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Emergency Radiology |
1 From the Departments of Radiology (M.A.B., G.C., C.B.S.) and Surgery (N.Y.P., D.B.H.), University of California, San Diego, 200 W Arbor Dr, San Diego, CA 92103-8756. From the 1998 RSNA scientific assembly. Received April 14, 2000; revision requested June 7; revision received June 27; accepted July 11. Address correspondence to G.C. (e-mail: gcasola@ucsd.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Patients with blunt abdominal trauma underwent US. The abdomen and pelvis were scanned for free fluid, the visceral organs were assessed for heterogeneity, and duplex US was performed if necessary. Empty bladders were filled with 200–300 mL of sterile saline through a Foley catheter. US findings were considered positive if free fluid was present or if parenchymal abnormalities that could be consistent with trauma were detected. Screening US results were compared with findings of diagnostic peritoneal lavage, repeat US, computed tomography (CT), cystography, surgery, and/or autopsy and/or the clinical course.
RESULTS: Findings from 2,693 US examinations were evaluated and were positive in 145 of 172 patients with injuries (sensitivity, 84%) and 64 (89%) of 72 patients who ultimately underwent laparotomy with surgical repair of injuries. False-negative findings were retroperitoneal injury, bowel injury, and intraperitoneal solid organ injury without hemoperitoneum. No patient with false-negative findings died. Specificity of US was 96% (2,429 of 2,521 patients), and overall accuracy was 96% (2,574 of 2,693 patients). Positive predictive value was 61% (145 of 237 patients), and negative predictive value was 99% (2,429 of 2,456 patients).
CONCLUSION: Abdominal US is useful in screening for injury in patients with blunt abdominal trauma, and its use represents a notable change in institutional practice. Diagnostic peritoneal lavage is rarely performed, and CT is used when screening US findings are positive, when injury is clinically suspected despite negative US findings, or when US is not available.
Index terms: Abdomen, CT, 70.12112, 70.12115 • Abdomen, injuries, 70.41, 70.43 • Abdomen, US, 70.1298, 70.12984 • Trauma, 70.41, 70.43 • Ultrasound (US), comparative studies
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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More recently, computed tomography (CT) became an equally important diagnostic tool and made nonsurgical treatment possible in many patients who would have undergone laparotomy on the basis of diagnostic peritoneal lavage findings (3–7).
Ultrasonographic (US) evaluation of patients with blunt abdominal trauma was described more than 25 years ago (8), and US is now the primary examination used in several trauma centers in Europe and Asia, as well as in select centers in the United States (9–17). Advantages of US are that it is fast, portable, and easily integrated into the resuscitation of patients with trauma without a delay of therapeutic measures. These features particularly facilitate its use in the evaluation of patients who are hemodynamically unstable. Unlike diagnostic peritoneal lavage, US is noninvasive and has no associated morbidity.
Limitations of US include its dependence on operator skill, which becomes particularly important if surgeons or emergency physicians with limited training perform the studies. Adequate training and experience are crucial to accurate US evaluation. At our institution, more than 3,000 abdominal US examinations have been performed since 1994 to evaluate trauma. The purpose of the present study was to evaluate the accuracy of abdominal US used as a screening examination in patients with blunt abdominal trauma.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Patient Selection
The study population consisted of all patients who underwent screening abdominal US with a mechanism of injury for blunt abdominal trauma who were included in the trauma registry database. Patients included in the registry met Multiple Trauma Outcomes Study criteria (18). At our institution, these criteria include a hospital stay of 72 hours or more; admission to the operating room, intensive care unit, or intermediate care unit; interfacility transfer; or death.
Patients were eligible for US if they were suspected of having blunt abdominal trauma and if they did not require immediate laparotomy. During the study period of 4 years 8 months, 2,699 patients who met criteria for the Multiple Trauma Outcomes Study (18) underwent abdominal US for evaluation of blunt trauma. Images were uninterpretable in three patients due to obesity (n = 1) or subcutaneous air (n = 2), and these patients were excluded. Three additional patients were excluded because they refused diagnostic peritoneal lavage or CT after US revealed a small amount of fluid; thus, their suspected injuries could not be confirmed or disproved.
