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1
Department of Radiology II, University Hospital Innsbruck, Anichstr. 35, 6020
Innsbruck, Austria.
2
Department of Radiology, Hospital Barmherzige Brueder, Kajetanerplatz 1, 5010
Salzburg, Austria.
3
Department of Surgery, Hospital Barmherzige Brueder, 5010 Salzburg,
Austria.
4
Institute of Medical Statistics, University of Vienna, Schwarzspanierstr. 17,
1090 Wien, Austria.
Received March 19, 2001;
accepted after revision May 11, 2001.
Address correspondence to T. Rettenbacher.
Abstract
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SUBJECTS AND METHODS. The sonographic appearance of 149 consecutive abdominal wall hernias was prospectively investigated and correlated with subsequent surgical results. Commercially available 4- to 10-MHz linear transducers and 2- to 5-MHz curved transducers were used to evaluate the hernias.
RESULTS. Surgery revealed 126 nonincarcerated and 23 incarcerated hernias. The sonographic signs suggestive of incarceration that we identified included free fluid in the hernia sac, which was observed in 91% of the incarcerated hernias and in 3% of the nonincarcerated hernias; bowel wall thickening in the hernia, which was detected in 88% of the incarcerated hernias and in none of the nonincarcerated hernias; fluid in the herniated bowel loop, which was detected in 82% of the incarcerated hernias and in 3% of the nonincarcerated hernias; and dilated bowel loops in the abdomen, which occurred in 65% of the incarcerated hernias and in none of the nonincarcerated hernias. These imaging findings allowed the identification of incarceration in all 23 cases and led to a false-positive result in two of 126 nonincarcerated hernias.
CONCLUSION. Cross-sectional imaging signs indicating hernial incarceration included free fluid in the hernial sac, bowel wall thickening in the hernia, fluid in the herniated bowel loop, and dilated bowel loops in the abdomen. Sonography is an appropriate cross-sectional imaging modality for detecting these signs that are helpful in diagnosing patients with atypical clinical presentations.
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Hernial complications such as incarceration are also usually detected at clinical presentation. Cross-sectional imaging, however, is required when the clinical presentation is misleading or inconclusive, or when the surgeon believes it is important to properatively assess the contents of an incarcerated hernia [1, 12, 13]. Radiologists, therefore, should be familiar with the imaging findings of both incarcerated and nonincarcerated hernias. Several years ago, we observed that an incarcerated abdominal wall hernia was usually associated with a sonographic appearance that clearly differed from the sonographic appearance of a nonincarcerated hernia. To our knowledge, no cross-sectional imaging studies in the literature have examined the spectrum of imaging signs of hernial incarceration and their frequencies in a substantial patient population. The aim of this study was to investigate with sonography whether distinct cross-sectional imaging signs exist to differentiate incarcerated and nonincarcerated abdominal wall hernias.
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Also included in our study were six patients in whom sonography revealed complicated abdominal wall hernias that were clinically not suspected. Three of the patients were referred for sonography because of abdominal pain and vomiting of unclear origin; two patients, because of a clinically unclear abdominal wall mass; and one patient, because of suspicion for acute appendicitis.
Imaging Techniques
All sonographic examinations included in this study were performed by four
radiologists experienced in gastrointestinal sonography. The patients were not
specially prepared for the sonographic examination. Sonography was performed
on patients in a supine position, and the Valsalva's or coughing maneuvers
were almost always performed during the examination to confirm or exclude a
hernia. At least four images on several planes were obtained of each hernia
and documented on film with a multiformat camera. Commercially available
sonographic equipment included an HDI 3000 unit with 2- to 4-MHz curved array,
4- to 7-MHz linear array, and 5- to 10-MHz linear array transducers (Advanced
Technology Laboratories, Bothell, WA) and an AU4 unit with 3.5- to 5.0-MHz
curved array and 7.5- to 10.0-MHz linear array transducers (Esaote, Florence,
Italy). Color Doppler sonography was performed using the previously mentioned
linear array transducers. Color Doppler settings were adjusted for maximal
blood flow sensitivity. These settings included a wall filter of 50 Hz, a
pulse repetition frequency of 1000, maximum power, and the highest possible
gain at a usable ratio of signal-to-background noise. To evaluate the
peristalsis of herniated bowel loops, the sonographic transducer was held
motionless over the hernia for approximately 2 min.
