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Emergency Radiology |
1 From the Departments of Radiology (C.B.S., M.A.B., O.A.A.B., G.C.), Family and Preventive Medicine (R.D.), and Surgery (D.A.F., D.B.H.), University of California, San Diego, UCSD Medical Center, 200 W Arbor Dr, MC 8756, San Diego, CA 92103. Presented at the 2002 RSNA scientific assembly. Received February 20, 2003; revision requested May 14; revision received June 4; accepted July 23. R.D. supported in part by National Institutes of Health grant M01 RR00827. Address correspondence to C.B.S. (e-mail: csirlin@ucsd.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: From a database of 4,000 patients screened with US for blunt abdominal trauma at a level 1 trauma center, the 3,679 patients with negative US findings were retrospectively classified by consensus of two authors into high-risk (n = 494) and low-risk (n = 3,185) groups based on the presence of hypothetical predictors of missed injury: hematuria (n = 96) or fracture of the sixth through 12th ribs (n = 216), lumbar spine (n = 105), or pelvis (n = 174). Outcome in each patient was determined by the same two authors consensually after retrospective review of the trauma registry and all radiologic, surgical, and autopsy reports. The risk for missed abdominal injury was determined for each patient risk group and for each hypothetical predictor. Risks were statistically compared by using the Pearson 
2, Fisher exact, or Fisher-Freeman-Halton exact test, depending on expected frequencies.
RESULTS: High-risk patients were 24 times more likely to have abdominal injuries after negative US findings (30 [6.1%] of 494) than were low-risk patients (eight [0.25%] of 3,185) (P < .001). Among high-risk patients, the absolute risks for missed abdominal injury associated with specific predictors were 15.6% (15 of 96 patients) for hematuria, 6.0% (13 of 216) for lower rib fractures, 7.6% (eight of 105) for lumbar spine fractures, and 5.2% (nine of 174) for pelvic fractures. Each of these risks was significantly higher for patients in the high-risk group than for those in the low-risk group (P < .001).
CONCLUSION: Hematuria and fracture of the lower ribs, lumbar spine, or pelvis are objective predictors of missed abdominal injury in patients with blunt abdominal trauma and negative US findings, and such patients may benefit from additional screening with computed tomography.
© RSNA, 2003
Index terms: Abdomen, injuries, 66.761, 70.41, 81.4134 • Abdomen, US, 70.1298 • Emergency radiology • Trauma, **.412
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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We hypothesized that hematuria and fractures of the sixth through the 12th ribs, lumbar spine, and pelvis might be predictors of missed abdominal injury in patients with negative US findings. Thus, the primary purpose of our study was to determine the risk for missed abdominal injury in patients with blunt abdominal trauma and negative findings at screening US and with coexistent hematuria or fracture of the sixth through the 12th ribs, lumbar spine, or pelvis.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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After screening US, patients were admitted to the trauma surgery service for observation and management of other injuries (median hospital stay, 1.8 days; range, 6 hours to 430 days). Patients who were discharged after screening US without being admitted did not meet MTOS criteria and were not included in the study. Anteroposterior chest radiography and urinalysis were performed in all patients at admission. CT and other tests—including repeat US, cystography, diagnostic peritoneal lavage, exploratory laparotomy, and radiography of the spine and pelvis—were performed at the discretion of the trauma service. The number of patients in whom radiographs of the spine or pelvis were obtained was not recorded. Because all study patients met MTOS entry criteria, all were considered to have nontrivial mechanisms of trauma. Survivors were examined at follow-up appointments in the trauma clinic 1 week after discharge. Autopsies were performed in all patients who died. The results of autopsy and any additional follow-up radiologic or surgical examinations served as the standard of reference for this study. If no autopsy or additional examination was performed, clinical outcome was used as the standard, as previously described (4,10–13,15). The date and time of imaging were not recorded. Urinalysis results were typically available within 15 minutes of urine specimen collection, but the exact time was not recorded.
