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Commentary |
1 Department of Radiology, New York University Medical Center, 560 First Ave., New York, NY 10016.
Received February 1, 2006; accepted after revision February 6, 2006.
Address correspondence to J. E. Jacobs.
Keywords: abdominal imaging • appendicitis • CT • sonography
Appendicitis was first described and reported by Reginald H. Fitz [1] at the 1886 meeting of the Association of American Physicians. Appendicitis is the most frequent cause of acute abdominal pain requiring surgical intervention in the Western world, and appendectomy is the most common abdominal operation performed on an emergency basis annually in this country [2, 3]. The current annual incidence of acute appendicitis is one case per 1,000 population in the United States and 86 cases per every 100,000 persons worldwide [4, 5]. Furthermore, missed appendicitis is one of the most common causes of successful malpractice claims against physicians [6-8].
Despite continued advances in clinical medicine, the diagnosis of acute appendicitis often remains elusive, with as many as 12-30% of patients undergoing unnecessary appendectomy [9]. The classic clinical triad of migrating abdominal pain, right lower quadrant tenderness, and leukocytosis is absent in up to 50% of patients [10]. The currently accepted negative laparotomy rate is 10-15%, but negative laparotomy rates can be much higher in women of child-bearing age [11, 12]. The physician's goal is to expeditiously and accurately confirm or exclude the diagnosis of acute appendicitis while minimizing diagnostic delays and hospital costs. Mortality and morbidity rates for removal of a normal appendix are 0.14% and 4.6%, respectively, but increase to 0.24% and 6.1% for acute appendicitis and to 1.7% and 19% for perforated appendicitis [13]. Prevention of appendiceal perforation mandates diagnosis in a timely fashion.
Clinical assessment remains an essential and critical part of the initial evaluation of patients with suspected acute appendicitis. However, advances in radiology have made preoperative noninvasive imaging an important diagnostic adjunct to clinical evaluation. Toward that end, radiology is now commonly used for elucidation of the cause of acute abdominal pain. The diagnosis or exclusion of acute appendicitis is greatly facilitated by accurate identification of the inflamed or normal appendix. In addition, radiology can often determine alternate conditions as the cause of the patient's pain when the appendix is normal. Graded compression sonography and MDCT are currently the preferred primary imaging methods throughout the world for evaluating the appendix in patients with right lower quadrant pain who are suspected of having appendicitis.
The technique of graded compression sonography, introduced by Puylaert [14] in the mid 1980s, has substantially aided sonographic identification of the appendix. The advantages of sonography are that it is widely available, relatively inexpensive, and noninvasive, and, most important, that it poses no ionizing radiation risk to the patient. This latter advantage is significant when evaluating pregnant patients. In addition, radiation is an important concern in pediatric and young adult patients, who are up to 10 times more sensitive to the effects of ionizing radiation than middleaged and elderly adults. Important limitations of sonography are that it is operator-dependent, it can be difficult to perform in patients with severe abdominal pain or in patients with large amounts of bowel gas, and it can be limited in muscular or obese patients. The limited ability of sonography to adequately penetrate the abdomen in obese patients has contributed to its lack of widespread use in North America and parts of Europe.
Identification of the normal or abnormal appendix is paramount for helping radiologists to confidently exclude or diagnose appendicitis. Identification may be limited using a high-frequency linear transducer in patients with poorly compressible right lower quadrant bowel, in large patients with the cecum and appendix located deep within the pelvis, in patients with poorly defined right lower quadrant anatomy, and in patients with a retrocecal or perforated appendix. The reported diagnostic accuracy of sonography varies depending on the patient population studied. A meta-analysis of pediatric and adult studies published between 1986 and 1994 showed an overall sonographic sensitivity of 85% and specificity of 92% [15].
