University of Pisa
Department of Translational Research and New Technologies
in Medicine and Surgery
Residency Program in Diagnostic Radiology
(2012-2017)
Chairman: Prof. Davide CaramellaAdhesive Small Bowel Occlusion: wich CT signs predict surgery?
Supervisor
Candidate
Prof. Davide Caramella, MD Dr. Francesca Pancrazi, MD Academic YearAbstract
- Objectives: To determine in adhesive small bowel occlusion (ASBO), which MDCT findings are predictive for the failure of a non-operative management (NOM), of Gastrografin® test outcome and finally for the need of surgery.
- Methods and Materials: From January 2015 to April 2017 we examined 137 admissions for ASBO: 71 Females, 66 Males, mean age 69 years (10-97), at our first level Emergency Department (ED). We excluded the patients without an adherential occlusion and those patients with spontaneous resolution of the bowel occlusion before the Gastrografin® test. This test is based on administration of the water soluble contrast (per os or via NGT) and to follow the progression of the contrast by means of seriated abdominal X-rays. It is considered successful if the cecum is opacified within eight hours.
The MDCT parameters, taken in consideration, were twelve: maximum caliber of the bowel; wall thickness greater than 5mm; parietal pneumatosis; presence or absence of peritoneal free abdominal fluid (FAF), peritoneal fluid density (measured in Hounsfield Units-H.U.); whirl sign; number of transition points; closed loop obstruction; small bowel feces sign; reduced bowel wall enhancement (RBE); mesenteric fluid congestion and fat notch sign.
The peritoneal fluid density has been considered measurable in the 84 admissions in which FAF, identified on MDCT, resulted to be sized >3cm2. The HU was measured by a round region of interest (ROI) positioned in the largest and lower pool of FAF (to include eventual blood stratifications) avoiding adjacent structures. A sensitivity analysis was performed to determine a high-density HU threshold. The FAF density in patients who underwent therapeutic laparotomy due to ischemia was compared with those successfully discharged without surgical approach. We evaluated the previously mentioned MDCT parameters: a) in all the patients that underwent surgery (n=86), b) in the group of patients treated surgically owing to ischemia (n=22), c) in the patients treated successfully with NOM implemented by Gastrografin® test (n=51) and d) in patients that underwent failed NOM implemented by Gastrografin® test (n=56) to find out any MDCT sign predictive of NOM failure.
- Results: The RBE resulted to be the more significant MDCT parameter (p value: <0,025) in predicting the surgical approach in general. Wall thickness greater than 5mm (p value: <0,0001); presence of peritoneal fluid (p value: 0,013); closed loop obstruction (p value: 0,044); RBE (p value: <0,0001) resulted to be significant in the prediction of surgery due to ischemia. Peritoneal fluid density (measured in HU) resulted to be significant for ischemia too and a best cut off value of 13,5-14,5 HU has been found with an AUC of 0,69 and a p value of 0,012. In fact, all patients with FAF density >14,5 UH resulted to need surgery due to ischemia with a sensitivity of 79% and specificity of 55%, PPV of 34% and NPV 90% an accuracy of 60% and a Youden Index of 0,34. The other MDCT parameters did not show any significant correlation. In our study no MDCT sign resulted to be significant in prediction of a failure of NOM performed with Gastrografin® test.
- Conclusions: In ASBO, MDCT is fundamental for the management of the patients. Among the MDCT findings, RBE resulted significant in predicting the need of a surgical approach in general; while thickness greater than 5mm, presence of peritoneal fluid, closed loop obstruction, RBE and an increased peritoneal fluid density ( >14,5 UH) are useful to elaborate a model to predict surgery due to ischemic complications. No MDCT parameter resulted to be significant in prediction of a failure of NOM implemented with Gastrografin® test.
- Key words Adhesive Small Bowel Occlusion, Surgery due to ischemia, Gastrografin® test, Non Operative Management, Emergency surgical treatment.
- Abbreviations Adhesive Small Bowel Occlusion (ASBO), Non Operative Management (NOM), Small Bowel Obstruction (SBO), Multi Detector Computed Tomography (MDCT), Ultra-Sonography (US), Magnetic Resonance Imaging (MRI), Magnetic resonance enterography (MRE), Nasogastric Tube (NGT), Emergency Department (ED), Free Abdominal Fluid (FAF), Reduced Bowel Wall Enhancement (RBE); Intra Venous Contrast Medium (IVCM).