The trauma service entered patients prospectively into the trauma registry. The registry contains information about all documented injuries and is updated regularly by the trauma service as new injuries are discovered. To identify any missed injuries after discharge from the hospital, the trauma service performed systematic monthly audits of all trauma centers in the county. In these monthly audits, all errors and delays in diagnosis of traumatic injuries in all county trauma centers were discussed, tabulated, and entered into the trauma registry of each institution. To our knowledge, this is the only systematic county-level trauma auditing system in the United States.
US Technique
US examinations were performed during trauma resuscitation by one of 15 certified sonographers (with 1–20 years of experience) using one of two scanners (ATL HDI 3000, Advanced Technologies Laboratories, Bothell, Wash; or 128-XP, Acuson, Mountain View, Calif) and a 2.25-, 3.5-, or 5.0-MHz sector transducer or a 5.0-MHz curved-array transducer with Doppler capabilities. Prior to scanning, the patients’ bladders, if empty, were distended with 200–300 mL of sterile saline solution administered via Foley catheter. Seven abdominal regions were examined for free fluid, including the upper quadrants, the pelvis, the paracolic gutters, and the renal fossae. Visceral organs were evaluated for parenchymal abnormalities indicative of injury. Total scanning time typically was 3–5 minutes, but the precise duration of each US examination was not recorded.
US Image Interpretation
Until April 2001, US examinations were recorded with an imaging system (Kodak Image Link; Eastman Kodak Company, Rochester, NY), and images were reviewed with an alternator. After April 2001, images were archived digitally with a picture archiving and communication system (IMPAX; Agfa Healthcare, Ridgfield Park, NJ) and reviewed on a 19-inch color monitor (MWD 421; Barco Display Systems, Kortrijk, Belgium) with resolution of 1,000 x 1,000 pixels.
US images were interpreted prospectively by the resident and staff radiologist on the US service at the time of the examination. Images were considered positive for abdominal injury only if they depicted free fluid, extraparenchymal hematoma, or parenchymal abnormality indicative of injury; all other images were considered negative. In the trauma setting, any parenchymal abnormalities except fatty infiltration of the liver or clearly visualized cysts, gallstones, renal calculi, or calcified granulomas are considered to indicate possible injury. On the basis of a previously reported study (12) and in the absence of other suspicious findings, we considered small accumulations of anechoic fluid measuring less than 3 cm in the anteroposterior dimension and isolated in the cul-de-sac in women of reproductive age to be physiologically normal and not indicative of traumatic injury.
Data Collection and Statistical Analysis
The trauma registry and all radiologic, surgical, and autopsy reports for each patient in the study were reviewed by two authors (C.B.S., M.A.B.) in consensus to determine the presence of fracture of the lower ribs, lumbar spine, or pelvis, or of hematuria at admission urinalysis. Lower rib fractures were defined as those involving one or more of the lower seven ribs (the sixth through 12th ribs). The exact portion of a rib that was fractured was not documented. Fractures involving the first through the fifth ribs were not considered because they did not satisfy our a priori hypothesis. Any documented fractures of the lumbar spine or pelvis qualified. For lumbar spine fractures, we recorded whether fractures were limited to the posterior elements (transverse or spinous processes, pedicles, and laminae) or involved at least one vertebral body. We did not record the exact site or type of pelvic fracture. We did not grade the severity or score the number of individual fractures involving the ribs, lumbar spine, or pelvis. Hematuria was defined as more than 50 red blood cells per high-power field, because this was the definition used in the trauma registry. The gross appearance of the urine specimen was not recorded in the trauma registry and therefore was not included in our analysis.
On the basis of our hypothesis, patients were retrospectively divided into two risk groups by the same two authors in consensus. The high-risk group (n = 494) consisted of patients with hypothetical predictors of missed injury: hematuria (n = 96) or one or more fractures in the lower ribs (n = 216), lumbar spine (n = 105), or pelvis (n = 174). The low-risk group (n = 3,185) had no hematuria or fractures.
The sum of the numbers of patients with each individual hypothetical predictor exceeded the number of patients in the high-risk group because some high-risk patients had more than one predictor. Whereas 408 patients had a single predictor, 76 patients simultaneously had two predictors, nine simultaneously had three predictors, and one simultaneously had all four predictors.
