Postoperative Pneumocephalus as a Predictor of Recurrence in Chronic Subdural Hematoma: A Propensity Score - Matched Retrospective Observational Study

Summary

This study was undertaken to systematically evaluate the relationship between postoperative pneumocephalus and recurrence in patients undergoing burr-hole drainage for chronic subdural hematoma (CSDH).

Abstract

Chronic subdural hematoma (CSDH) is a common neurosurgical condition with a high recurrence rate after burr‑hole drainage. Postoperative pneumocephalus is frequently observed, but its relationship with recurrence remains unclear. This retrospective observational study evaluated whether postoperative pneumocephalus volume predicts CSDH recurrence. Patients who underwent first‑time burr‑hole drainage for CSDH between January 2021 and June 2025 were included and classified according to head CT findings within 24 h after surgery. Propensity score matching (PSM; 1:1, caliper 0.02) balanced baseline characteristics (age, sex, hematoma side, antiplatelet/anticoagulant use), yielding 150 matched patients (75 per group). The primary outcome was ipsilateral hematoma recurrence requiring reoperation within 6 months. Pneumocephalus volume was measured using the Tada formula, with inter‑observer reliability assessed by the intraclass correlation coefficient (ICC = 0.964, 95% CI: 0.941–0.978). The recurrence rate was significantly higher in the pneumocephalus group than in the non‑pneumocephalus group (22.7% vs. 5.3%; OR = 5.20, 95% CI: 1.66–16.32; p = 0.002). Multivariate logistic regression showed that the presence of pneumocephalus was independently associated with recurrence (OR = 3.26, 95% CI: 1.45–7.30, p = 0.004), and each 1 mL increase in volume was associated with a 9% higher risk (OR = 1.09, 95% CI: 1.02–1.15, p = 0.008). ROC analysis yielded an AUC of 0.754 (95% CI: 0.645–0.864), with an optimal cut‑off of 12.5 mL (sensitivity 76.2%, specificity 70.5%). Bootstrap internal validation confirmed stability (mean cut‑off 12.6 mL, optimism‑corrected AUC 0.745). In conclusion, postoperative pneumocephalus volume is associated with CSDH recurrence after burr‑hole drainage and may help identify patients needing closer follow‑up; however, prospective multicenter validation is required before any specific volume threshold can be adopted in clinical practice.

Introduction

Chronic subdural hematoma (CSDH) is a common neurosurgical condition whose incidence has been increasing with the aging population^1,2. Its pathophysiology is thought to involve tearing of bridging veins after mild head trauma, leading to blood accumulation in the subdural space, local inflammation, and eventual formation of a hematoma capsule^3,4. Because of repeated microbleeding and fibrinolysis hyperactivity, most patients cannot absorb the hematoma spontaneously and require surgical intervention^5,6. Single‑ or double‑hole burr‑hole drainage is the first-line treatment due to its minimal invasiveness and proven efficacy^7,8. However, postoperative recurrence remains a major challenge, with reported rates ranging from 5% to 30%^9,10.

Numerous risk factors for recurrence have been identified, including older age, antiplatelet or anticoagulant use, bilateral hematoma, and imaging features such as mixed density or septation on preoperative CT^11,12. These factors suggest that recurrence is multifactorial, involving coagulation status, hematoma structure, and the postoperative healing environment^4,13. In postoperative imaging, pneumocephalus—accumulation of air in the cranial cavity—is frequently observed, mainly due to residual irrigation fluid and communication between the drainage system and the external environment^14,15. However, its clinical significance remains debated. Some studies suggest that small amounts of pneumocephalus are benign and resolve spontaneously^16,17; while others report that large volumes may delay brain re‑expansion and predispose to recurrence^16,18. These conflicting findings likely stem from small sample sizes, inadequate confounding control, or a lack of quantitative volumetric analysis¹⁹.

The optimal management of postoperative pneumocephalus is also uncertain, with practice varying between active positioning to promote gas expulsion and more conservative approaches. Clarifying the relationship between pneumocephalus volume and recurrence would allow early postoperative CT to guide risk stratification and follow‑up intensity. Against this background, the current study was undertaken to systematically evaluate the relationship between postoperative pneumocephalus and recurrence in patients undergoing burr-hole drainage for CSDH. This study incorporates several methodological features that build upon prior investigations, including propensity score matching to minimize baseline confounding, quantitative volume measurement to explore a potential threshold, and inclusion of the neutrophil-to-lymphocyte ratio (NLR) to assess inflammatory pathways. In addition, bootstrap internal validation and sensitivity analyses using Firth penalized and mixed-effects logistic regression were applied to address overfitting, rare events, and potential clustering. These analytical choices were intended to provide a controlled local replication of the known association and to generate hypotheses for future prospective studies.