Introduction
Adhesive small bowel occlusion (ASBO) is the most common cause of intestinal obstruction especially in patients with a history of previous abdominal surgery, and one of the most frequent causes of hospital admission. It is estimated that at least 60% of SBO is due to adhesions syndrome [1, 2]. Some studies showed that ASBO might appear after any abdominal operation, but it is more frequent after appendectomy, peritonitis, gynecological and colorectal surgery [1]. Numerous risk factors have been reported such as: age over 65, omental resection, gynecological surgery, penetrating abdominal trauma, and previous episodes of adhesive syndrome [1, 3-6]. The ASBO diagnosis is based on clinical evaluation, laboratoristic examination (leukocytosis, elevated C-reactive protein, serum amylase and lactic acid), abdominal X-rays, US abdominal study and MDCT scan. Clinical, laboratoristic and radiological informations must be integrated each other for the appropriate management.
The detection of ischemic complications is crucial to warrant a surgical decision and the imaging, especially MDCT, has a pivotal role because a diagnostic delay may result in an increased morbidity and mortality [7, 8].
Teixeira et al. [9] reported that a NOM should not extend beyond 3– 5 days for a non-resolving SBO, even in the absence of clinical deterioration and that surgery delayed >72 hours increases mortality threefold, and systemic infectious complications twofold, compared to surgery performed <24 hours after presentation. Schraufnagel et al. [10] reported higher rates of complications (bowel resection, longer hospital stay) and increased mortality in patients operated for ASBO after ≥4 days. Another recent study by Bauer et al. reported no differences in the outcomes between immediate and delayed groups regardless of duration of surgery delay, that was at least of 24 hours [11]. Abdominal X-rays The assessment for suspected ASBO should include (whenever possible for the clinical condition of patients) supine and erect plain abdominal films, that can show [12-14]: • dilated stomach (Figure 1); • distension of small bowel (>3cm) with multiple air- fluid levels (Figure 2); • absence or reduction of the colonic gas and rectal gas (Figure 2);
• the "stretch sign" or "string of beads sign" (small-bowel gas arranged as low-attenuation stripes perpendicular to the long axis of the bowel); this finding is due to fluid-filled loops of bowel, with a small amount of remaining gas trapped in folds between valvulae conniventes;
• the "pseudotumor sign": which represents obstructed, dilated, and fluid- filled loops of small bowel in supine position;
• presence of more than two air fluid levels wider than 2.5 cm and air- fluid levels differing more than 2 cm in height from one another within the same small bowel loop.
Figure 1. Supine abdominal X-rays of a patient with ASBO, anterior-posterior (A) and lateral view (B) showing marked gastric (stars in A and B) and bowel dilatation (dot in A and B) with air fluid levels (arrows in B), and reduced representation of colonic gas. Figure 2. Distended bowel loops (stars in A and B) and multiple air-fluid levels (black arrows in A) of two different patients in upright posterior-anterior (A), and supine position, anterior-posterior view (B); gastric air-fluid level (white arrow in A); air underlying valvulae conniventes (arrows in B) and reduced
representation of rectal (dot in B) and colonic gas.
In ASBO, the sensitivity, specificity, and accuracy of abdominal X-rays varies respectively from 79% to 83%, from 67% to 83%, and from 64% to 82%. The cause and the site of SBO usually is not clearly visualizable on plain abdominal films and a MDCT examination is often needed [15].
Multi Detector Computed Tomography
MDCT should be performed when clinical history, physical examination or abdominal radiography are not completely exhaustive regarding SBO [1, 16] and to determine if a prompt surgical intervention is needed.