Outcomes in high- and low-risk groups were then compared by calculating the absolute and relative risks for missed abdominal injury (any, surgical, and nonsurgical). Absolute risk was defined as the percentage of patients with negative findings and missed injury at screening US. Relative risk was calculated by dividing the absolute risk in a given group by the absolute risk in the low-risk group. Comparisons were repeated with data stratification according to patient sex. Abdominal injuries were defined as contusion, laceration, or hematoma of intraabdominal structures, or as unexplained intraabdominal fluid if no discrete injury was identified. Surgical injuries were defined as those for which laparotomy was performed. Nonsurgical injuries were defined as those treated successfully without surgical intervention or considered minor at autopsy. The general location of injuries (intraperitoneal, extraperitoneal, or both) and the specific organs involved were documented. The delay in hours between negative findings at screening US and eventual diagnosis of missed injury was recorded.
Each of the four hypothetical predictors (hematuria, rib fracture, lumbar spine fracture, and pelvic fracture) was analyzed as an independent risk factor for missed injury. This analysis was performed in two separate stages. In the first stage, absolute risk and relative risk were calculated for missed injury associated with each of the parameters. Patients were included in the risk calculation for each hypothetical predictor that they had; thus, patients who had more than one predictor were counted more than once in the analysis. The specific sites and general locations (intraperitoneal, extraperitoneal, or both) of missed abdominal injuries associated with each predictor were recorded.
In the second stage of analysis, the individual hypothetical predictors were statistically compared. To achieve accurate statistical comparison, we had to ensure that patients with more than one predictor were not counted more than once. In contrast to the first stage, in which patients were counted as many times as the number of predictors they had, in the second stage, patients with more than one predictor were excluded from analysis. For example, to compare the three categories of fracture, we restricted our analysis to the 378 patients who had fractures involving only one part of the skeleton (ribs [n = 182], lumbar spine [n = 71], and pelvis [n = 125]), regardless of whether they had hematuria. To compare hematuria with the various fractures, we restricted our analysis to 408 patients: the 60 patients with hematuria and no fracture, and the 348 patients with only one type of fracture and no hematuria (169 with fracture of the ribs, 70 with fracture of the lumbar spine, and 109 with pelvic fracture). To further evaluate lumbar fracture as a predictor of missed abdominal injury, we restricted our analysis to the 105 patients with lumbar fracture (independent of hematuria or other fractures): 39 with one or more fractures limited to the posterior elements of the spine, and 66 with one or more fractures involving at least one vertebral body.
Finally, each patient was assigned a numeric risk score from 0 to 3+, according to the number of hypothetical predictors present in that patient. The risk for missed injury was then calculated for each risk score.
For relative risk, 95% CIs were calculated. Proportions were statistically compared by using the Pearson 2 test, Fisher exact test, or Fisher-Freeman-Halton exact test, depending on the expected frequencies. A Wilcoxon rank sum test was used to determine differences in number of injuries per patient between risk groups. The Somer D test for association was applied to assess for a trend of increasing risk for missed injury as the risk score increased from 0 to 3+. P values less than .05 were assumed to indicate statistically significant difference for each analysis, with no adjustments being made for multiple comparisons. Power to detect reported differences was calculated post hoc by using two-tailed tests. Statistical software was used for these analyses (JMP version 5.0, SAS Institute, Cary, NC; SPSS version 10.1.0 for Windows, SPSS, Chicago, Ill; and StatXact-5 version 5.0.3, Cytel Software, Cambridge, Mass).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Patients in the high-risk group were 24 times more likely than patients in the low-risk group to have a missed abdominal injury (relative risk, 24; 95% CI: 11, 52) (Fig 2). The relative risk was 16 (95% CI: 5, 51) for surgical injuries and 32 (95% CI: 11, 94) for nonsurgical injuries (Fig 2). The differences in risk for missed injury (any, surgical, or nonsurgical) between high-risk and low-risk patient groups were statistically significant (P < .001 for each comparison). Our study had higher than 99% power to detect these reported differences.
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The delay between negative findings at screening US and eventual diagnosis of missed abdominal injuries in patients of each risk group is shown graphically in Figure 3. All missed injuries in the eight low-risk patients were diagnosed within 24 hours of initial US. By contrast, missed injuries in 26 (87%) of the 30 high-risk patients were diagnosed within 24 hours; missed injuries in the other four high-risk patients were diagnosed at 31, 63, 72, and 144 hours.
