Year : 2006 | Volume
: 16 | Issue : 4 | Page : 523--532
Imaging in acute appendicitis: A review
RK Jain, M Jain, CL Rajak, S Mukherjee, PP Bhattacharyya, MR Shah
Department of Radiology, Quadra Medical Services Pvt. Ltd. Kolkata, India
R K Jain
Quadra Medical Services Pvt. Ltd. 41, Hazra Road, Kolkata 700019
|How to cite this article:|
Jain R K, Jain M, Rajak C L, Mukherjee S, Bhattacharyya P P, Shah M R. Imaging in acute appendicitis: A review.Indian J Radiol Imaging 2006;16:523-532
|How to cite this URL:|
Jain R K, Jain M, Rajak C L, Mukherjee S, Bhattacharyya P P, Shah M R. Imaging in acute appendicitis: A review. Indian J Radiol Imaging [serial online] 2006 [cited 2020 Sep 19 ];16:523-532
Available from: http://www.ijri.org/text.asp?2006/16/4/523/32261
Acute appendicitis is the most common cause of emergency abdominal surgery . While the diagnosis of acute appendicitis is still largely thought to be a clinical one, a meaningful number of patients are found to have normal appendices at surgery. The erroneous diagnosis of this acute condition has led to a high rate (8-30%) of inappropriate removal of the normal appendix. This high rate needs to be balanced with the problem of being over restrictive in the diagnosis of acute appendicitis, which may allow uncomplicated appendices to progress to perforation and peritonitis .
However the incidence of acute appendicitis requiring appendectomy has significantly decreased over the past three or four decade, and the trend appears to continue. Some of the decrease in the number of appendectomies is attributable to better diagnosis . With the availability of high-resolution sonography and spiral CT it is possible to bring down these high rates of false positivity significantly.
The vermiform appendix, a blind-ending tubular structure, arises from the posteromedial aspect the cecum inferior to the ileocecal junction. It varies considerably in length and circumference, the average length being between 7.5 and 10 cms. The position of the base of the appendix is essentially constant, being found at the confluence of the three taeniae coli of the cecum, which lies deep to the Mc Burney's point. The free end of the appendix is however found in variety of locations. [Figure 1]. The difference in appendiceal position influences clinical findings considerably . In unusual cases of malrotation of the gut, or failure of decent of cecum, the appendix is not in the right lower quadrant .
The appendix has its own mesentry, the mesoappendix, arising from the inferior part of the mesentry of the terminal ileum, which attaches to the cecum and proximal part of the appendix. The mesoappendix contains the appendicular artery, a branch of the ileocolic artery. Venous drainage of the appendix is via the ileocolic veins and the right colic vein into the portal system. The lymphatic drainage occurs to the ileocolic nodes along the course of the superior mesenteric artery to the celiac nodes and cisterna chyli. The afferent nerve fibers from the appendix accompany the sympathetic nerves to the T10 segment of the spinal cord, which explains why in appendicitis is sometimes referred to the periumbilical area.
On histology, the submucosa contains numerous lymphatic aggregations or follicles. There is a rough parallel between the amount of lymphoid tissue in the appendix and the incidence of acute appendicitis, the peak for both occurring the mid teens .
Appendicitis is commonly associated with obstruction of the appendiceal lumen due to fecalith. Obstruction may also be secondary to hypertrophy of lymphoid tissue, inspissated barium, gallstones, worms (ascaris), foreign bodies, or tumor.
Following obstruction of the appendiceal lumen, continued mucus secretion and inflammatory exudation leads to distension, mucosal edema and mucosal ulceration along with translocation of bacteria to the submucosa. The swelling of appendix stimulates the nerve endings of visceral afferent fibers and the patient perceives visceral periumbilical or epigastric pain.
