Several factors affect the recurrence of CSDH after surgery, including the general clinical characteristics of the patient, surgical skills, perioperative management, and imaging characteristics, which are closely related to the recurrence5,17,21,22,28. Owing to advancements in research, clinicians can reduce surgical complications by improving the surgical skills and strengthening perioperative management, yet cases of postoperative recurrence continue to be reported. This may be attributed to the structural characteristics of the brain tissue and the pathological characteristics of the hematoma.
The brain tissue is similar to an elastic sponge. A high-quality sponge has good resilience and can re-expand quickly after decompression, whereas poor quality sponges rebound slowly after compression and have poor recruitment effects9. The cord separation of the hematoma cavity is likely to cause poor drainage. Furthermore, the presence of fresh blood in the hematoma fluid indicates that the disease is in the active phase and might be associated with postoperative recurrence6,21. These characteristics can be observed by analyzing the imaging parameters during the perioperative period.
The clinical factors related to RrR were retrospectively analyzed in this study. As reported previously, age (> 65 years) was related to recurrence3,12. The imaging characteristics during the perioperative period play an important role in the assessment of the RrR. The univariate analysis shows that the preoperative CT classification, volume of effusion, midline shift, effusion thickness, and cerebral re-expansion rate after surgery were related to recurrence. These results are consistent with those reported previously2,9,24,28.
Cerebral re-expansion rate is calculated as the change in brain tissue volume before and after surgery. Under the condition of a fixed cranial cavity volume, the effusion and volume change of a hematoma before and after surgery can indirectly reflect the cerebral re-expansion rate11,15. However, some studies defined brain re-expansion rate as the change in the maximal thickness of the hematoma and the maximal thickness of the effusion before and after surgery19,20,25 (Supplemental Table 2). The volumes of the hematoma and the effusion in the formula for the brain re-expansion rate need to be calculated using software, which is not convenient for clinical application. In this study, three formulas were compared based on the volume, maximal thickness, and midline shift ratios. The formulas demonstrated similar predictive effects, especially the ones based on the maximal thickness ratio and the volume ratio (AUC difference = 0.002; p = 0.983; Supplemental Table 5 and Fig. 2). The calculation of the maximal thickness of the hematoma and effusion does not require software assistance and can be conveniently applied in the clinical setting. Therefore, we use the formula based on the maximal thickness ratio as that for the cerebral re-expansion rate in the grading system.
Cerebral atrophy was found to affect the re-expansion of the brain tissue (p = 0.002). In addition, age (> 65 years) and injury time (> 30 days) had a tendency to influence the re-expansion (p = 0.091 and p = 0.057). Multivariate analysis revealed that atrophy was the only factor that affected the re-expansion rate (p = 0.015). Previous studies have found that cerebral atrophy, long injury time (> 30 days), and old age are high-risk factors for CSDH recurrence, and these factors are related to the cerebral re-expansion rate19,20,25. This means that the cerebral re-expansion rate is a hub in the collection of the above risk factors.
The pathological characteristics of CSDH are closely related to the characteristics of the CT imaging23. The density of the images on the CT scan is closely related to hematoma recurrence18. The homogeneous type includes three subtypes (hypodense, isodense, and hyperdense). The separated type is defined as a higher density component under a lower density component, and there is a clear boundary between them. If two components are mixed without a boundary, it is called the gradation type. The laminar type is defined as a hematoma that presents with a dense layer running along the inner membrane. The trabecular type is defined as a hematoma with a low iso-density component and a high-density septum that separates the inner and outer membranes. In the pathophysiology of CSDH, the hypodense and gradation subtypes are considered to have a moderate tendency to re-bleed, and the trabecular type is considered to be the regression stage of these lesions23. The isodense, hyperdense, laminar, and separated types have a high-risk of recurrence28. Conversely, the hypodense, gradation, and trabecular types have a low risk of recurrence.
The data of 242 patients were used to establish a model scoring system to assess the recurrence of CSDH following which another group (119 patients, June 2015 to July 2016) was used to verify the grading system. The factors predicting postoperative recurrence were screened out, and those that met the criteria were incorporated into the multiple regression analysis model. It was concluded that the cerebral re-expansion rate and preoperative CT imaging classification were important independent predictors of RrR. According to the ROC curve analysis, the critical threshold of the postoperative cerebral re-expansion rate (cut-off point, 40%) was determined. According to the intensity and regression coefficients associated with RrR to assign scores to establish a grading system. The prediction performance of the new model was compared with those of the previously published models in the validation group, and the new model had a better evaluation effect (AUC = 0.856). The main parameters of the model were easy to collect clinically and could quickly screen the RrR high-risk patients, thereby providing a reference for guiding the treatment.
Past studies have established models to evaluate high-risk patients for RrR. They have good clinical application values but are associated with some shortcomings. The Alberta grading system only includes preoperative clinical and imaging parameters without the postoperative factors and cannot fully reflect the perioperative imaging changes8. In the Oslo grading system, the preoperative hematoma volume cut-off point is 130 mL, and the postoperative residual cavity volume cut-off points are 80 and 120 mL29. in the Xining grading system, the thresholds for the volume of the hematoma before the operation and the volume of the postoperative residual cavity are 121 and 72 mL, respectively33. However, it is not suitable for different races to use the same fixed volume parameter threshold due to limitations in clinical application. The Xining grading system adopts the Nomogram Model, which is a relatively innovative method, but the outcome of predicting the recurrence is binary. The proportion of postoperative gas accumulation in the Wuhu grading system is an important factor27. However, with improvements in surgical skills, the amount of postoperative gas produced is reduced and does not affect the patient's prognosis7. On the contrary, the cerebral re-expansion rate can more accurately reflect the changes in the patient’s perioperative imaging.
Comparisons of the relationship between bilateral CSDH and cerebral re-expansion rate indicated that the postoperative re-expansion ability of bilateral CSDH was weaker (p = 0.028; Supplemental Table 3). This result was consistent with those reported in previous studies15. Consequently, the unilateral grading system cannot be applied to patients with bilateral CSDH. We need develop a model to predict the recurrence of bilateral CSDH32.
The parameters of postoperative day 1 were mostly used in some studies29,32. In the current study, the imaging parameters of postoperative day 1 were compared with those of days 7–9. The 7–9th day parameters demonstrated better predictive abilities of the recurrence of CSDH (Supplemental Table 1 and Fig. 1). The re-expansion is greatest during the first week after surgery and slows down considerably after that9.
Cerebral re-expansion is very important to reduce recurrence. The current methods used to increase the re-expansion rate of the brain tissue include the following: intraoperative aspiration of pneumocephalus via a subdural drain following evacuation4, neuroendoscopic removal of the residual septa and trabecula structures to promote brain expansion14, postoperatively performed supervised Valsalva maneuver (SVM)31, administration of at least 2000 mL per 3 days10, and early mobilization16. These methods reduce potential subdural space and promote cerebral expansion, thereby decreasing RrR.
One of the limitations of this study is that it is a single-center study. Hence, further verifications using multicenter studies are warranted.