Interpretation of Sonographic Findings
In this study, the sonographic detection of bowel, other abdominal organs,
or fatty tissue in the abdominal wall, as well as the sonographic
visualization of a lesion connecting with the abdomen, was required to
interpret a lesion as an abdominal wall hernia. Several years before the study
began, the authors observed on sonography that incarcerated abdominal wall
hernias were often associated with distinct imaging signs. On the basis of
this experience, a study protocol was designed in which the investigators were
requested to prospectively evaluate the following points by sonography: the
contents of the hernias, such as fatty tissue, bowel, or other organs; the
presence or absence of free fluid in the hernial sac; the wall thickness of
the herniated bowel loop; the presence or absence of fluid in the herniated
bowel loop; the diameter of small bowel in the abdomen; the presence or
absence of color Doppler signals in the hernial contents; and the presence or
absence of peristalsis in the herniated bowel loop. Immediately after the
imaging was performed, the cross-sectional signs were noted on a prepared
sheet of paper. In addition, the sonographic signs were then independently
reviewed by the four investigating radiologists, and a consensus opinion
regarding the signs was made. The study protocol was approved by the ethics
committee.
Statistical Analysis
Surgical results were considered the gold standard for this study. A hernia
was diagnosed as incarcerated at surgery if there was evidence of a compromise
in the blood supply of the hernial contents. To enable statistical correlation
of imaging signs with surgical results in all patients, we evaluated only
those hernias in which sonography was performed and the diagnosis was
confirmed by surgery.
For each sonographic sign, 2 x 2 contingency tables were used to calculate the estimates for sensitivity, specificity, positive and negative predictive values, and accuracy. Data analyses were performed using the statistical software package version 8.0 (SAS Institute, Cary, NC) for Windows (Microsoft, Redmond, CA).
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In 77 of 236 prospectively imaged patients, sonography did not reveal any abnormal findings. Sixteen of the patients underwent surgery despite normal findings on sonography because the clinical presentation remained suggestive of a hernia. Surgery revealed a nonincarcerated hernia in six patients and no abnormal findings in 10 patients.
In 18 of 236 patients, sonography revealed an abdominal wall lesion other than a hernia, including a lipoma in five patients, an enlarged lymph node in five patients, a metastasis in three patients, a suture granuloma in three patients, and an abscess in two patients.
The statistical analysis of imaging signs included 149 hernias in 131 patients in whom sonography and surgery were performed. A final diagnosis based on the surgical results included 126 nonincarcerated hernias and 23 incarcerated hernias. The location of all 149 abdominal wall hernias is given in Table 1. The contents of the nonincarcerated hernias were fatty tissue alone in 54 patients, fatty tissue and bowel in 47 patients (Fig. 1), bowel alone in 16 patients, and urinary bladder in three patients. The contents of the incarcerated hernias were fatty tissue alone in six patients, fatty tissue and bowel in four patients, and bowel alone in 13 patients. In 15 of 17 patients with incarcerated hernias containing bowel, blood circulation improved during surgery. Therefore, resection of the incarcerated bowel loop was necessary in only two patients. In three of six patients with incarcerated hernias containing only fatty tissue, resection of the fatty tissue was performed.
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The sonographic signs of incarceration investigated for this study and their frequencies are given in Table 2.
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The sensitivity, specificity, positive and negative predictive values, and accuracy of each sonographic sign as a finding positive for hernial incarceration are shown in Table 3.
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Concomitant free fluid in the hernia was the only sonographic sign of incarceration that we found when fatty tissue alone was present in a hernial sac (Fig. 2). In these cases, this finding indicated incarceration with a sensitivity of 100% (6/6) and a specificity of 96% (52/54) because free fluid was found in two of 54 nonincarcerated hernias containing fatty tissue alone. If bowel was present in a hernia, all four sonographic signs of incarceration could be applied (Figs. 3 and 4A,4B). Each of the incarcerated hernias with bowel displayed at least two signs of incarceration. The combination of at least two of the four identified signs allowed the detection of incarceration with a sensitivity of 100% (17/17) and a specificity of 100% (63/63) because the combined signs were never observed in nonincarcerated hernias in our series.
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All 23 incarcerated abdominal wall hernias were correctly interpreted preoperatively as incarcerated by the investigating radiologists, whereas two of 126 nonincarcerated hernias were considered incarcerated and were therefore false-positive for incarceration on sonography. During the time of the study, no incarcerated hernias were overlooked on sonography.
In the six patients in whom sonography diagnosed an incarcerated hernia that was clinically not suspected and who underwent surgical repair, the sonographic signs included free fluid in the hernia sac in five patients, bowel wall thickening in the hernia in four patients, fluid in the herniated bowel loop in five patients, dilated bowel loops in the abdomen in three patients, absence of blood flow in the hernial contents in two patients, and absence of peristalsis in the hernia in four patients (Fig. 3).
