Complete hemispheric exposure vs. superior sagittal sinus sparing craniectomy: incidence of shear-bleeding and shunt-dependency

Purpose Decompressive hemicraniectomy (DC) has been established as a standard therapeutical procedure for raised intracranial pressure. However, the size of the DC remains unspecified. The aim of this study was to analyze size related complications following DC. Methods Between 2013 and 2019, 306 patients underwent DC for elevated intracranial pressure at author´s institution. Anteroposterior and craniocaudal DC size was measured according to the postoperative CT scans. Patients were divided into two groups with (1) exposed superior sagittal sinus (SE) and (2) without superior sagittal sinus exposure (SC). DC related complications e.g. shear-bleeding at the margins of craniectomy and secondary hydrocephalus were evaluated and compared. Results Craniectomy size according to anteroposterior diameter and surface was larger in the SE group; 14.1 ± 1 cm vs. 13.7 ± 1.2 cm, p = 0.003, resp. 222.5 ± 40 cm2 vs. 182.7 ± 36.9 cm2, p < 0.0001. The SE group had significantly lower rates of shear-bleeding: 20/176 patients; (11%), compared to patients of the SC group; 36/130 patients (27%), p = 0.0003, OR 2.9, 95% CI 1.6–5.5. There was no significant difference in the incidence of shunt-dependent hydrocephalus; 19/130 patients (14.6%) vs. 24/176 patients (13.6%), p = 0.9. Conclusions Complete hemispheric exposure in terms of DC with SE was associated with significantly lower levels of iatrogenic shear-bleedings compared to a SC-surgical regime. Although we did not find significant outcome difference, our findings suggest aggressive craniectomy regimes including SE to constitute the surgical treatment strategy of choice for malignant intracranial pressure.

optimal craniocaudal diameter remains unspecified. An agressive craniectomy, including maximal decompression with exposure of superior sagittal sinus, might be a risk factor for shunt-dependency. This study aims to analyze the size related complications of two different surgical techniques of DC according to anatomical landmarks: patients undergoing complete hemispheric exposure vs. those without exposure of the superior sagittal sinus (SSS). We mainly focused on the incidence of shunt-dependent hydrocephalus and the incidence of shear-bleeding at the edge of DC.

Methods
A retrospective single center study of patients undergoing DC from 02/2013 to 10/2019 was performed. The extent of the craniectomy was analyzed using early postoperative CT scans with DICOM Viewer software. We measured the anteroposterior diameter as proposed by Flint et al. [8]. The surface of the craniectomy was calculated using the De Bonis equation [9]. We divided the patients in two groups based on anatomical landmarks: (1) patients with complete exposure of superior sagittal sinus (SE) and (2) patients without exposure of superior sagittal sinus (SC) (Figs. 1 and 2).

Surgical techniques
The head of the patient is positioned with his head rotated parallel to the floor, the side of the craniectomy pointing upwards. Mayfield skull clamp is used to fix the head. By unilateral decompressive hemicraniectomy, the half of the head is shaved. The incision in the shape of a reverse question mark is starting at the tragus and continuing slightly across the midline. The trauma flap is created [10].
Group 1 (SE)-after exposing the skull, sagittal suture is identified and two burrholes are placed on the sagittal suture, where SSS is expected, determining the medial craniectomy edge. The sagittal sinus was exposed by craniectomy on sagittal suture. Usually, full exposure of the hemisphere including the exposure of the SSS is reached. Duraplasty is not performed.
Group 2 (SC)-after identifying the sagittal suture, the burrholes are placed ipsilateral on the site of the craniectomy, leaving 1-2.5 cm distance between the edge of the craniectomy and sagittal suture. Both sinus and bridging veins are covered with bone. Duraplasty is also not performed.
The sinus exposure was performed according to attending neurosurgeon.

Shear-bleeding
All available postoperative CT scans were analyzed for the incidence of newly developed intraparenchymal hemorrhage at the edge of the craniectomy (Fig. 3).

Shunt-dependent hydrocephalus
The focus was given on patients who developed shuntdependent hydrocephalus after DC and underwent secondary shunt implantation.

Further postoperative complications
We retrospectively analyzed and compared perioperative blood-loss, the need of red blood cell transfusions, wound healing disturbances, operating time, air embolism, sinus thrombosis and intraoperative sinus injury. We divided the outcome of the patients as favorable (mRS ≤ 3) and unfavorable (mRS > 3).

Statistics
A Fisher's exact test was used to demonstrate the association between the sinus exposure and development of shearbleeding and shunt-dependent hydrocephalus. Values with p < 0.05 were considered to be significant. Finally, propensity score adjustment was performed to analyze the risk-bleeding risk factors between the groups.

