Applied Radiology

Dr. McGahan is a Professor of Radiology and the Director of Abdominal Imaging and Ultrasound at the University of California Davis Medical Center, Davis, CA. He is also a member of the Editorial Advisory Board of this journal.

Appendicitis is one of the most common conditions in children and adults. ^1,2 In most situations, signs and symptoms can establish the diagnosis reliably. Patients usually present with periumbilical pain that later localizes to the right lower quadrant (RLQ) at McBurney's point. In most cases, pain is accompanied by mild anorexia and nausea or vomiting. In a recent series, the combination of abdominal pain and tenderness accompanied by leukocytosis >12,000 cells/mm ³ had the highest predictive value for acute appendicitis. ^2,3 However, in some patients there is a delay in diagnosis resulting in increased risk of perforation, sepsis, and even death. In a recent series of childhood appendicitis, all 10 patients younger than 10 years of age had a perforated appendix at the time of operation. In 35 children between 1 and 5 years of age with appendicitis, there was a perforation rate of 69%. ² Recent series have demonstrated a 19.8% morbidity rate among patients with perforated appendicitis. ³

The surgical literature has accepted that up to 25% of patients undergoing appendectomy will have a normal appendix. ⁴ This is the so-called negative appendectomy rate. Therefore, the goal of any further studies would be threefold. The first goal would be to diagnose appendicitis promptly in hope of reducing the rate of perforation and associated complications. The second goal would be to reduce the number of patients undergoing unnecessary removal of a normal appendix. After exclusion of appendicitis, the final goal would be to establish the etiology of the patient's RLQ pain.

Computed tomography versus ultrasound

Acute appendicitis and its complications may be imaged with either computed tomography (CT) or ultrasound (US). Several questions remain concerning the use of these imaging modalities. The first question is "Should any imaging be used?" The second question is "If imaging is performed, should it be CT, US, or any other imaging modality?"

These questions have no clear answer. A recent controversial report by Lee ³ indicated that neither the use of CT nor US led to improved diagnostic accuracy for appendicitis, but, in fact, these procedures actually might delay surgical consultation and necessary appendectomy. However, many individuals do not agree with this view. Most institutions have reached a general consensus that for patients with suspicion of acute appendicitis, selective, rather than routine, use of imaging studies is recommended. ⁵ In many, if not most, patients, the diagnosis or exclusion of appendicitis may be made clinically. However, when imaging is used, it may reduce the negative appendectomy rate. ⁶ In a report by Schuler et al, ⁷ the use of CT reduced the negative appendectomy rate from 21% without the use of preoperative CT, to 6% with the use of CT. Therefore, most would utilize imaging, either CT or US, in cases in which the diagnosis of appendicitis is equivocal.

In those cases in which selective use of imaging is performed, should it be CT or US? Original publications comparing CT to US usually showed that CT had a higher sensitivity than US. For instance, in 1994, Balthazar et al ⁸ published data showing US sensitivity of 76% and CT sensitivity of 96%, with US accuracy of 83% compared with the CT accuracy of 94%. Similar comparative sensitivities have been reported with spiral CT compared with compression US, with CT sensitivity ranging from 85% to 100% and US sensitivity ranging from 74% to 100%. ^9-11 Computed tomography is usually associated with a greater decrease in the negative appendectomy rate than is US. In most studies, use of both CT and US led to a decrease in the negative appendectomy rate compared with clinical evaluation. ¹⁰

However, in patients for whom CT is used as the imaging modality, a higher rate of perforation is often reported than is reported in patients for whom US is used. Appendiceal perforation at appendectomy was found in 23% when patients were examined by US alone, 54% for CT alone, and 71% when both examinations were used. ¹⁰ This result is similar to a report presented at the 2001 RSNA meeting that reported a perforation rate of 23% in patients examined by US and 55% in those in which CT was utilized to diagnose appendicitis. ¹²

To further complicate the issue, some such as Lee ³ have shown an average delay in diagnosis of 7.8 hours when CT was utilized for evaluation of patients with RLQ pain. This delay may be a cause for higher perforation rates when using preoperative CT imaging. However, Lee did not believe that this delay was necessarily associated with the increased perforation rate. It is possible that the more complicated or equivocal cases are more diagnostically difficult and therefore require imaging studies. The extended workup delays diagnosis and thus may lead to an increased perforation rate. More straightforward cases may go directly to surgery, and thus exhibit a lower perforation rate at appendectomy.

Currently, most individuals advocate CT as the most adequate method for imaging appendicitis. However, there are certain situations in which US may be especially helpful. These typically would include many women and children.

Diagnostic CT

CT technique

Within the literature there are multiple combinations of CT techniques that have been used. Some of these techniques advocate use of intravenous (IV) contrast, while others do not use IV contrast. Some include a complete examination of the abdomen and pelvis, while others include only a focused examination of the abdomen or pelvis. Some advocate use of oral contrast and others advocate use of rectal contrast. Some of the advantages and disadvantages of each of these techniques are listed in Table 1.