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The risk for missed abdominal injury was significantly higher in patients with any of the hypothetical predictors than in patients with no predictors (P < .001 for each comparison). As shown in Table 2 and Figure 4, 5.2%–7.6% of patients with fractures had missed abdominal injuries (ie, the percentage varied according to the type of fracture), including 1.2%–2.9% with surgical injuries and 4.0%– 4.8% with nonsurgical injuries. Moreover, 15.6% of patients with hematuria had missed injuries, including 5.2% with surgical injuries and 10.4% with nonsurgical injuries (Table 2, Fig 4). Thus, compared with the risks for patients with no predictors, the relative risks of missed injury were 24 for patients with rib fracture (95% CI: 10, 57), 30 for patients with lumbar spine fracture (95% CI: 12, 79), 21 for patients with pelvic fracture (95% CI: 8, 53), and 62 for patients with hematuria (95% CI: 27, 143) (Fig 5). Our study had a power of 98%–99% for detection of these reported differences.
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In the calculations of risk and relative risk and in the tabulation of specific injuries associated with each hypothetical predictor, patients with more than one predictor were counted more than once. In contrast, in the statistical comparison of individual predictors, patients with more than one predictor were excluded so that they would not be compared with themselves.
Assessment of Risk Score
A total of 3,185 patients had no hypothetical predictors (risk score of 0), 408 had one predictor (risk score of 1), 76 had two predictors (risk score of 2), and 10 had three or more predictors (risk score of 3+). The percentage of patients with missed injuries increased in a stepwise fashion as the risk score increased (Fig 6): Eight (0.25%) of 3,185 patients with missed injuries had a risk score of 0, 19 (4.7%) of 408 had a risk score of 1, eight (10.5%) of 76 had a risk score of 2, and three (30.0%) of 10 had a risk score of 3+. A similar trend was observed in the group of injured patients with regard to the need for surgery: four (0.1%) of 3,185 patients with a score of 0, seven (1.7%) of 408 with a score of 1, and—with the two upper risk score categories combined to compensate for the small sample size—three (3.5%) of 86 with a score of 2–3+ had injuries that required surgery. These trends were statistically significant (P < .001 and P < .01, respectively). Our study had a power higher than 99% for detection of these reported trends.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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The results of this study confirm our hypothesis that hematuria and fractures of the lower ribs, lumbar spine, and pelvis are predictors of missed injury in patients with negative findings at screening US for blunt abdominal trauma. Patients with at least one of these predictors of injury were significantly more likely to have a missed injury than were patients without any predictors (6.1% vs 0.25%, relative risk of 24). The differences remained statistically significant when injuries were stratified as surgical (2.0% vs 0.1%, relative risk of 16) or nonsurgical (4.0% vs 0.1%, relative risk of 32). Thus, although high-risk patients constituted only 13% (494 of 3,679) of all patients in the study, they accounted for 79% (30 of 38) of those with missed injuries, including 71% (10 of 14) of those with surgical injuries and 83% (20 of 24) of those with nonsurgical injuries. In addition to their prognostic value, a distinct advantage of these predictors is that they are objective criteria that can be assessed at initial work-up.
As opposed to patients with coexistent predictors of injury, patients with negative findings at screening US and no predictors had abdominal injuries extremely rarely. In our study, only one in 398 low-risk patients with negative US findings had a missed abdominal injury and only one in 796 had a missed injury requiring surgery. It should be emphasized, however, that screening US examinations at our institution are performed by experienced certified sonographers using high-end US scanners and include a thorough evaluation of peritoneal and retroperitoneal compartments for free fluid and for parenchymal abnormalities. Because equipment, sonographer experience, and examination protocols may vary from institution to institution, our results may not be generalizable to all trauma centers.