Anteroposterior diameter and surface
Anteroposterior diameter of the DC was overall of 13.7 cm ± 1.2 cm. The AP Diameter was significantly larger in SE Group compared to SC Group; 14.1 ± 1.1 cm vs. 13.7 ± 1.2 cm, p = 0.003.
There was a significant difference in surface of the resected bone between both groups; SE Group 222.5 ± 40.0 cm 2 vs. SC Group 182.7 ± 36.9 cm 2 , p < 0.0001. For detailed information, see Table 2. Above right-axial CT scan of patient from SC group, the arrow is pointing on shearbleeding. Below left-preoperative CT scan of patient from SC group. Below right-postoperative CT scan with newly developed shear-bleeding In exploratory analysis, the craniectomy type seems to be the only statistically significant factor associated with the incidence of shear-bleeding (Table 3).
Most of the shear-bleeding lesions were localized near the medial boundary of the craniectomy. The incidence of these medially localized lesions was significantly lower in SE group with 6.8% vs. 20.8% in SC group; p = 0.0003 (Fig. 4).

Underlying diagnosis
SC was associated with higher incidence of shear-bleeding in all underlying conditions leading to DC. The limitation of this analysis is the low number of patients in each subgroup. See Fig. 5 and Table 4 Shunt-dependency 43 patients developed secondary drainage-dependent hydrocephalus and underwent a shunt implantation.

Intraoperative blood-loss
None of the analyzed groups presented itself with higher intraoperative bleeding volume. The blood loss was lower as 500 ml in 26.7% of cases.
In the logistic regression with shear-bleeding as dependent variable, the type of craniectomy as well as the propensity score as independent variables. The type of craniectomy remained significantly associated (Table 5).

Discussion
We retrospectively analyzed patients that had undergone DC in the course of surgical treatment of pathologically raised intracranial pressure at our institution. We compared two surgical approaches of craniectomies with or without exposure of SSS. The SE group showed significantly lower incidence of shear-bleeding. No difference in shunt-dependency was noted between the groups.

Shear-bleeding
Our study confirmed the experimentally discussed association between the parenchyma injury around the craniectomy edges and the size of the DC [18]. We observed higher rates of shear-bleedings localized medially in the SC group. As demonstrated above, centrally localized shear-bleeding can lead to severe clinical symptomatic.
Compared to Wagner et al. [5], we did not include the patients with the increased size of ICH.
We did not classify the progression of already presented ICH as a shear-bleeding, but as "relief-effect" bleeding. "Relief-effect" bleeding is not a complication corresponding directly to the size of the DC. It is rather associated with the procedure itself as a result of sudden pressure relief during the DC, loss of tamponade effect and rapid expansion of the cerebral parenchyma [7,19].
As expected, the SE group showed significantly lower incidence of the shear-bleeding than the SC group.

Shunt-dependency
DC has been previously reported to be a risk factor for hydrocephalus development. The idea suggested by De Bonis et al. [15] that the incidence of shunt-dependent hydrocephalus increases if the SSS and the bridging veins are exposed has not been confirmed in our cohort. We observed the same distribution of shunt-dependency among both groups regardless of the DC size. The incidence of shunt implantation corresponds to results already reported by other authors reaching from 14.8 to 22.5% according to underlying pathology [14,20,21].
The proposed mechanism of hydrocephalus by DCs going near to the midline is the interference with pulsatile CSF resorption [22] . DCs being "too big" are reported to interfere with the CSF flow and the occurrence of post-DC hydrocephalus signs are described in 88% of such cases [23]. In our cohort, the exposure of SSS was not accompanied with elevated level of shunt-dependency.

Limitations
The study has several limitations. Data acquisition was retrospective, based on a single-center experience. Furthermore, non-randomized setting left the decision about the DC-size on the attending neurosurgeon. The statements of clinical outcome are limited by heterogeneous cause and seriousness of the underlying diagnosis that lead to the DC [18].

Conclusions
Complete hemispheric exposure, and therefore larger DC size, seems to be associated with smaller likelihood of shear-bleeding, without the elevation of the incidence of procedure-related complications.
Author contributions MV: study design, data collection and interpretation, manuscript writing. MS, VB, PS and HV: critical revision. CB: statistical evaluation. EG: study design, data interpretation, proofreading, literature search.
Funding Open Access funding enabled and organized by Projekt DEAL. There are no financial conflicts of interest to disclose.
Availability of data and materials Data available on request due to privacy/ethical restrictions.
Code availability Not applicable.

Declarations
Conflict of interest The authors whose names are listed below certify that they have NO affiliations with or involvement in any organization or entity with any financial or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Consent for publication Not applicable.
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