Protocol

Ethical statement
The study protocol was reviewed and approved by the Sanming First Hospital Affiliated to Fujian Medical University Ethics Committee (No. 2026-38), and the investigation was conducted in accordance with the ethical standards set forth in the Declaration of Helsinki. As this is a retrospective observational study without any intervention, the collected data were generated during routine clinical diagnosis and treatment, without adding any additional burden or risk to patients’ examinations. Therefore, an application for exemption from informed consent was submitted, and all patients’ personal information has been kept strictly confidential. The research results will be used only for academic publication and will not disclose any information that can identify an individual. All materials and software used in this study are listed in the Table of Materials. For items marked “generic” or “standard hospital supply,” specific catalog numbers are not applicable because these are routine consumables available from multiple suppliers. The CT scanner model and software versions are specified to ensure reproducibility. For manual calculations (e.g., the Tada formula) and standard statistical tests performed in SPSS/R, no separate catalog number is required.

Research subjects
A single-center retrospective observational design was adopted for this study. Patients with CSDH who received burr-hole drainage in the neurosurgery department from January 1, 2021, to June 30, 2025, constituted the study population. After applying strict inclusion and exclusion criteria, data from qualified patients were retrieved and prepared for subsequent statistical analysis. To evaluate the relationship between postoperative pneumocephalus and hematoma recurrence, and to mitigate confounding bias, PSM was applied to achieve baseline balance in the original cohort. Propensity scores were derived from a logistic regression model with postoperative pneumocephalus as the dependent variable, adjusted for age, sex, hematoma side, and history of antiplatelet or anticoagulant medication. These covariates were selected based on their known associations with either CSDH recurrence or postoperative pneumocephalus, as documented in prior studies. The matching method used was nearest-neighbor matching with a cutoff of 0.02, and a 1:1 non-replacement matching was performed. After matching, a total of 150 patients were included (75 in the pneumocephalus group and 75 in the non-pneumocephalus group), as shown in Figure 1.

Exclusion and inclusion criteria
Inclusion criteria^20,21: (1) age 18 years and above; (2) confirmed diagnosis of unilateral or bilateral CSDH through head CT or MRI examination; (3) underwent the first burr-hole drainage surgery in the hospital; (4) had a re-examination of head CT within 24 h after the surgery and the imaging data were completely preserved; (5) complete postoperative clinical follow-up data were available, and the follow-up period was no less than 6 months.

Exclusion criteria^22,23: (1) patients with intracranial tumors, aneurysms, or arteriovenous malformations and other intracranial space-occupying or vascular diseases; (2) acute or subacute subdural hematoma (defined as the time from injury to surgery being less than 14 days); (3) those with a history of ipsilateral burr hole drainage or craniotomy; (4) those who underwent reoperation due to non-hematoma recurrence after surgery; (5) those with missing key variables in clinical or imaging data, making analysis impossible. Patients with bilateral hematomas who undergo concurrent bilateral burr hole drainage are included as research subjects, with each patient contributing a single observation. Bilateral status (unilateral vs. bilateral) is recorded as a covariate and is included in the propensity score matching and regression analyses. No patient contributed more than one data point, thereby preserving statistical independence of observations.

Handling of bilateral hematomas
Patients with bilateral hematomas who underwent concurrent bilateral burr hole drainage were included as research subjects, with each patient contributing a single observation. Bilateral status (unilateral vs. bilateral) was recorded as a covariate and included in the propensity score matching and regression analyses. No patient contributed more than one data point, thereby preserving statistical independence of observations. To further address any potential residual clustering, a sensitivity analysis using mixed‑effects logistic regression with a random intercept for each patient was performed (see Statistical methods).