Abdominopelvic MDCT is executed not only to confirm a diagnosis of ASBO but also to determine the etiology, the level of obstruction and the presence of complications such as a closed loop occlusion or a bowel ischemia (Figure3). After administration of intravenous contrast medium (IVCM) and with multiplanar reconstruction MPR (coronal and/or sagittal), MDCT is superior than plain films in depicting the transition point, in evaluating the severity of SBO (complete vs partial), in identifying its cause, and in recognizing complications (ischemia, necrosis, and perforation). The sensitivity, specificity, and accuracy of CT scans for ASBO diagnosis are, respectively, 90-94%, 96%; and 95% [17]. Figure 3. MDCT axial venous scan of a patient with a closed loop ischemic obstruction (proximal loop-black arrow and distal loop-arrowhead in A) with whirl sign (short white arrow in A), mesenteric edema, thin black arrow and free peritoneal fluid (long white arrow in A). Surgical specimen of the same case confirming the ischemic bowel loop.
The diagnosis of ASBO is primarily of exclusion because adhesive bands are not directly visualizable on MDCT; only an abrupt change in the caliber of the bowel is seen without any associated mass lesion, significant inflammation, or bowel wall thickening at the transition point. This finding combined with a history of abdominal surgery, associated kinking and tethering of the adjacent non obstructed bowel usually suggests the diagnosis [14].
Many MDCT signs of occlusion are discussed in literature [12, 18, 19]:
- Transition point: is the point of transition between proximal dilated gas-filled and distal collapsed small bowel loops (Figure 4);
- Beak sign: the tapering of the bowel loop at the point of obstruction (Figure 4);
- Closed loop obstruction: it occurs when a segment of bowel is obstructed at two points along its course, resulting in progressive accumulation of fluid in gas within the isolated loop, two adjacent beaks, C-shaped bowel and radial distribution of mesenteric vessels becomes visible on imaging (Figure 5);
- Small bowel feces sign: the presence of feces in the small bowel is indicative of longer stagnation and proximity to the transition point (Figure 4); - Whirl sign: describing an omental vessel twisting (Figure 4); - Fat notch sign: extra-luminal compression made by a band on the bowel at the transition point (Figure 4). MDCT findings of ischemic bowel or necrosis include: - bowel wall thickening: it is due to edema, hemorrhage, or both, on non contrast- enhanced CT scans, hemorrhage may determine a high attenuation layer in the bowel wall (Figure 5); - abnormal or decreased bowel wall enhancement; it may appear as: a decreased
enhancement relative to the uninvolved bowel (Figure 5), a hyper-attenuation of the mucosa relative to the remainder of the bowel wall creating a “target” appearance, or as heterogeneous enhancement. RBE is the result of blockage of the bowel wall arteriovenous microcirculation (by the extent of bowel dilatation or by torsion of the occluded bowel loop vascular pedicle), with bowel wall vessel engorgement, exudation and final mural hemorrhage. This dynamic process leads to alteration of bowel wall infusion;
- pneumatosis (intramural air), with or without associated gas in mesenteric or portal veins,* is a relatively late sign (Figure 5);
- mesenteric edema and/or fluid in the adjacent mesentery or peritoneal space: they result from venous congestion and transudation of fluid across serosa by mesenteric venous outflow obstruction (Figure 6).
Potential limitations of Contrast Enhanced MDCT may be: severe vomiting, kidney failure or allergy. On MDCT examination the adhesions themselves are generally not directly identified, but their presence is suspected by finding an abrupt transition point from dilated to collapsed bowel loops without an otherwise identified cause (Figure 4). Adhesions compress the bowel extrinsically, and they often cause an abrupt tapering or “beak sign” at the site of obstruction and a fat notch sign [12, 18]. Free peritoneal Abdominal fluid (FAF) density has been evaluated by Matsushima et al. [20] who considered its utility in determining the need for a surgical intervention; they demonstrated that ASBO patients, who require surgical intervention, have significantly higher FAF density than those who are managed non-operatively.
Figure 4. MDCT signs of SBO: whirl sign (star in A) and beak sign (arrow in A); maximum loop diameter measured in B (calipers), small bowel feces sign (star in B); fat notch sign, extra-luminal compression made
Figure 5. MDCT signs of advanced SBO: parietal pneumatosis of the small bowel (arrows in A); closed loop obstruction of small bowel loops (arrow in B); bowel wall thickening >5mm (caliper in C); reduced bowel wall contrast enhancement of small bowel loops in the left abdominal quadrant (arrow and star in D). Figure 6. Peritoneal fluid (star in A) and peritoneal fluid density measurement using a ROI positioned in the non-enhanced scan, in the largest and lower pool of fluid to include eventual blood stratifications and avoiding adjacent structures (ROI in A); mesenteric striated stranding due to congestion (star in B).