Our choice of specific predictors was made a priori and was based on several observations and assumptions. We chose hematuria as a potential predictor, for example, because we assumed that in the setting of trauma, hematuria should be considered a surrogate marker of urinary tract injury. This assumption is supported by our data: The most commonly injured organs in patients with hematuria were the kidneys and the neighboring adrenal glands. We chose the various fractures as predictors because fractured bones could lacerate tissues and cause direct injury to organs. We chose the sixth through the 12th ribs because these ribs are in contact with portions of the diaphragmatic pleura and would be most likely, when fractured, to puncture the abdomen. Similarly, we chose lumbar spine and pelvic fractures because of the proximity of these bones to abdominal structures. In our study, the most commonly injured organs in patients with lower rib fractures were the liver and spleen, and the most common injury in patients with lumbar spine or pelvic fractures was retroperitoneal hematoma. However, not all abdominal injuries could be directly explained by a specific predictor. Intraperitoneal solid organ and bowel or mesenteric injuries, for example, occurred in patients with hematuria and lumbar or pelvic fractures, although not as commonly as did extraperitoneal injuries. Nonetheless, these particular fractures were important surrogate markers of important injury, regardless of direct causality.
Among the individual predictors, hematuria had a higher association with missed abdominal injury than did any of the fractures, whereas the various fractures conveyed similar risks. These statistical results should be interpreted cautiously. One might conclude that hematuria is a better marker of missed injury than the other predictors; however, the higher risk associated with hematuria could be related to the somewhat arbitrary definition of hematuria in the trauma registry (>50 red blood cells per high-power field), which served as the source of urinalysis data in our study. It is likely that hematuria, if defined by a lower threshold number of red blood cells, would have had a lower association with missed injury. This hypothesis was not analyzed in our study because the trauma registry did not contain data on such urinalysis specimens. Moreover, for the sake of simplicity, we made no attempt to grade the severity of individual fractures. It is likely that displaced, comminuted, or multiple fractures may have increased the risk for abdominal injury, but this hypothesis also was not evaluated. Although the sample sizes are small, our data indicate that patients with isolated posterior element fractures had a risk for missed abdominal injury similar to that of patients with vertebral body fractures; thus, patients with even relatively minor lumbar fractures should undergo additional testing to exclude occult abdominal injury. This finding supports previous observations regarding the importance of transverse process fractures in the assessment of trauma patients (17).
As we expected, the risk for missed abdominal injury increased as the number of predictors increased. Although patients with multiple predictors were relatively uncommon (only 2.0% [76 of 3,679] had a risk score of 2 and only 0.3% [10 of 3,679] had a risk score of 3+), they were at particularly high risk for missed injury (10.5% and 30.0%, respectively). These findings lead us to conclude that such patients should receive a more thorough initial assessment. It also should be emphasized, however, that nearly 5% of patients with only one predictor (risk score of 1) had missed abdominal injuries. Thus, even patients with a single predictor should undergo additional testing.
On the basis of these results, we propose a new procedure for triaging blunt abdominal trauma patients (Fig 7): If abdominal injury predictors are known or suspected to be present in a patient before screening US is performed, we suggest that the patient be screened instead with CT. If no predictors are known or suspected, the patient should be screened with US. However, because screening US is not a definitive test for blunt abdominal trauma injuries, patients with negative findings at US and with no known or suspected predictors should be admitted for observation. The required admittance of trauma patients with negative US findings would not impose an additional burden on most level 1 trauma centers, because trauma patients—even those with negative findings at CT—are routinely admitted. If predictors are discovered during observation, patients with negative US findings should undergo further evaluation with CT.
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If we had followed the proposed triage procedure from the inception of screening US at our institution, only eight (0.2%) of the 3,679 patients with negative US findings who were included in this study would have been inappropriately triaged (ie, admitted for observation without undergoing CT). The missed injuries in all eight of these patients were discovered during the first 24 hours of observation, so any negative effect of applying this procedure would presumably have been small.
Two limitations of our study should be emphasized. Because of the retrospective nature of the study, some data could not be reliably collected. For example, the exact times at which urinalysis and radiographic examinations of the chest, lumbar spine, and pelvis were performed and interpreted were not recorded prospectively. Typically, patients underwent chest radiography initially, followed by abdominal US. Additional radiographic examinations were performed in some patients after screening US, on the basis of clinical suspicion of spine, pelvic, or other fractures. Urine specimens were usually obtained prior to the performance of US.