Research plan
Data were extracted from the hospital medical record system and the imaging archive system, including: (1) general demographic characteristics (age and gender); (2) clinical data (history of hypertension, diabetes, antiplatelet/anticoagulant use, preoperative Markwalder neurological function grading score); (3) imaging data (hematoma side, CT density type, preoperative midline displacement distance, and presence, volume, and distribution of pneumocephalus on postoperative 24 h CT); (4) laboratory indicators (neutrophil-to-lymphocyte ratio in peripheral blood 24 h after surgery); and (5) perioperative data (surgery duration, irrigation volume, postoperative drainage tube retention time, and total drainage volume). The volume of pneumocephalus was quantified using the Tada formula

      (1)

where A, B, and C are the largest diameters on axial, sagittal, and coronal planes. For multiple separate gas pockets, each was measured individually, and the volumes were summed. All measurements were performed independently by two attending neurosurgeons; the mean value was used as the final data point. If the difference between measurements exceeded 10%, a third attending physician reviewed the case to determine the final measurement.

Surgical technique and postoperative management
All surgeries were performed by one of three attending neurosurgeons following a standardized institutional protocol. A single burr-hole was placed at the point of maximal hematoma thickness as identified on preoperative CT (using intraoperative CT or neuronavigation when available). The dura and outer membrane were coagulated and opened in a cruciate fashion. The hematoma cavity was irrigated with warmed normal saline using a continuous siphon‑assisted irrigation technique until the effluent was clear; the total irrigation volume was recorded for each patient. A 14-Fr silicone subdural drain was then inserted and tunneled subcutaneously. The drain was connected to a passive gravity drainage system (no active suction). Postoperatively, patients were positioned supine or flat with the head of the bed maintained at 0–15° for the first 24 h. The drainage bag was kept at the level of the external auditory meatus to avoid excessive negative pressure. The drain was clamped for 4 h every 8 h starting from the first postoperative day and was removed when the daily drainage volume was less than 30 mL for two consecutive days. No closed drainage system (e.g., CDS) was used in this cohort.

Follow-up and outcome definition
The primary outcome was ipsilateral CSDH recurrence requiring reoperation within 6 months of the index surgery. Recurrence was diagnosed if patients developed new or worsened neurological symptoms and CT showed reaccumulation or significant enlargement of the residual hematoma, with a clinical decision to reoperate^24,25. Follow-up data were collected for all patients for at least 6 months. The study database was locked on June 30, 2026; by that date, the last enrolled patient (June 30, 2025) had completed at least 6 months of follow‑up.

Surgical workflow
The key surgical and imaging procedures described in this protocol are summarized below in six sequential steps: (1) Patient positioning and burr-hole placement - positioning of the patient under general anesthesia, identification of the point of maximal hematoma thickness on preoperative CT (using intraoperative CT or neuronavigation when available), and creation of a single burr-hole. (2) Irrigation technique - cruciate opening of the dura and outer membrane, followed by continuous siphon-assisted irrigation of the hematoma cavity with warmed normal saline until the effluent is clear. (3) Subdural drain insertion - insertion of a 14-Fr silicone drain into the subdural space, subcutaneous tunneling, and connection to a passive gravity drainage system. (4) Postoperative imaging assessment – acquisition of a non‑contrast head CT scan within 24 h after surgery, identification of pneumocephalus, and interpretation of its distribution and extent. (5) Volumetric measurement using the Tada formula - step-by-step demonstration of measuring the three orthogonal diameters (A, B, C) on axial, sagittal, and coronal planes, followed by calculation of volume using the formula

     (2)

For multiple separated gas pockets, each collection is measured individually, and the volumes are summed. (6) Clinical follow‑up protocol – neurological assessment using the Markwalder grading scale and criteria for reoperation in case of recurrence. The video includes on‑screen annotations and narration to facilitate reproducibility. Total video duration is approximately 6 minutes.

Key research indicators
Postoperative pneumocephalus is the core exposure factor in this study. It is defined as the appearance of gas-density shadows in any part of the brain on a plain CT scan performed within 24 h after craniotomy and drainage. The volume of postoperative pneumocephalus is calculated using the Tada formula. For multiple pneumocephalus, the sum of the gas volumes in each area is taken as the total pneumocephalus volume. All imaging measurements were independently completed by two neurosurgical attending physicians, and the average value was used as the final analysis data. Inter-observer reliability was assessed using the intraclass correlation coefficient (ICC, two-way random effects model for absolute agreement). Recurrence events are the primary outcome measure in this study. It is defined as the reaccumulation of CSDH on the same side within 6 months after surgery, leading to aggravation of neurological symptoms and requiring reoperation based on clinical judgment. The recurrence was determined from outpatient follow-up or hospital records during the follow-up period and was confirmed by at least two neurosurgical attending physicians to ensure the accuracy of the outcome determination.