Ultrasonography and Magnetic Resonance Imaging
US is able to reveal differences in mucosal folds around transition point (the pattern of the valvulae conniventes) and presence of peristalsis, thus differentiating between ileus and mechanical obstruction. In contrast to these findings, the Bologna Guidelines 2013 state that there is limited value for US (level 2c), in SBO or in patients with distended bowel, because the air may obscure the underlying findings and extra-luminal fluid might be difficult to detect, making it a useful diagnostic tool only when applied by skillful operators [3].
Magnetic resonance enterography (MRE) may be employed for ASBO only in selected patients and its use should be restricted to those patients having MDCT or iodine contrast contraindications. Even if MRE provides similar sensitivity and specificity, than MDCT, it has a limited role in diagnosing ASBO in the acute setting.
ASBO decisional algorithm
Since January 2015, a decisional algorithm has been applied in our E.D. for evaluating the patients with clinical symptoms and signs of SBO: crampy abdominal pain and nausea, vomiting, bowel closed to feces and gas, abdominal distention, absent bowel sounds (Figure 7).
This algorithm includes an accurate anamnestic evaluation, a complete objective examination, an early gastric decompression by nasogastric tube (NGT) placement and routine laboratory examinations.
The primary evaluation performed in ED is fundamental to provide early resuscitation by administration of parenteral liquids, to correct hydro-electrolytic alterations and to evaluate the presence of clinical signs of peritonitis, sepsis or intestinal ischemia. The next step is to perform US and abdominal plain films in order to assess radiological signs of SBO. If they are confirmed, a MDCT, with and without intravenous contrast media administration, is warranted to determine the cause, the site and possible complications. In the acute phase no oral contrast is administered to the patients for the MDCT scan. MDCT is performed in absence of vomiting or contraindication to contrast medium administration and allows to differentiate non-adherential causes of SBO, such as neoplasms, inguinal hernias, and inflammatory forms, precluding the possibility of conservative treatment. In cases of signs of ischemia surgery is recommended [21].
The ASBO decisional algorithm considered for these patients an implementation of the traditional NOM (fasting, hydro-electrolyte imbalance correction, pain and vomiting control) applying a systematic Gastrografin® test.
In case of failure of this test (absence of cecal opacification within eight hours after oral administration of water soluble contrast) a surgical intervention is needed.
The choice of laparotomic or laparoscopic approach is decided considering the different clinical condition and the experience of the operating surgeon [5].
Gastrografin® test
In the absence of suggestive signs of strangulation, peritonitis or intestinal ischemia in primary and secondary evaluation, there is good evidence [22] to support NOM (based on resuscitation by intra-venous hydro-electrolyte integration, and continuous clinical and laboratory re-evaluation) implemented by the Gastrografin® test, according to a standardized protocol (Figure 7).
Many studies address Gastrografin® test as a diagnostic and therapeutic option at the same time for the NOM in ASBO [16, 23-27].
Gastrografin® is a radiopaque hyperosmolar solution containing diatrizoate sodium and diatrizoate meglumine (10 g e 66 g per 100 mL respectively). Its high osmolarity (2150 mOsm/l, six times higher than the extracellular fluid), recalls liquids in the intestinal lumen, reduces edema of the bowel wall and activates the contraction of the smooth muscle fibers of the bowel wall. All their effects together may stimulate peristaltic movements able to overcome the occlusion [28].
In the clinical practice, a dosage between 50 and 150 mL of Gastrografin® is administered per os or trough NGT, immediately after the admission or after an initial conservative treatment of 48 hours. The arrival of Gastrografin® in the cecum is assessed by means of an abdominal X-ray performed at least 8 hours after its administration and it can select the patients candidates to NOM.
This protocol stipulates that 100 mL of Gastrografin® are administered via the NGT, at least 6 hours after its positioning and after further manual aspiration from the tube (in order to ensure complete gastro-jejunal detention). The NGT is then closed for at least 2-3 hours (or until the onset of nausea or vomiting) and a series of abdominal X-rays is programmed.