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Free fluid in the hernia sac was a sensitive and specific criterion of incarceration in our series (Tables 2 and 3 and Figs. 2,3,4A,4B,5). Free fluid is an eye-catching finding on sonography because of the great difference in echogenicity between the usually echo-free fluid and the other hernial contents or surrounding tissue (Figs. 2,3,4A,4B,5). This single sign immediately indicated to the investigator that a hernial complication had a high probability of being present. In the literature, we found two sonographic studies mentioning that an incarcerated hernia may contain free fluid [1, 15]. Motta et al. [27] reported three cases determined by sonography of torsion of an inguinal hernial sac filled with only a multiseptate fluid. Several cross-sectional imaging studies dealing with imaging signs of hernial incarceration [1, 2, 12, 17, 28] did not mention free fluid as a sign, although three of them [1, 2, 17] showed images of incarcerated hernias with a considerable amount of free fluid in the hernial sac without describing this as a criterion of incarceration. In our series, two of four patients with free fluid in a nonincarcerated hernia had free fluid in the abdomen and also in the hernia caused by liver cirrhosis. This finding can be a pitfall. Free fluid in an incarcerated hernia may be explained by transudation into the hernial sac caused by the compromised blood supply of hernial contents. The fluid in the hernial sac may be clear or sanguineous at surgery [27].
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Wall thickening of herniated bowel was another important sonographic sign of incarceration in our patients (Fig. 3). Wall thickening indicated incarceration of the hernias containing bowel with an excellent specificity but with limited sensitivity (Table 3). A cutoff point of 4 mm for wall thickening in this study was obtained for nondilated bowel loops in particular. In dilated bowel loops, wall thickening may sometimes remain undiagnosed with a cutoff point of 4 mm (Fig. 4A,4B) and result in a reduced sensitivity. Some sonographic and CT studies reported in the literature state that wall thickening of the herniated bowel loop is suggestive of hernial incarceration [1, 15, 17, 28]. Percentages of occurrence, to our knowledge, are not available. Wall thickening of incarcerated bowel may be predominantly explained by blood congestion and edema.
Another sign of hernial incarceration that we investigated was fluid in the herniated bowel loop (Figs. 3 and 4A,4B). The free fluid indicated incarceration of the hernias containing bowel with high specificity but limited sensitivity (Table 3). To our knowledge, this sign has not been reported previously in the literature. However, we found a sonographic image in a study showing fluid in an incarcerated bowel loop that was not mentioned in the text [2]. This sonographic image of an incarcerated hernia shows fluid in the herniated bowel loop with bowel wall thickening and free fluid in the hernial sac [2]. The reason for the presence of fluid in an incarcerated bowel loop may be exudation into the bowel lumen, causing excess fluid in the bowel in cases of bowel obstruction.
An indirect sign of an incarcerated hernia was evidence of dilated, fluid-filled bowel loops in the abdomen. This sign indicated incarceration of the hernias containing bowel with excellent specificity but limited sensitivity (Table 3). Despite the relatively low percentage of occurrence in our series, dilated, fluid-filled bowel loops are worth mentioning as an important sign because they may be the only conspicuous cross-sectional imaging finding in patients who clinically present exclusively with vomiting, diffuse abdominal pain, or both. We observed this situation in three patients. Almost all patients with an incarcerated hernia containing bowel can be expected to have complete bowel obstruction. For the absence of this sign in a relatively high percentage of patients (35% in the present study), we suggest the following explanations: incarceration is not complete and some chyme passes through, the time from the onset of incarceration to diagnostic imaging is too short to develop dilatation of bowel loops in the abdomen, and a Richter's hernia is present in which only part of the bowel wall is herniated. Evidence of bowel obstruction in the abdomen in patients with incarcerated hernias containing bowel is described in detail in the literature [12, 17, 18, 28,29,30,31]. To our knowledge, however, percentages of occurrence are not available.
Our data suggest that the absence of blood flow in the contents of a hernia should not be taken as a sign of incarceration because most incarcerated hernias in our series (78%) had detectable blood flow on color Doppler sonography (Tables 2 and 3). The patients in whom color Doppler sonography did not detect blood flow in the incarcerated bowel loop included the two patients with incarcerated hernia and bowel necrosis.
Our study results show that the absence of peristalsis should not be considered a sign of incarceration because nonincarcerated hernias did not show peristalsis during the sonographic investigation in a relatively high percentage of patients (Table 2). Our data, however, suggest that if peristalsis is present in an incarcerated hernia on sonography, bowel resection at surgery is probably not necessary.
Gas in the bowel wall or free gas, either in the abdomen or the hernia sac, was considered a sign of a complicated hernia [17, 28]. In our series, however, no patient had such an advanced stage of an incarcerated hernia.
A limitation of our study was that we had only two incarcerated hernias with bowel necrosis. The spectrum of imaging signs of incarcerated hernias with this advanced stage of disease could therefore not be assessed adequately. Another limitation could be that the investigators were aware of the results of the physical examination and the clinical presentation of patients at the time of sonographic imaging.
In summary, several cross-sectional imaging signs of hernial incarceration, such as free fluid in the hernial sac, bowel, wall thickening in the hernia, fluid in the herniated bowel loop, and dilated bowel loops in the abdomen are identifiable on sonography. These signs, indicating hernial incarceration in a high percentage of cases, are helpful in patients whose clinical presentations are inconclusive or misleading.
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