not statistically significant when performing the confrontation between the two groups.
Further complicating the matter, it has to be pointed out that a significantly higher proportion of patients that were recorded during the procedure had bilateral implantations compared to the group of individuals who had direct image-implantation. Considering that bilateral implantations presented a greater mean air volume than monolateral, the side of surgery could potentially act as a confounding factor. From a theoretical point of view, it is possible that the falx could act as a physical barrier preventing part of liquoral egress from the side controlateral to the burr hole to the outside of the skull during monolateral implants. Nonetheless, because right and left subarachnoid spaces are in direct communication, such block has limited influence on the total amount of CSF lost with the patient in supine position. In support of this thesis, the difference in volume of gas between monolateral and bilateral implants did not reach statistical significance in our study.
With the second approach, we performed a separate analysis on patients of Institute 2, where the operative times in the surgical reports are reported from the moment of the first skin incision to the termination of the skin suture. Because the amount of time needed to perforate the dura from the initial incision may be considered (in a first approximation) similar, such computation is more robust to this bias. Indeed, also with this second approach no correlation was found between volume and time, confirming previous results. It is important to note however, that the number of cases was significantly lower compared to Institute 1.
Another important problem is the one related to brain atrophy. As mentioned earlier, because the volume of CSF above the burr holes is the one being subject to outflow, patients with higher degrees of brain atrophy are at increased risk for developing pneumocephalus, as they have a higher proportion of CSF/brain parenchyma than their normotrophic counterparts .
Because CSF egress is a gravity-dependent phenomenon, planning the surgical procedure placing the burr holes at the highest point of the skull, may be a useful solution to minimize the quantity of gas which fills the subdural space . Although the position of the burr hole can vary only up to a certain point, it is possible to regulate the inclination of the head by changing the patient positioning. Utilizing a semi-sitting position may be advantageous from this point of view because it relocates the burr holes on top of the skull. In addition to minimizing the quantity of CSF on top of the burr hole, the semi-sitting position changes the orientation of the brain in relation to the force of gravity. Consequently, air would accumulate on top of the skull concavity resulting in a superior-to-inferior force.