The first radiograph is performed 6-8 hours after Gastrografin® administration [22]: patients in which the water-soluble contrast have opacified the cecum within 8 hours (successful test), are feeded with a liquid diet, have the NGT removed and are candidates to surveillance and discharge. After the initial successful outcome of the Gastrografin® test, if a clinical suspicion of SBO occurs again in the subsequent hours or in the following days, surgery becomes necessary. When Gastrografin® has not opacified the cecum, the NGT is kept open and a further abdominal X-ray is performed after 24 hours [24]. If the contrast medium has not yet reached the colon, the test is considered unsuccessful and surgery is recommended.
The complete resolution of the SBO and therefore the patient's definitive discharge requires disappearance of symptoms and signs of occlusion, canalization to feces and gas and complete oral feeding.
The Gastrografin® test protocol and the NOM implies a close clinical evaluation and surveillance of the patient and need to be interrupted in case of intensification of abdominal pain, signs of peritonitis, fever, vomiting persistence, and failure to opacify the cecum 24 hours after its administration. In all these cases there is indication to surgery.
Gastrografin® test allows to perform the diagnosis of complete occlusion, helping to make a relative quick decision for surgery [29-32]. The prediction of patients who may not respond to treatment with Gastrografin® has not yet been determined, although many factors associated with unsuccessful NOM have been analyzed [3, 33, 34].
Objectives
To evaluate in adhesive small bowel occlusion (ASBO) which MDCT findings are predictive for the failure of a non-operative management (NOM), of Gastrografin® test outcome and finally for the need of surgery.Methods and Materials
The study consisted in a retrospective evaluation of 137 admissions for ASBO in the period from January 2015 to April 2017. We excluded those patients which were not evaluated with the ASBO decisional algorithm, which didn't perform MDCT examination before surgery and with a cause of SBO different from adhesions; patients with a spontaneous resolution of the SBO before the Gastrografin® test were excluded too.Two radiologists (one in training and one experienced) evaluated the MDCT scans without knowing the MDCT findings considered by the other radiologist, surgery results and/or the eventual presence of ischemia, or if the patient underwent to successful NOM implemented by Gastrografin® test.
MDCT was performed by using a 64–detector row scanner (LightSpeed VCT; GE Healthcare, Milwaukee, Wis) before and after administration of 100-150mL of iodinated contrast material injected at 3-4 mL/sec. Technical parameter were: 120 kVp, and the amperage setting ranged from 100 to 600 mA, according to the body habitus. The images were reconstructed at a 2.5mm section thickness in the axial plane, with native images available for interpretation. Coronal and sagittal MPR reconstructions were obtained. No oral contrast was administered for the CT examination in acute phase.
MDCT parameters evaluated were: maximum caliber of the bowel; wall thickness greater than 5mm; parietal pneumatosis; peritoneal free abdominal fluid (FAF) and peritoneal fluid density (measured in Hounsfield Units-H.U.); whirl sign; number of transition points; closed loop obstruction; small bowel feces sign; reduced bowel wall enhancement (RBE); mesenteric fluid congestion and fat notch sign.
The peritoneal fluid density has been considered measurable in the 84 admissions in which FAF, identified on MDCT, resulted to be sized >3cm2. The HU was measured by a round region of interest (ROI) positioned in the largest and lower pool of FAF (to include eventual blood stratifications) avoiding adjacent structures (Figure 6). A sensitivity analysis was performed to determine a high-density HU threshold. The FAF density in patients who underwent therapeutic laparotomy due to ischemia was compared with those successfully discharged without surgical approach.
We evaluated the previously mentioned MDCT parameters: a) in all the patients that underwent surgery (n=86), b) in the group of patients treated surgically owing to ischemia (n=22), c) in the patients treated successfully with NOM implemented by Gastrografin® test (n=51) and d) in patients that underwent failed NOM implemented by Gastrografin® test (n=56) to find out any MDCT sign predictive of NOM failure.
Statistical Analysis
Before testing of inferential statistics, an exploration phase was performed. All variables were described by statistical characteristics: categorical data were described by frequency, whereas continuous data by mean and range.