Secondary research indicators
It covers multiple dimensions, including general demographic characteristics, clinical baseline data, preoperative imaging features, laboratory indicators, and perioperative data, primarily to describe the study population, conduct PSM, and explore potential confounding factors or effect modifiers. General demographic characteristics include age and gender. Clinical baseline data include a history of hypertension and diabetes, a history of antiplatelet or anticoagulant medication use, and a preoperative Markwalder neurological function grading score, which assesses the degree of preoperative neurological dysfunction, ranging from 0 to 4^26,27. Preoperative imaging features include the side of the hematoma, the hematoma density on CT, and the preoperative midline shift distance. The hematoma density type is classified based on imaging findings as low density, equal density, high density, or mixed density; the midline shift distance is measured relative to the septum pellucidum or the pineal gland, and the vertical distance of its deviation from the midline is measured.

The laboratory indicator was the 24 h postoperative neutrophil-to-lymphocyte ratio derived from peripheral blood counts. This indicator serves as a surrogate marker for the overall inflammatory response and is calculated from routine blood test results. Perioperative data includes the duration of surgery, the volume of intraoperative irrigation, the retention time of the postoperative drainage tube, and the total volume of postoperative drainage. The duration of the surgery starts from the incision until the skin is sutured; the amount of intraoperative irrigation is recorded as the total volume of saline used to irrigate the hematoma cavity; the retention time of the postoperative drainage tube refers to the number of h from the end of the surgery to the removal of the drainage tube; the total volume of postoperative drainage is recorded as the total volume of fluid drained from the end of the surgery to the removal of the drainage tube. The collection and recording of all these indicators follow unified operational norms and data-collection forms to ensure data completeness and consistency.

Sample size calculation
This investigation included 150 patients, equally divided into the pneumocephalus and non-pneumocephalus groups (n = 75 per group). The observed between-group difference in recurrence rates was 17.4 percentage points (22.7% compared with 5.3%), with a corresponding 95% confidence interval of 7.8% to 27.0%. The lower limit of the interval exceeded the clinically significant minimum difference (typically set at 5%), indicating that the effect was clinically significant and the estimate was relatively robust. In the multivariate analysis, the OR for postoperative pneumocephalus was 3.26 (95% CI, 1.45–7.30). The interval was consistently above 1.0, indicating a strong positive association between pneumocephalus and recurrence and ruling out accidental factors. For every 1 mL increase in pneumocephalus volume, the corresponding OR value was 1.09, with a 95% confidence interval of 1.02 to 1.15. The interval was narrow and did not include 1.0, suggesting a precise and reliable dose-response relationship. Based on the observed effect size, the post hoc test power calculation was performed. The power of this study was 0.92, indicating that when the true effect size matched the observed value, there was a 92% probability of detecting a difference between the groups with the current sample size. ROC analysis demonstrated an area under the curve of 0.754, with a 95% confidence interval of 0.645–0.864 and an interval width of 0.219. This indicates that the predictive efficacy estimate was moderate and that the accuracy was acceptable. Based on the three indicators of effect size estimation, confidence interval width, and test power, the core conclusion of this study has good statistical reliability and clinical applicability.

Statistical methods
All statistical analyses were conducted using SPSS. Normality was tested for continuous variables: normally distributed data are reported as mean ± SD and compared using independent t-tests; non-normally distributed data are reported as median (IQR) and compared using Mann-Whitney U tests. Categorical variables are expressed as n (%) and compared using chi‑square or Fisher’s exact tests. Because only 21 recurrence events occurred, the multivariate models were deliberately kept parsimonious to avoid overfitting. Based on clinical knowledge, each model included the primary exposure variable (either binary pneumocephalus or continuous volume) and two pre‑specified covariates: bilateral hematoma and history of antiplatelet/anticoagulant medication. No automated variable selection was used.

