In this study, we established a prediction model for irreversible II necessitating bowel resection in cases of SBO. Our model is based on two independent objective predictors: the WBC count and the value of free peritoneal fluid on unenhanced CT.
Systemic inflammatory response syndrome is reportedly associated with SBO [6, 8–12]. Particularly, only high WBC counts have been associated with irreversible II, reflecting the severity of inflammation due to the irreversible ischemic changes found in SBO [10]. Consistent with previous studies, the WBC count was significantly higher in the resection than in the non-resection group in the present study, and the AUC was 0.741, indicating a relatively accurate prediction of irreversible II.
Various CT findings, such as reduced bowel wall enhancement, increased unenhanced bowel wall attenuation, and the presence of mesenteric fluid, have been associated with irreversible II in cases of SBO [5–7, 13–15]. In our study, the value of the free peritoneal fluid on unenhanced CT was a significant predictor of irreversible II. SBO is caused by venous occlusion due to compression of the mesentery, causing transmural hemorrhage following congestion, edema, and mucosal hemorrhage. Thus, hemorrhagic peritoneal fluid is often observed in SBO. Kobayashi et al. reported that the presence of red blood cells in the free peritoneal fluid in SBO increases according to the degree of strangulation, and the count was higher in patients with bowel resection than in those without [16]. In our study, hemorrhagic peritoneal fluid during surgery was observed more frequently in the resection than in the non-resection group, which may reflect the transmural hemorrhage due to strangulation. Previous studies have reported CT values of the exudative body fluids < 10 HU [17], while those of hemorrhagic peritoneal fluid range from 15–75 HU [18]. A recent study reported that CT values of peritoneal fluid > 10 HU in cases of SBO indicate the need for bowel resection [19]. It has been suggested that the greater the hemorrhage associated with the progression of II due to strangulation, the higher the CT values of free peritoneal fluid. In our study, the AUC was 0.750, indicating a relatively accurate prediction of irreversible II.
The two indicators in our prediction model are objective indicators, allowing for objective and reproducible prediction of irreversible II in SBO. Additionally, these indicators are readily available at most hospitals. Furthermore, the CT value of free peritoneal fluid can be quickly evaluated using only unenhanced CT scans. Reduced bowel wall enhancement on contrast-enhanced CT has been reported as being helpful in predicting irreversible II in SBO, with a sensitivity of 75–81% and specificity of 19–74% [5, 7, 14]. However, contrast-enhanced CT may be contraindicated in patients with severe renal dysfunction or iodine allergy. In contrast, our model is useful even when contrast agents are contraindicated.
In our model, higher scores were associated with a higher probability of bowel resection. At a score of 0 (with a probability of 15.2%), immediate surgery may avoid a bowel resection for many patients with SBO. Moreover, it may be sufficient to release the strangulation, allowing to choose laparoscopic surgery. In this study, laparoscopic surgery was performed significantly more often in the non-resection group than in the resection group. Conversely, at a score of 4 (with a probability of 85%), open surgery may be preferable considering the need for emergency resection of the ischemic bowel. In addition, respiratory and circulatory management may be considered as a precaution against further deterioration in patients with progressive intestinal ischemia. Furthermore, higher scores were also associated with higher rates of necrosis in the resection specimens. All 17 specimens at a score of 4 in the resection group showed pathological hemorrhagic necrosis, and of them, 14 (82.4%) specimens showed transluminal hemorrhagic necrosis (data not shown).
Our study has several limitations. This was a single-center retrospective study with a small sample size. Multi-center prospective studies with large sample sizes are required to support the findings. To reduce overfitting, 5-fold cross-validation was used to assess the internal validation of our prediction model. However, additional external validation is necessary to verify its application.