The predictive parameters of "surgery" and "surgery due to ischemia" were performed using a univariate binary logistic regression model. All variables significantly influencing in the univariate
assess the independent contribution of each risk factor. The results of the regression model were calculated by Wald test and expressed using the odds ratio (OR) with its related confidence interval (CI) and p-value. In the multivariate analysis, the regression coefficient was indicated and the sensitivity and specificity of the diagnostic model were also determined.
To find the best cut-off of Free Abdominal Fluid Density (FAF) as predictive factor of "Surgery due to ischemia", a receiver-operating-characteristic (ROC) analysis was performed and area under curve (AUC) analyzed by a non-parametric test. Once the optimal cut-off value for density of FAF was determined, the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy were also calculated for the need for surgical intervention. Differences were considered significant at p<0.05. All statistical analysis, descriptive and inferential, were carried out with SPSS version 23 (SPSS Inc. SPSS® Chicago, IL, USA).
Results
The patients population resulted to be representative and homogeneous regarding gender, age, BMI and WBC at the moment of admission (Table 1). Characteristic Statistics Female 71 Male 66 Age 69 (10-97) BMI 24,2 (16,5-35,7) WBC (X1000) 10,63 (1,52-37,03) Table 1. Clinical characteristics of the 137 admissions. Time of hospital stay resulted to be significantly higher (p>0,001) in patients that underwent to surgery, and significantly lower in patients underwent to successful NOM. Of the 137 admissions, 30 patients underwent to early surgery (14 due to ischemia); and 107 patients were treated with NOM. A subgroup of 56 had delayed surgery, in 8 cases due to ischemia, while NOM for ASBO was successful in 51 admissions. A total amount of 86 patients required surgical intervention, and a sub-group of 22 underwent to small bowel resection due to ischemia (Figure 8). Only 84 patients resulted to have measurable density (in UH) of the FAF.
In the evaluation of MDCT signs in patients that underwent to surgery (n=86) and patients that underwent to successful NOM implemented by Gastrografin® test (n=51), RBE resulted to be significant (p value: <0,025) in predicting the surgical approach. All patients with RBE underwent to surgical resection (Table 2). Table 2. Univariate analysis of the “surgery” predictive factors. In the evaluation of MDCT signs in patients that underwent to surgery due to ischemia (n=22) and patient that underwent to successful NOM implemented by Gastrografin® test (n=51) a univariate and multivariate analysis have been performed to compare the MDCT signs in these two groups of patients. Univariate analysis
Factor OR (95%CI) p-value
Max loop diameter 1,14 (0,83-1,56) 0,407 Wall thickness >5mm 3,59 (0,76-16,9) 0,106 Parietal pneumatosis 1,19 (0,11-13,5) 0,888 Peritoneal fluid 1,46 (0,72-2,95) 0,296 Whirl sign 2,09 (0,92-4,75) 0,080 Transition point 1,37 (0,40-4,71) 0,614 Closed loop obstruction 5,84 (0,72-47,5) 0,099 Small bowel feces sign 0,57 (0,28-1,15) 0,116 Reduced bowel wall enhancement 5,13 (0,62-42,5) 0,025 Mesenteric fluid congestion 1,32 (0,51-3,39) 0,563 Notch sign 0,83 (0,29-2,33) 0,719
Table 3. Univariate and multivariate analysis (logistic regression) of the "surgery due to ischemia" predictive factors. OR: Odds Ratio; RC: regression coefficient.
All these parameters: wall thickness greater than 5mm (p value: <0,0001); presence of peritoneal fluid (p value: 0,013); closed loop obstruction (p value: 0,044) and RBE (p value: <0,0001), resulted to be significant in the prediction of surgery due to ischemia (Table 3).