There are proponents of virtually every variation in technique. However, in reviewing the literature, it appears that 5-mm, rather than 10-mm, sections have resulted in identifying 94% versus 69% of abnormal appendices. ¹³ Second, it seems that using CT of the entire abdomen and pelvis has higher sensitivity in diagnosing appendicitis and perhaps, more importantly, other causes of abdominal pain, rather than a more focused examination. For instance, a complete study of the abdomen attained a 99% sensitivity compared with 88% sensitivity for a focused study for diagnosing appendicitis. ¹⁴ There are a number of advocates of oral or rectal contrast. ¹⁵ Wise et al ¹⁶ take the approach of using rectal contrast only in those cases in which a CT without contrast is not definitive. They reserve rectal contrast for problem cases. The author's preference for CT is, in general, to administer IV contrast and scan the entire abdomen and pelvis. However, the approach can vary depending on the patient's particular situation. If the patient has a possible bowel obstruction, then oral contrast is given. For those patients who may require emergency surgery, no oral or rectal contrast is used initially. Rectal contrast is reserved for equivocal cases, although some pediatric radiologists feel it should be used in all children suspected of acute appendicitis. ¹⁵

CT findings

CT is very useful to visualize the bowel and localize the normal appendix once the ileocecal valve is identified. The appendix is usually located arising from the cecum approximately 3 cm below the ileocecal valve. The appendix itself may be up to 10 cm in length. It may be collapsed, air-filled, or contrast-filled if oral or rectal contrast is used (figures 1 through 3). There are a number of anatomic variations and locations of the cecum; the cecum itself typically may lie in the right upper quadrant or deep within the pelvis. To compound this problem, there are various anatomic possibilities for the location of the appendix. The appendix itself may lie, among other locations, deep within the pelvis, paraumbilically in the RLQ, or retrocecally. Computed tomography is very helpful to identify the location of the appendix. Typically on CT, the normal appendix is 1 to 2 mm in thickness with an overall diameter of 6 mm. The lumen is usually collapsed and the mesentery around the appendix exhibits a uniform fat density with crossing linear vessels. The abnormal appendix will show a thickened wall >3 mm and a distended lumen >= 6 to 7 mm. There may be an associated appendolith, mesenteric fat infiltration or fluid, and there may be associated adjacent adenopathy (figure 4). More complex appendicitis may be associated with abscess formation and cecal thickening, the so-called "arrowhead sign" (figure 5). In a recent report on the CT diagnosis of appendicitis, the most common signs of acute appendicitis included fat stranding (100%), an enlarged appendix >6 mm in lumen width (97%), adenopathy (63%), and an appendolith (43%). ¹⁷

Diagnostic ultrasound

Ultrasound accuracy

There is little doubt that technique and experience of the examiner plays a key role in the US diagnosis of appendicitis. When dedicated individuals perform sonography, the sensitivity and overall accuracy of US remain quite high. Dedicated radiologists available 24 hours a day to perform appendiceal US had a sensitivity of 89%, a specificity of 95%, a positive predictive value of 86%, and a negative predictive value of 96% in a recent report by Dilley. ¹⁸

When surgeons performed both the US and the clinical examination, they attained a sensitivity of 98%. ¹⁹ Can such high sensitivities be obtained in routine practice? In many instances, the answer is "yes," and in other instances the answer is "no." Much depends on the expertise of the individual performing the study and a number of other factors, such as patient selection. Thin patients are better candidates for US than are obese individuals. Pediatric patients may be better candidates for US since there is a substantial radiation dose associated with thin-cut CT. This is an important consideration in young patients. If possible, this increased radiation should be avoided, but not at the expense of a missed diagnosis.

Ultrasound technique

The author finds several points helpful when performing US for RLQ pain. The technique of the abdomen examination is modified slightly from that described by Baldisserotto et al. ²⁰ Using their technique, they reported US sensitivity of 98.5%, specificity of 98.2%, positive predictive value of 98%, and negative predictive value of 98%.

First, the entire abdomen is examined with a 4- to 5-MHz curved sector scanner. There are a number of other etiologies of abdominal or RLQ pain, ranging from mesenteric adenitis, to obstructive uropathy, to ovarian torsion. Thus, an examination of the entire abdomen is clearly warranted. In women, one must examine the pelvis through a distended bladder with a sector or curved ray scanner. Do not fail to perform an endovaginal examination in female patients if possible pelvic disease exists or if possible abnormalities are identified on the transabdominal scan.

Second, the compression examination is performed with a high-resolution (>= 7.5 MHz) linear array transducer (figure 6). Patient localization can be added to the study. It has been shown that if the patient points to the location of the pain, this sonographic "self localization" helps reduce examination time and is a valuable addition to the standard graded compression US of the appendix. ^21,22

When performing the graded compression, the common femoral artery and vein is identified in order to orient to the RLQ. The patient is then scanned cephalad from this position. The transducer is then used to compress the RLQ, including the terminal ileum and the cecum, to identify any possible appendix. Better compression is obtained if the left hand is placed behind the patient's flank.