With increasing intraluminal pressures, further distension results in obstructed lymphatic and venous drainage and allows vascular congestion of the appendix. The inflammatory process soon involves the serosa. When the inflamed serosa of the appendix comes in contact with the parietal peritoneum, patients typically experience the classic shift of pain to the right lower quadrant. Intramural venous and arterial thromboses ensue, resulting in gangrenous appendicitis.
Unrelenting tissue ischemia results in appendiceal infarction and perforation. Rupture of the appendix with spillage of pus into the peritoneal cavity results in localized or generalized peritonitis. More commonly, inflamed or perforated appendix can be walled off by the adjacent greater omentum and loops of small bowel resulting in phlegmonous mass or paracecal abscess.
This sequence is not inevitable and some episodes of acute appendicitis may resolve spontaneously if the obstruction is relieved. Rarely, appendiceal inflammation resolves leaving a distended mucus-filled organ termed mucocele of the appendix.[Figure 2]
Appendicitis occurs in all age groups. It is rare in infants but becomes increasingly common in childhood and reaches peak incidence in the late teenage years and early twenties. Sex ratio is equal before puberty and male-to-female ratio is 3:2 in teenagers and young adults. The ratio again equalizes by the time patients reach their midthirties. No racial predilection exists for appendicitis.
A diagnosis of appendicitis usually can be made on the basis of history and physical examination.
Pain is the prime symptom of appendicitis and initially is located in the lower epigastrium or periumbilical area. The pain subsequently localizes to the right lower quadrant, where it becomes progressively more severe. This classic pain sequence is usual but not invariable. The difference in appendiceal position, age of the patient, and degree of inflammation, accounts for variations in the clinical presentation.
Anorexia nearly always accompanies appendicitis. Nausea, vomiting, and low-grade fever are common. Uncommonly, diarrhea or constipation may be seen. The sequence of appearance of symptom that is anorexia followed by pain and then vomiting has great differential diagnostic significance 3. If vomiting precedes the onset of pain, the diagnosis should be questioned.
The cardinal features of acute appendicitis are localized abdominal tenderness, rigidity, muscle guarding, pain on percussion, and rebound tenderness. Pain in right lower quadrant with palpation of the left lower quadrant (Rovsing sign) is helpful in supporting a clinical diagnosis. Asking the patient to cough will elicit a sharp pain in the right lower quadrant (positive cough sign).
With a retrocecal appendix the anterior abdominal findings are less striking and tenderness may be more marked in the flank. Pain in right lower quadrant with hyperextension of the right hip (psoas sign) demonstrates nearby inflammation when stretching the ileopsoas. Pain in the right lower quadrant with passive internal rotation of the flexed right hip (obturator sign) indicates that an inflamed appendix is contact with the obturator internus.
High level of C-reactive protein (>0.8 mg/dL) with leukocytosis and neutrophilia are the most important laboratory findings 6.
The clinical presentation of appendicitis is variable. While the clinical diagnosis may be straightforward in patients who present with classic signs and symptoms, atypical presentations may result in diagnostic confusion and delay in treatment 4. Clinical diagnosis is more confusing in young and elderly patients. In addition, many other clinical disorders present with symptoms similar to those of appendicitis and the differential diagnosis 3includes the following:
Acute Mesenteric Adenitis
Perforated peptic ulcer
Urinary tract infection
Diseases of the Male: Testicular torsion
Gynecologic disorders: Pelvic inflammatory disease (PID)
Ovarian cyst or torsion
Ruptured ectopic pregnancy
Rectus sheath hematoma
Since accurate clinical diagnosis of appendicitis is difficult, negative appendectomy rate7 can be as high as 20%. Unnecessary surgery for suspected appendicitis exposes patients to increased risks, morbidity, and expense 8. Radiological examination can reduce the number of misdiagnoses and negative laparotomies and help in treatment of appendiceal abscesses and in postoperative complications. Judicious use of graded compression US & CT in patients with equivocal clinical findings results in lower false-negative appendectomy rates .