The multivariate analysis has been performed to evaluate all of these significant qualitative parameters and: wall thickness greater than 5mm (p value: 0,001); closed loop obstruction (p value: 0,016) and RBE (p value: 0,011) resulted to be significant in the prediction of surgery due to ischemia (Table 3). A logistic regression model has been evaluated considering all of this qualitative parameters: it resulted to have a sensitivity and specificity respectively of 79% and 91% in the prediction of surgery due to ischemia. Cohen's Kappa coefficient reveal an moderate agreement of the model with our patients population with a p value <0,0001 (Table 4). Table 4. Sensitivity and Specificity of the regression model. Cohen’s Kappa coefficient reveal a moderate agreement. Univariate analysis Multivariate analysis
Factor OR (95%CI) p-value RC OR (95%CI) p-value
Max loop diameter 0,63 (0,37-1,08) 0,095 Wall thickness >5mm 19,2 (5,18-71,2) <0,0001 2,56 12,9 (2,75-61,2) 0,001 Parietal pneumatosis 11,4 (0,99-131) 0,051 Peritoneal fluid 5,05 (1,41-18,1) 0,013 1,16 3,18 (0,78-12,8) 0,106 Whirl sign 2,47 (0,96-6,32) 0,059 Transition point 0,41 (0,05-3,32) 0,402 Closed loop obstruction 4,04 (1,04-15,7) 0,044 1,98 7,26 (1,44-36,4) 0,016 Small bowel feces sign 0,93 (0,37-2,36) 0,882 Reduced bowel wall enhancement 53,2 (6,11-462) <0,0001 3,11 22,4 (2,06-243) 0,011 Mesenteric fluid congestion 1,17 (0,32-4,39) 0,810 Notch sign 0,29 (0,04-2,35) 0,248 Constant -3,37 0,03 (0,01-0,13) <0,0001 Surgery due to ischemia
No Yes Tot
Model expected group No 112 11 123 Yes 3 11 14 Tot 115 22 137 Sensitivity 79% Specificity 91% Cohen’s Kappa 0,560 p-value <0,0001
In Figure 9 associations between presence of the predictive parameters and surgery due to ischemia has been demonstrated in percentage; in Figure 10 the associations between absence of the predictive parameters and surgery due to ischemia are shown. Figure 9. Association between presence of the predictive parameters and "surgery due to ischemia". Figure 10. Association between absence of the predictive parameters and "surgery due to ischemia".
The FAF density (measured in HU) resulted to be a significant predictor of surgery due to ischemia, and this quantitative parameter, has been evaluated using ROC curve. A best cut-off value of 13,5-14,5 HU has been found with an AUC of 0,69 and a p value of 0,012 (Figure 11). The parameter has been evaluated apart from the logistic regression model due to the fact that only in 84 admissions the FAF density resulted measurable. The best cut off that resulted to have sensitivity of 79% and specificity of 55%, PPV of 34% and NPV 90% an accuracy of 60% and a Youden Index of 0,34 (Table 5). Figure 11. ROC curve to evaluate the best cut-off of the Free Abdominal Fluid Density as predictive factor of the “surgery due to ischemia” (84 admissions). Best cut-off is between this range: 13,5-14,5 HU (sensitivity: 79%; specificity: 55%). Predictive parameters Value Sensitivity 79% Specificity 55% PPV 34% NPV 90% Accuracy 60% Youden index 0,34 Table 5. Predictive parameters of High Density FAF for surgical intervention due to ischemia. PPV: positive predictive value; NPV: negative predictive value.
To find out any MDCT signs predictive of NOM failure, we analyzed the MDCT signs in patients that underwent to conservative treatment implemented by Gastrografin ® test (n=107), the group was comprehensive of patients underwent to successful NOM and patients underwent to NOM failure. In our studies no MDCT findings resulted to be significant to discriminate patients that could benefit from NOM.
Discussion
The previous literature has reported that none of the commonly used clinical signs, laboratory data, or MDCT findings are predictive for the need of surgical intervention in ASBO [12, 18, 35]. Previous studies have attempted to create a clinical-radiological model to reliably predict the likelihood of surgical intervention and many of these reported FAF as a finding associated with the need for surgical intervention in the ASBO patients [25, 36-39].