Initially, US was available 24 hours a day. Later in the study, due to a sonographer shortage, US was available from 7:00 AM to 11:30 PM, Monday through Friday, and from 8:00 AM to 4:30 PM, Saturday and Sunday. If US was not available, patients underwent CT or diagnostic peritoneal lavage and were not included in the study. If US findings were positive and if the patient remained hemodynamically stable, CT was performed to better define the extent of injury. Unstable patients with positive US findings underwent laparotomy without CT.
Early in the study, many patients underwent diagnostic peritoneal lavage prior to laparotomy for confirmation of US results. As the trauma surgeons became more confident in the use of US, laparotomy was performed more frequently only on the basis of positive US findings. If there was clinical suspicion of injury despite negative US findings, either during the initial evaluation or later, further investigation was performed by using CT, diagnostic peritoneal lavage, repeat US, or cystography at the discretion of the trauma surgeon.
Technique
All US examinations were performed by registered diagnostic medical sonographers with general US experience of 2–30 years. Studies were completed in the resuscitation suite or operating room in the presence of the trauma fellow and a staff or resident radiologist. Residents had between 6 months and 4 years of experience interpreting US images.
The equipment was either an HDI 3000 (Advanced Technologies Laboratories, Bothell, Wash) or a 128-XP (Acuson, Mountain View, Calif) machine. In most cases, a 3.5-MHz sector probe was used, although when indicated for better imaging, 2.25- or 5.0-MHz sector probes or 5.0-MHz curved-array probes were used. Duplex US was performed if indicated by clinical suspicion or findings at gray-scale imaging. Images were obtained with the Image Link system (Eastman Kodak, Rochester, NY) for subsequent interpretation by radiologists.
The US trauma protocol, which was used for all patients in the present study, consisted of evaluation of the right and left upper quadrants of the abdomen, epigastrium, paracolic gutters, retroperitoneal space, and pelvis. Empty bladders were filled with 200–300 mL of sterile saline through a Foley catheter if there was no contraindication to catheterization, such as suspected urethral injury. Attention was directed to the presence of free fluid and the US appearance of the abdominal organs and the heart. Pleural effusion, pericardial effusion, and cardiac findings were diagnosed and treated accordingly; however, these findings are not reported in the present study, which addresses abdominal injury. While length of examination was not consistently recorded, a typical abdominal US trauma protocol required approximately 10 minutes to complete.
CT examinations were performed by using a 9800 scanner (GE Medical Systems, Milwaukee, Wis) prior to 1995 and a HiSpeed Advantage scanner (GE Medical Systems) after March 1995. Initially, nonhelical scans were obtained with 10-mm collimation. After 1995, helical scans were obtained with 7-mm collimation and a table speed of 7 mm/sec, and images were reconstructed at 7-mm intervals. Patients received 500 mL of water orally or through a nasogastric tube in the resuscitation suite. In adults, a total of 125 mL of ioversol (Optiray 320; Mallinckrodt, St Louis, Mo) was administered intravenously at a rate of 3–4 mL/sec by using a power injector (Liebel-Flarsheim, Cincinnati, Ohio). In children, the contrast agent dose was calculated according to weight and was administered by means of hand injection.
Peritoneal lavage was performed in the resuscitation suite by a resident or attending surgeon with a standard technique. Usual criteria for positive diagnostic peritoneal lavage findings are designed to aid in the determination of whether a patient has a large amount of hemoperitoneum that requires prompt laparotomy; these criteria include a red blood cell count greater than 100,000 per cubic millimeter (1 x 1011 per liter). Small amounts of hemoperitoneum, which can be detected at US, would not typically produce such a high red blood cell count. By using this criterion as a standard, a small amount of blood in the peritoneal space depicted at US would be counted as a false-positive finding. For this reason, a lower red blood cell count of 15,000 per cubic millimeter (1.5 x 1010 per liter) was arbitrarily chosen to represent a positive test result when a comparison was made with US results. This lower red blood cell count suggests some intraperitoneal bleeding but would not necessarily indicate an urgent need for laparotomy.
Definitions
For statistical analysis, US findings were considered positive if free fluid was present or if a parenchymal abnormality that could be consistent with trauma was identified. Free fluid in the presence of a known medical cause of ascites was considered a positive US finding because hemoperitoneum could not be excluded, and further investigation was necessary to rule out injury. All indeterminate parenchymal lesions were considered positive US findings and prompted CT even in the absence of free fluid. Nontraumatic lesions, such as well-visualized simple cysts, that allowed definitive diagnosis at US were considered negative findings. For the purposes of the present study, pleural and pericardial effusions were considered negative findings for abdominal injury.