Though plain films are reported to reveal abnormalities in 50% of patients with appendicitis , they are not specific, not cost effective, and can be misleading . Plain films are indicated for the evaluation of a patient with suspected appendicitis only when other diagnostic probabilities (e.g., perforation, intestinal obstruction, ureteral calculus) are also considered ,.
The various plain film findings that have been described in appendicitis are as follows: ,,,
" Right lower quadrant gas
" Increased soft tissue density of the right lower quadrant
" Separation of the cecum from right extraperitoneal fat planes
" Deformity of the cecal and ascending colon gas shadow occurring due to adjacent inflammatory mass
" Localized ileus with gas in the cecum, ascending colon and terminal ileum
" Effacement of the right extraperitoneal fat line
" Gas in peritoneum and retroperitoneum
" Gas filled appendix
Barium enema examination may be helpful in selected patients. Barium enema is performed on an unprepared bowel gently without any external pressure. Complete filling of a normal appendix effectively excludes the diagnosis of appendicitis. Nonfilling or incomplete filling of the appendix along with mass effect on the cecum suggests appendicitis, the mass effect being due to abscess/ inflammatory reactions surrounding the inflamed appendix. The terminal ileum may be displaced or narrowed by the adjacent inflammatory mass and there may be thickening of the mucosal folds of the terminal ileum. However, non-filling of appendix may be seen in as many as 10-20% of normal patients.
It has been shown by Sehey that appendix fills in 92% of normal children and hence failure of the appendix to fill in symptomatic children is a significant finding.
Barium enema examination may also be useful in evaluating complex colonic abnormalities detected with cross-sectional imaging .
Ultrasonography (US) is valuable in the diagnosis of doubtful cases of appendicitis and is a cost-efficient adjunct to the clinical evaluation. US is inexpensive, safe, and widely available. Because US involves no ionizing radiation and excels in the depiction of acute gynecologic conditions, it is recommended as the initial imaging study in children, in young women, and during pregnancy8. It has reported sensitivities of 75%-90%, specificities of 86%-100%, accuracies of 87%-96%, positive predictive values of 91%-94%, and negative predictive values of 89%-97% for the diagnosis of acute appendicitis . Use of preoperative ultrasonography is also associated with overall lower negative appendectomy rate .
Graded compression technique described by Puylaert  is the standard method for sonographic evaluation of acute appendicitis. Graded compression US, with slow and gentle maintained pressure, allows for a lengthy and successful evaluation of the area of interest and shows obstructed appendix as a noncompressible loop of gut .
Recently Baldisserotto et al  has described the use of the noncompressive technique before the graded compression study. This may successfully establish the diagnosis in some cases, thereby avoiding compression in patients with abdominal pain. Change of the patient's position to displace the bowel gas may also help in visualization of the appendix deeply set in the abdominal cavity without compression. Compression study is however, useful in identifying the cases of appendicitis not visualized at the noncompressive examination.
It is very important to standardize the examination technique for identification of appendix and thereby avoiding false negative diagnosis. Baldisserotto has suggested an excellent routine for the actual US examination of the right lower quadrant, which we have found very useful in our daily practice. The US examination of the right lower quadrant should start in the transverse plane from the tip of the liver and proceed towards the pelvic brim. The ascending colon usually is appreciated by its gas content and haustral pattern. In the region of the cecum, careful attention should be paid to inflammatory changes in the perienteric fat and the appendix itself. Sagittal and oblique images should then be obtained until the entire region of interest has been scanned. Detailed images are obtained of the appendix, if it is seen. The examination is generally begun with a curvilinear transducer appropriate for the patient: a 3.5-MHz transducer for large patients and a 5-MHz transducer for thin patients. The linear transducer is used latter for more detailed study. The retrocecal appendicitis is best studied by the examination through the right flank .
The inflamed appendix is seen as a blind-ended, aperistaltic, noncompressible, tubular structure that arises from the base of the cecum having a diameter greater than 6 mms.[Figure 3] Presence of a fecalith [Figure 4] may aid in arriving at a positive diagnosis.