ROC curve analysis evaluated pneumocephalus volume for predicting recurrence, with AUC and 95% CI reported; the optimal cut-off was determined by the Youden index. For the propensity score‑matched cohort, paired analyses were performed: McNemar test for categorical data and paired t-test or Wilcoxon signed‑rank test for continuous data. Conditional logistic regression was applied to account for matching. Two‑sided tests were used, with statistical significance set at p < 0.05. To internally validate the optimal cut‑off and reduce optimism, bootstrap resampling with 1,000 iterations was performed. In each bootstrap sample, the ROC curve was re‑estimated and the optimal cut‑off recalculated. The 95% CI for the cut‑off was derived from the bootstrap distribution, and the optimism-corrected AUC was computed. To address concerns about rare events (n = 21) and potential overfitting, a sensitivity analysis using Firth’s penalized maximum‑likelihood logistic regression (logistf package in R) was performed. Results were compared with the standard conditional logistic regression.

To address potential clustering from bilateral hematomas, a sensitivity analysis was performed using a mixed‑effects logistic regression model with a random intercept for each patient. The results were consistent with the primary conditional logistic regression (presence of pneumocephalus: OR = 3.18, 95% CI: 1.42–7.12, p = 0.005; pneumocephalus volume: OR = 1.08, 95% CI: 1.02–1.14, p = 0.009), confirming robustness.

Inter-observer reliability for volumetric measurements was assessed using the intraclass correlation coefficient (ICC, two‑way random effects model for absolute agreement). An ICC above 0.75 was considered excellent. The propensity score model included age, sex, hematoma laterality, and antiplatelet/anticoagulant history because these are preoperative factors known to influence both pneumocephalus formation and recurrence. Other potential confounders (e.g., hematoma density, Markwalder grade, perioperative antithrombotic timing) were either adjusted for in multivariate models or examined in univariate analyses. Postoperative or intraoperative variables (e.g., drainage duration, residual hematoma volume, surgical technique, drainage type) were excluded from PSM as they may be mediators rather than pure confounders.

Results

Baseline information
The inter‑observer reliability for postoperative pneumocephalus volume measurement was excellent, with an intraclass correlation coefficient (ICC) of 0.964 (95% CI: 0.941–0.978). Table 1 presents a comparison of baseline characteristics between the two groups after PSM (n = 75 per group). There were no statistically significant differences between the pneumocephalus group and the non‑pneumocephalus group in terms of age (68.45 ± 10.23 years vs 67.89 ± 11.07 years...

Discussion

Postoperative pneumocephalus after CSDH burr-hole drainage has long been controversial. This propensity-score-matched study of 150 patients showed that the presence of pneumocephalus was associated with a 3.26 - fold higher recurrence risk (OR = 3.26, 95% CI: 1.45–7.30) and a dose-response relationship (each 1 mL increase in volume corresponded to a 9% higher risk, OR = 1.09, 95% CI: 1.02–1.15). The optimal cut-off for pneumocephalus volume was 12.5 mL (AUC = 0.754; sensitivity = 76.2%; specificity = 70.5%). ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
14Fr silicone subdural drain	Medical-grade silicone drainage catheter (generic)	14 Fr; used with passive gravity drainage system
Computed Tomography (CT) scanner	Siemens Healthineers	SOMATOM Definition AS; FDA cleared
Data dictionary (variable definitions)	Study team	N/A (created for this study)
Excel (Microsoft Office)	Microsoft Corporation	Version 16.0+; https://www.microsoft.com/microsoft-365/excel
Intraoperative CT / neuronavigation	Siemens Healthineers	Somatom Definition AS with navigation option; used when available
logistf package (Firth penalized logistic regression)	CRAN (R package)	Version 1.26.1; https://CRAN.R-project.org/package=logistf[reference:3]
Mixed effects logistic regression	R (lme4 package)	lme4 package; https://CRAN.R-project.org/package=lme4
Passive gravity drainage system	Standard subdural drainage bag	250 mL capacity; connected to 14Fr drain
R software environment	R Foundation	Version 4.4.x; https://www.r-project.org[reference:5]
SPSS Statistics	IBM Corporation	Version 26.0; https://www.ibm.com/spss[reference:6]
Tada formula (volume calculation)	Manual calculation	N/A (method described in Protocol)
Warmed normal saline (irrigation fluid)	Standard hospital supply	0.9% NaCl, warmed to 37–38 °C