Another limitation of our study is that definitive findings of missed abdominal injury (eg, findings at CT, laparotomy, or autopsy) were available for only a minority of the patients included in our study. As a result, clinical suspicion and findings at follow-up diagnostic testing were used as the reference standard for the majority of our patients (3,343 [90.9%] of 3,679). To optimize the validity of a clinical reference standard, we limited our study to patients who met MTOS entry criteria. In other words, all surviving patients were admitted either for a minimum of 6 hours to the intensive care unit or the intermediate care unit, where continuous monitoring is possible, or for a minimum of 72 hours to the surgical ward. Patients who were discharged after screening US without admission did not meet MTOS entry criteria and were not included in the study. Finally, all county trauma centers were audited monthly to detect potential missed injuries after discharge. Although some discharged patients with missed injuries might have received subsequent treatment at institutions outside the county, such patients would likely have been few.
Similarly, although urinalysis and chest radiography were performed in all patients, radiographs of the lumbar spine and pelvis were obtained on a case-by-case basis at the discretion of the trauma service, because our institution’s trauma assessment procedure includes selective radiographic screening of the lumbar spine and pelvis on the basis of clinical suspicion (18). Thus, an imaging reference standard for lumbar and pelvic fractures was not available in all study subjects, and it is conceivable that some patients in the low-risk group may have had undiagnosed fractures. Moreover, because radiographs were not obtained in all patients, lumbar transverse process fractures were detected at CT in three patients who did not undergo spinal radiography. In our opinion, inclusion of these fractures in our analysis was warranted because the fractures were grossly displaced and likely would have been detected on radiographs. Radiography was not performed in these patients, however, because it would have provided no benefit with regard to patient care. Moreover, our study was designed to retrospectively assess the predictive value of various fractures and included no prospective assessment of radiographically detectable fractures. Finally, because the criteria for obtaining ancillary radiographs in trauma victims may vary among institutions, our results may not be generalizable to all trauma centers.
It is noteworthy that the male patients in our study population, on average, were much younger than the female patients. One possible explanation for this divergence in age is that boys and young men are more likely than girls and young women to be trauma victims. In addition, because of the large sample size, a small absolute difference (4 years) in mean age achieved high statistical significance. Nevertheless, the age difference between the male and female patient subgroups did not bias our results, because the risk for missed injury was unaffected by patient sex.
In conclusion, the results of our study show that hematuria and fractures of the lower ribs, lumbar spine, and pelvis are objective predictors of missed abdominal injury in blunt abdominal trauma patients with negative findings at initial screening with US. It is important to note the presence of these predictors in trauma patients, because the rapid diagnosis and treatment of abdominal injury is a key factor in decreasing morbidity and mortality associated with blunt trauma. These objective predictors can be assessed immediately after the patient’s arrival in the resuscitation suite. We believe that patients with these predictors for missed injury should be screened immediately with CT rather than US, whereas appropriate care for low-risk patients can be determined on the basis of US findings alone. Patients who have no predictors for missed abdominal injury need not undergo CT or diagnostic peritoneal lavage, both of which involve a risk of associated complications. Lastly, the presence of one or more of these predictors in patients with positive US findings could indicate an increased likelihood that their injuries will require surgery, but additional study is needed to test this hypothesis.
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FOOTNOTES |
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Abbreviation: MTOS = Major Trauma Outcome Study
Author contributions: Guarantor of integrity of entire study, C.B.S.; study concepts, C.B.S., M.A.B., O.A.A.B., D.B.H., G.C.; study design, C.B.S., M.A.B., G.C.; literature research, C.B.S., M.A.B., G.C.; clinical studies, C.B.S., M.A.B., G.C., D.B.H.; data acquisition, C.B.S., M.A.B., G.C., O.A.A.B., D.A.F.; data analysis/interpretation, C.B.S., D.A.F., R.D.; statistical analysis, C.B.S., R.D.; manuscript preparation and manuscript editing, C.B.S., R.D., G.C.; manuscript definition of intellectual content and manuscript revision/review and final version approval, all authors
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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