Because of the limitations of sonographic evaluation, CT has emerged in much of the United States as the preferred imaging technique for assessing appendicitis. Current helical CT protocols recognize that diagnosis is greatly facilitated with the use of prospective thin-section (3- to 5-mm collimation) scanning through the right lower quadrant to maximize z-axis resolution and to improve identification of the appendix. The administration of IV contrast material aids in the diagnosis of appendicitis by increasing detection of the inflamed appendix [16]. Administration of oral or rectal contrast material to opacify the cecum and terminal ileum is useful for differentiating the appendix from adjacent bowel. Performing data acquisition through the entire abdomen and pelvis facilitates identification of the appendix if it lies outside the right lower quadrant and also facilitates alternative diagnoses. Helical CT has proven to be an effective and accurate means of diagnosing acute appendicitis, with reported sensitivities of 90-100%, specificities of 91-99%, accuracies of 94-98%, positive predictive values of 92-98%, and negative predictive values of 95-100% [17-22].
Studies comparing the use of sonography with CT in patients suspected of having acute appendicitis have generally favored CT for providing greater diagnostic accuracy, superior detection and staging of complications, and higher accuracy for establishing alternative diagnoses [23-25]. One of the reasons CT is preferred for evaluation of appendicitis was emphasized in a study performed by Peña and Taylor [26]. Those authors prospectively evaluated 139 children and young adults who had equivocal clinical findings of appendicitis and found that radiologists were significantly more confident of their CT interpretations than their sonographic interpretations. Furthermore, they found that radiologists' level of experience did not significantly affect the diagnostic confidence of their interpretations. Studies have also shown that CT may help to decrease hospital costs and negative laparotomy rates [27, 28].
In this month's AJR there are three interesting articles discussing imaging features helpful for diagnosing acute appendicitis or its mimics. These articles emphasize the importance of identifying both appendiceal and periappendiceal abnormalities for more accurately determining the cause of right lower quadrant pain.
The pictorial essay by van Breda Vriesman and Puylaert [29] illustrates CT and sonographic features of common alternative diagnoses in patients with right lower quadrant pain and suspected appendicitis.
Ripolles et al. [30] evaluated a large series of patients with Crohn's disease and appendiceal involvement and showed that ileal wall thickening greater than 5 mm and color Doppler flow in the terminal ileum were the most useful sonographic indicators for discriminating appendiceal involvement in Crohn's disease from acute appendicitis. Secondary helpful features were involvement of intestinal segments outside the ileocecal region, the presence of irregular thickening of the submucosal layer of the terminal ileum, and fibrofatty proliferation of the mesentery adjacent to the inflamed ileum.
Rodriguez et al. [31] retrospectively reviewed a large series of children (18 years or younger) who underwent CT or sonography before appendectomy and compared the imaging characteristics and outcomes of those younger than 5 years with those older than 5 years [31]. Those authors showed that the rate of appendiceal perforation is markedly higher for children younger than 5 years, that the risk for perforation is inversely proportional to the patient's age, and that echogenic periappendiceal fat (thought to be due to edema related to transmural appendiceal inflammation) is more commonly seen in younger patients. It is telling that although the study was performed at a dedicated urban pediatric teaching hospital, most of the children (68%) were imaged only with CT.
Sonography has historically been advocated for evaluating right lower quadrant pain in children, young adults, and pregnant women because of its lack of ionizing radiation and its ability to aid the diagnosis of alternative conditions such as gynecologic abnormalities. However, radiologists' frequent inability to confidently identify the appendix sonographically because of the technical factors previously discussed has led to the preferential use of CT in most cases. The ability to rapidly and confidently make an accurate diagnosis using CT is thought to outweigh the necessity of administering ionizing radiation in these patients. It remains to be seen whether continued improvements in sonographic equipment, color and power Doppler imaging, 3D sonography, and radiology training methods can significantly affect the diagnostic accuracy of sonography.
Analysis of volumetric CT or 3D sonographic data sets using workstations and multiplanar reformation techniques may also increase the conspicuity of the appendix and periappendiceal abnormalities, thereby improving diagnostic accuracy. Alternatively, recent studies have shown the potential efficacy of MRI for the evaluation of patients with right lower quadrant pain, and MRI may ultimately prove to be the most appropriate imaging technique for assessing children, young adults, and pregnant women [32, 33].
In conclusion, preoperative radiology assessment has proven to be an important ancillary diagnostic function for the accurate evaluation of patients with right lower quadrant pain. Advances in imaging technology will hopefully continue to improve identification of the normal or abnormal appendix and associated ancillary findings that are useful for confirming or excluding the many causes of right lower quadrant pain.
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