In the Schwenter and colleagues study [36], the clinical factors of pain, guarding
(involuntary muscle spasm elicited by fulminant acute peritonitis), leukocytosis, and elevated C- reactive protein were combined with reduced wall enhancement and greater than 500 mL of FAF on MDCT into a scoring system. The presence of three factors yielded specificity of 91% and sensitivity of 68% for surgical resection of ischemic bowel within 24 hours in comparison with successful conservative management. With four factors present, the specificity reached 100% [36]. Zielinski et al. initially created a prediction model which incorporates vomiting, mesenteric edema, FAF, and absence of small bowel feces sign on MDCT [37, 40] Kulvatunyou et al. recently evaluate a model with three selected factors (no flatus, FAF, and high-grade obstruction) with a PPV of 56%. [36, 38]. Chang et al. found that intraperitoneal fluid, high-grade or complete obstruction, mesenteric fatty stranding, and absence of feces sign were the most significant predictors. When all of the four criteria were used in combination, high sensitivity of 98.4% and specificity of 90.9% were achieved for the prediction for surgery [39]. Considering MDCT signs of ischemia, Santillan et al. achieve that when more than one finding is present in the same patient, their diagnostic accuracy is higher [41]. Another study by Zalcman et al. demonstrated that the specificity of mesenteric fluid, mesenteric congestion, and free fluid for ischemia was 90%, 79%, and 76%, respectively, and that the specificity for ischemia increased to 94% when two or more of these findings were present [42].
In our study we attempted to create a radiological model considering all MDCT findings that resulted to be significant in the multivariate analysis: wall thickness greater than 5mm (p value:
0,001); closed loop obstruction (p value: 0,016) and RBE (p value: 0,011).
RBE, resulted to be the best predictive MDCT finding for surgery. In a recent meta-analysis of nine studies by Millet et al., RBE was the best predictive MDCT sign of strangulation. The same meta-analysis confirmed a moderate sensitivity and specificity of FAF for the diagnosis of bowel ischemia in the SBO patients (69 and 61 %, respectively) [35]. However, in a study by O’Daly et al.[43], 54 % of the patients with FAF did not require any surgical intervention for ASBO. Similarly, nearly half (n=27) of the patients that underwent successful NOM in our study (n=51), had measurable density FAF. In our study just 61% (n=84) of the patients had measurable density of FAF, at least in one pouch (FAF >3cm2). Previous studies categorized FAF by volume; in those studies, patients with a larger volume of FAF were more likely to require surgical intervention. However, these volume cutoffs were arbitrary and subjectively determined by radiologists [36, 43]. By adding the density data in predicting the need for surgical intervention due to ischemia, and considering high-density FAF a measured a value > than the cut-off- 13,5-14,5 HU, sensitivity and specificity of improved to of 79%and 55% respectively. Limitations of our study are that: a) the surgeon's final opinion was the gold standard to select
patients attending surgery; b) even if sensitivity, specificity, PPV, and NPV of FAF density measurement may be utilized to determine the need for surgical intervention in the patients with ASBO, further perspective studies regarding the FAF density, analyzed on MDCT are required; c) we were able to determine whether a small bowel resection was performed due to a strangulated segment of the small bowel but we did not evaluate the impact of high-density FAF on the need of a small bowel resection for other reasons, including severe adhesions or stricture, and further evaluation could be done on this events.
In patients that have no signs of ischemia a still open concern is how to distinguish patients with adhesive small bowel occlusion (ASBO), that need a surgical intervention, from those that could benefit from conservative treatment with Gastrografin ® test. For this reason a part of our study consisted in the evaluation of MDCT signs in patients that underwent to conservative treatment implemented by Gastrografin ® test to find out any MDCT signs predictive of NOM failure. Unfortunately in our studies no MDCT signs resulted to be significant to discriminate patients that could benefit from NOM.
Millet et al. in 2014 [44] found that the presence of multiple beak sign and a transition zone not contiguous to the anterior peritoneal layer are independent predictors of the need for surgery in patients who initially had undergone NOM for adhesive SBO, in our study more than one transition point do not result to be predictive of NOM failure.
Conclusions
In ASBO, MDCT is fundamental for the therapeutic management of the patients. Among the CT findings, RBE resulted significant in predicting the need of a surgical approach in general; while thickness greater than 5mm, presence of peritoneal fluid, closed loop obstruction, RBE and an increased peritoneal fluid density ( >14,5 UH) are useful to elaborate a model to predict surgery due to ischemic complications. No MDCT parameter resulted to be significant in prediction of a failure of NOM performed with Gastrografin® test.
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