A positive US finding was considered true-positive if CT, diagnostic peritoneal lavage, repeat US, cystography, laparotomy, or autopsy also revealed evidence of abdominal injury. Positive US findings were considered false-positive if injury was not confirmed at subsequent studies. In cases of medical ascites or physiologic pelvic fluid in female patients, US findings were considered false-positive because injury could not satisfactorily be excluded with US alone, and additional studies were required to guide further treatment.
Negative US findings were counted as true-negative if all other findings were negative and/or if the patient had an uneventful clinical course. Of 2,693 patients, 2,456 had negative screening US findings, and of those, 2,286 were followed up clinically without further imaging. According to the Multiple Trauma Outcomes Study inclusion criteria (18), all patients in this study were observed for 72 hours in a surgical ward or were admitted to the intensive care unit or the intermediate care unit, where most patients (1,921 of 2,286 patients) with negative US findings and who did not undergo further imaging were observed for at least 20 hours.
The remaining 365 patients were admitted to the intensive care unit or intermediate care unit and were discharged prior to 20 hours due to low clinical suspicion of occult injury (n = 292) or were discharged against medical advice (n = 73). The minimum intensive care unit or intermediate care unit observation period in a patient who did not leave against medical advice was 7 hours. Seventeen patients left the hospital against medical advice within 7 hours of admission.
In addition to this inpatient observation period, all patients had a clinic appointment 1 week after discharge. Clinical data, including those of any imaging studies performed, from subsequent visits to the emergency department were also available for review. Monthly surveys of all trauma centers in the county were performed to screen for patients presenting to other hospitals with delayed complications of a missed injury.
US findings were considered false-negative if a subsequent study revealed free fluid, hemoperitoneum, or any visceral abdominal injury. Such studies included diagnostic peritoneal lavage, laparotomy, autopsy, CT, repeat US, or cystography performed during the initial hospitalization or later.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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One hundred seventy patients with negative US findings underwent additional abdominal studies performed for clinical indications. Typical indications for further imaging or diagnostic peritoneal lavage were high clinical suspicion (eg, seatbelt sign), transient hypotension, unexplained decrease in the hematocrit level, or persistent abdominal pain. Most of these patients (n = 119) underwent CT, 17 underwent diagnostic peritoneal lavage, 30 underwent repeat US, and 22 underwent cystography. Seventeen patients underwent laparoscopy or laparotomy, and in 15, autopsy was performed after they died of nonabdominal injuries or complications. Findings from these studies were negative for traumatic abdominal injury in 143 patients, leaving 27 with false-negative results.
Of 237 patients with positive US findings, 28 directly underwent laparotomy with or without diagnostic peritoneal lavage. Injuries were found in all 28. The remaining 209 underwent CT, cystography, and/or diagnostic peritoneal lavage; of these, 117 had positive findings (Figs 1, 2), leaving 92 with false-positive findings. Of the 145 patients with true-positive US findings, 62 underwent laparotomy (which yielded positive findings according to surgery reports), and two died of abdominal injuries on the way to the operating room. Overall sensitivity was 84% (145 of 172 patients), specificity was 96% (2,429 of 2,521 patients), and accuracy was 96% (2,574 of 2,693 patients). The positive predictive value was 61% (145 of 237 patients), and the negative predictive value was 99% (2,429 of 2,456 patients).
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False-Positive Findings
There were 92 false-positive findings (Table 2). The most common false-positive US finding was free fluid depicted or questioned at US that was not confirmed at CT or with positive diagnostic peritoneal lavage findings. This particular discrepancy accounted for 31 of 92 false-positive findings. At US, many women had fluid in the pelvis, which was ultimately believed to be physiologic. Free pelvic fluid in women of reproductive age was thought to represent a positive US finding because a traumatic cause could not entirely be excluded on the basis of US findings alone. If subsequent CT findings and if the clinical course were otherwise unremarkable, US findings were considered false-positive. This finding accounted for 26 of 92 false-positive findings.