The normal appendix may be commonly identified. ²³ When visualized, the normal appendix should be a blind-ended, tubular structure with a maximum wall thickness of ¾ 2 mm with an outer diameter of ¾ 6 mm, have no peristalsis, and originate from the base of the cecum. Normal bowel will have peristalsis and thus change lumen size (figure 7). In most instances, the inner-echogenic submucosal ring and the outer hypoechoic ring may also be helpful to identify the normal appendix, although this pattern may be seen with other segments of the bowel.

Ultrasound findings of acute appendicitis are listed in Table 2. These include a blind-ended, noncompressible fluid-filled structure with a wall thickness of >= 3 mm and an outer diameter of >= 7 mm identified around the appendix (figure 8). There may be a circumferential color-flow identified around the appendix (figure 9). There may also be free fluid noted in the RLQ of the abdomen and the pelvis, and there may be echogenic mesenteric fat (figure 10). If appendiceal perforation has occurred, an ill-defined and/or fluid-filled abscess may be identified. While 6 mm is usually identified as the cutoff between normal and abnormal appendix, in some instances, 5 mm is used as the upper limits of normal, while 7 mm is considered to be positive for acute appendicitis. Thus, in some instances, a measurement between 5 and 7 mm is considered to be equivocal. ²⁴

Diagnostic pitfalls

There are several pitfalls in diagnosing appendicitis. These can include the following: tip appendicitis; spontaneous resolution of appendicitis; chronic appendicitis; abnormal appendix location; and other etiologies of RLQ pain.

Tip appendicitis-- Appendicitis localizing to the tip of the appendix in an early inflammatory response. Thus, if only the base of the appendix is examined, appendicitis may be missed.

Spontaneous resolution of appendicitis-- It is estimated that appendicitis will spontaneously resolve in approximately 10% of cases.

Chronic appendicitis-- While in most instances acute appendicitis will eventually result in perforation and abscess formation, there are documented cases in which appendicitis has been chronic with a patient's having persistent, and/or intermittent RLQ pain.

Abnormal appendix location-- This is perhaps the most common pitfall in diagnosing appendicitis. The appendix may be in locations that are not readily accessible by US. These may include a retrocecal, mid-abdominal, deep pelvic, or even right upper quadrant location. As such, it is important to examine the patient initially with a curved or sector scanner, and if necessary, use an endovaginal scan to diagnose appendicitis.

Other etiologies of RLQ pain-- There are a number of other etiologies of RLQ pain, including other bowel disease (figures 11 and 12), disease of the mesentery, or other processes involving the appendix, including appendiceal tumors. In addition, organs that surround the appendix may be etiologies of RLQ pain. This could include gynecological disease (figure 13), gallbladder disease, renal disease (figure 14), or disease of the retroperitoneum. ^25,26

Complications of appendicitis

Appendicitis may have a variable course. Acute appendicitis may resolve spontaneously in ¾ 10% of cases. Appendicitis may be a chronic process with the patient having indolent, long-standing RLQ pain. However, in most cases of acute appendicitis, if left untreated, the condition will progress to gangrenous appendicitis, resulting in periappendiceal phlegmon or a periappendiceal abscess. In most situations, an appendiceal abscess will be walled off (figure 15). However, in some cases, an appendiceal abscess may localize to the dependent portion of the pelvis within the cul-de-sac. In other situations, appendiceal abscesses may result in widespread abscesses throughout the abdomen.

Treatment of appendiceal abscesses

Either US and/or CT guidance may be used for treating periappendiceal or pelvic abscesses. In some situations, a combined use of CT and US may be required. This is usually performed using a guidewire exchange technique. In many circumstances, percutaneous drainage alone may be curative if the abscess is well localized in the periappendiceal area and the appendix has been resorbed in the inflammatory response. In other situations, the abscess may localize in the pelvis.

Previously, the authors found that CT, using the transgluteal approach, was helpful in draining these abscesses. However, more recently, we have drained these abscesses through an endorectal route (figure 16). This is best performed using a trocar technique and a single step method, although this procedure also may be performed under sonographic guidance using a guidewire exchange technique. Ultrasound is very useful in guiding drainage of deep pelvic abscesses associated with appendicitis.

Summary

In most situations, the diagnosis or exclusion of acute appendicitis will be apparent clinically. However, in equivocal cases, either CT or US may be utilized to assist in definitive diagnosis or establish an alternative etiology for the patients' RLQ pain. In most adult patients, when CT is performed at the authors' institution, the entire abdomen is examined using 5-mm slice increments with intravenous contrast enhancement. However, in women and children, especially those who are thin, US may prove very useful. Ultrasound is best performed using a 4- to 5-MHz curved sector scanner examining the entire abdomen. After performing this portion of the study, graded compression US using a 7.5-MHz linear probe is performed in the right lower abdomen. If a gynecological process is suspected or cannot be excluded, endovaginal scanning may be performed. By a meticulous examination performed by well-trained personnel, US will have a fairly high accuracy in diagnosing appendicitis. Either sonography or CT may be used to drain periappendiceal or pelvic abscesses associated with appendicitis. AR