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Other false-positive US findings were due to parenchymal abnormalities (Fig 4), perirenal fat mimicking a clot, or fluid-filled bowel mimicking injury. All examinations with false-positive findings were followed by CT or, in a minority of patients, by repeat US, diagnostic peritoneal lavage, or cystography. No false-positive findings resulted in nontherapeutic laparotomy.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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There is also variability as to the standard against which US is measured. When available, surgery or autopsy is used. In other patients, the standard may be CT, diagnostic peritoneal lavage, or clinical follow-up, none of which have perfect sensitivity. Because cost and practicality prohibit the performance of routine CT or diagnostic peritoneal lavage in all patients undergoing US at most institutions, the standard cannot always be consistent among patients.
The definition of a true- or false-positive or a true- or false-negative finding also varies, which affects the calculated accuracy of US. In some studies (13), the presence of free fluid at US and CT indicates a true-positive US finding. With this definition, the presence of medical ascites in the absence of acute abdominal injury would be included.
To place our findings in the perspective of existing data, several distinctions deserve emphasis. At our institution, studies are performed by experienced sonographers and are interpreted in real time by a radiology resident and a trauma surgeon. US staff interpret images when they are printed or the following morning if studies are performed after hours. The examination includes assessment of free fluid and parenchymal echotexture. Visualization of the pelvis is optimized by distending the urinary bladder.
The standard used in the present study was laparotomy, autopsy, additional imaging, diagnostic peritoneal lavage, or clinical follow-up. One limitation of our study is that the cases were not consecutive. The reasons that they were not consecutive are the following: First, US was not available 24 hours a day for the entire study period. Second, we chose to limit patients to those who met the Multiple Trauma Outcomes Study criteria (18) to optimize available clinical follow-up since it was used as a standard. Another limitation that applies to this and most other studies of trauma US is the use of clinical follow-up as a standard. More objective data are not always available for practical reasons. Some less severe injuries are likely missed because the patient improves clinically.
Because our inclusion criteria were those of the trauma department registry database, clinical follow-up was observation of a minimum of 72 hours in the surgical ward or was admission to the intensive care unit or intermediate care unit for surviving patients. All trauma-related deaths are referred to the coroner, and autopsy is performed. We relied strictly on the most definitive data available for each patient after images, patient charts, and autopsy findings were reviewed.
Our results differ from those of previous authors (11–17,19–23) in that we had a larger proportion of false-positive study findings. Because some traumatic and nontraumatic lesions can have similar appearances, illustrated by the lesions in two patients in Figure 4, we used US as a screening examination and regarded any suspected abnormality as an indication for further evaluation. For this reason, we considered such a finding to represent a positive US finding. Because we were interested in detecting actual injury, as opposed to fluid, we counted as false-positive those instances in which CT results confirmed the presence of fluid but suggested a nontraumatic cause. Some previous authors have considered these findings to be true-positive although no injury had actually occurred.
Initially at our trauma center, all women with pelvic fluid at US underwent CT, and our experience suggested that in many of these patients, the fluid was physiologic. It may be that women with isolated pelvic fluid do not require further evaluation in the appropriate clinical situation (27). The false-positive criteria described previously served to maximize the number of false-positive study findings, which decreased the specificity and positive predictive value. The most common cause of a false-positive finding in our series was a small amount of fluid seen or questioned at US but not confirmed at CT. Although in certain cases, the US finding represented hypoechoic fat or another misleading structure, it may be that US is more sensitive than CT with small amounts of fluid. However, because CT was used as a standard, these findings were also counted as false-positive in our series. If findings of physiologic fluid, medical ascites, and minute amounts of free fluid were not counted as false-positive, specificity would change from 96% to 99% (2,490 of 2,521 patients), and the positive predictive value would change from 61% to 84% (145 of 173 patients).
Initial US images did not depict injuries in 27 patients in our series. Five of these patients had bowel injuries, which are known to be diagnostically challenging with US or CT (28). No patient with bowel injury and false-negative US findings underwent CT within 6 hours of US, and while delayed CT demonstrated the injury, it is uncertain whether immediate CT findings would have been diagnostic. Speed and portability are qualities that make US appealing in the setting of trauma, where rapid diagnosis is critical. However, because of the development of hemoperitoneum over time, those same qualities can make it difficult to detect injuries with slower bleeding at US. In four of five patients with bowel injury and false-negative findings, initial US was performed before fluid accumulated, and injuries were detected at repeat US. Some authors (11) have considered findings in such cases to be true-positive and advocate repeat examination in all patients. We do not routinely perform repeat examination without a clinical indication, and findings in these cases underscore the importance of having a low threshold for repeat US or CT, particularly if initial US is performed soon after the injury.
All parenchymal injuries missed were those without early hemoperitoneum. This failure in detection has been shown to be a limitation of focused abdominal sonography for trauma (22,23), and our experience suggests that even with close attention to organ parenchyma, these injuries are more difficult to identify than injuries with hemoperitoneum. One of the patients with false-negative screening US findings died of a head injury soon after admission, and small visceral lacerations were encountered at autopsy. It is questionable whether CT would have revealed these injuries.
Another limitation of US lies in the depiction of the retroperitoneal space (29,30). In our series, 10 isolated retroperitoneal injuries were missed at US. The other missed extraperitoneal injury involved the uterus in a pregnant patient. The injury was found at cesarean delivery performed because of fetal distress. Additional imaging, such as CT, would not likely have revealed the injury, and CT would have been inappropriate in this patient. Pregnant women suspected of having blunt abdominal trauma are a subset of patients with trauma who are particularly well served by screening abdominal US.
In one patient with false-negative findings, CT demonstrated no definite visceral injury but depicted a small amount of fluid that was not seen at US. Again, timing may be more responsible for the discrepancy in this patient than imaging modality because of ongoing bleeding and active fluid resuscitation in the interval between US and CT. This patient, and the majority of patients with false-negative US findings in our series, did not require surgical intervention for abdominal injuries.
Regarding clinical outcome, it could be argued that the detection of minor injuries is less crucial to the surgeon. Seventy-two patients in our series underwent surgical repair of injuries. US depicted all 28 injuries requiring immediate laparotomy and 64 of 72 injuries requiring surgery. Thus, the sensitivity of screening US in the detection of injury requiring laparotomy was 89% (64 of 72 patients), and the sensitivity in the detection of injury requiring emergent laparotomy was 100% (28 of 28 patients).
In addition to the diagnostic limitations of US, there are practical considerations. Providing 24-hour coverage for a busy level 1 trauma center is best accomplished if sonographers are in house. Initially at our institution, due to hospital budget constraints, sonographers were on call from home during the night and returned many times in one night to evaluate patients with trauma. Delay in scanning, as well as cost, was increased. Many sonographers found the on-call demands intolerable, and several left our institution. As a consequence, hours during which US was available for evaluation of patients with trauma were temporarily reduced. Portability and expediency have been cited as advantages of US, and they are certainly important attributes at our institution, where the trauma suite is some distance from the CT scanner. At other institutions, if a scanner is located in the emergency department, CT requires minimal patient transport or delay in therapy. While US works well in our situation, CT may be a better choice for centers where the CT scanner is adjacent to the trauma suite, especially if there is no in-house sonographer.
A type of patient particularly well suited for evaluation with US is the unstable patient. US is the only noninvasive means of evaluating patients who cannot be transported. US can be performed simultaneously with emergent surgical intervention. Using US, the surgeon can know if immediate laparotomy is necessary while the patient is still in the operating room or resuscitation suite undergoing treatment for other injuries. In addition, all stable patients with blunt abdominal trauma could be regarded as potentially unstable. Further, US can help minimize time spent in the CT scanner, where the condition of a patient could potentially deteriorate.
The use of US to screen for abdominal injury in patients with blunt abdominal trauma has substantially changed our institutional practice. Diagnostic peritoneal lavage is now rarely performed; CT is used only if screening US findings are positive, if there is clinical suspicion of injury despite negative US findings, or if US is not available. As with any screening examination, the limitations of US must be recognized, and it must be used in conjunction with all clinical data. Its success at our institution is partly due to the fact that all patients believed to be at risk for occult abdominal injury, even those with negative US screening findings, are admitted to the hospital and observed. This protocol facilitates the use of additional studies if a patient’s clinical status deteriorates.
At our institution, the period of observation is similar for patients who have negative abdominal CT findings. There is no consensus among trauma surgeons as to the minimum observation period necessary after abdominal US or CT with negative findings, and we do not recommend the sole use of US at institutions where an observation period is not routine. We believe that US is an excellent screening modality in the setting of blunt abdominal trauma, but it should be used only in centers where a period of clinical observation is part of the trauma protocol.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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