The ovoid shape15 of appendix in transverse section on US over the entire appendiceal length reliably rules out acute appendicitis while in acute inflammation the appendiceal wall thickening causes an increase of the outer appendiceal diameter and a rounding of the shape.
In early acute appendicitis (catarrhal stage) five layers can be identified- [Figure 5]
central, thin hyperechoic line representing the collapsed lumen and superficial lining of the mucosa of the appendix, hypoechoic layer (2-3mms) representing edematous lamina propria and muscularis mucosa. hyperechoic submucosa (2-3 mms). hypoechoic muscular layer (2-3-mms).outer thin hyperechoic line representing the serosa.
In late (suppurative) stage the lumen of the appendix is distended with pus/ fluid and there is increased thickening of the submucosa and muscular wall in the range of 3-6 mms.
Circumferential color in the wall of the inflamed appendix on color Doppler US images is strongly supportive evidence of active inflammation . [Figure 6].
Loculated pericecal fluid, phlegmon or abscess, prominent pericecal fat and circumferential loss of the submucosal layer of appendix are associated with appendiceal perforation 16. [Figure 7].
A significant disadvantage of sonography is that it is operator dependent. Difficulties with ultrasonography also include the fact that a normal appendix must be identified to rule out acute appendicitis. Visualization of a normal appendix is more difficult in patients with a large body habitus and when there is an associated ileus, which produces shadowing secondary to overlying gas-filled loops of bowel. It may also be not possible on US to differentiate between appendiceal phlegmon from an abscess and CT may be helpful in this setting.
CT has become increasingly popular as an effective cross-sectional imaging technique for diagnosing and staging acute appendicitis. It is a quick and accurate examination that is operator-independent, is relatively easy to perform and provides images that are easy to interpret., Helical CT has 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% for the diagnosis of acute appendicitis. Its use has decreased the rate of negative appendectomies and has decreased the number of cases of appendiceal perforation.,
Disadvantages of CT include possible iodinated-contrast-media allergy, patient discomfort from administration of contrast media (especially if rectal contrast media is used), exposure to ionizing radiation, and cost. However, the cost is considerably less than that of removing a normal appendix or hospital observation.
Technique- there is no consensus on the ideal CT technique for studying appendix. There are different CT protocols depending upon the generation of CT scanners used as well as varying from center to center. While nonfocused CT performed for entire abdomen and pelvis with intravenous and oral contrast material is the most popular approach,, CT evaluation of appendicitis without the use of intravenously administered contrast material is also a growing trend ,,. Opacification of the terminal ileum and cecum with oral and/or rectal contrast material alone or in combination has been advocated4. However lane et al19 do not recommend the use of any contrast material. Weltman et al20 has shown that the use of thin-section (5mms) CT significantly improves the diagnosis of acute appendicitis compared to 10 mm sections. We at our clinic prefer to opacify the bowel using oral and / or rectal contrast along with IV contrast, and use thinner sections.
Image interpretation- the evaluation starts with the identification of appendix. Since the position of the cecum and ascending colon is highly variable, identification of the fatty lips of the ileocecal valve is helpful. Careful scrutiny of the entire cecum then frequently allows identification of the appendix as it arises from the posteromedial border. The appendix is frequently seen draped over the right external iliac artery and vein. The right common and external iliac artery and vein are therefore carefully evaluated from their origins at the bifurcation of the aorta into the femoral canal to identify the overlying appendix. This usually helps to avoid the pitfall of not seeing a pelvic appendix.
Once the appendix is identified, it is evaluated for sign of acute appendicitis as described to confirm or exclude the diagnosis of acute appendicitis. Once the appendiceal region is cleared, the cecum and ascending colon are carefully examined for potential involvement by cecal neoplasm [Figure 8], cecal diverticulitis, typhlitis, or segmental colitis. Diseases that involve primarily the pericolonic fat, such as primary epiploic appendagitis and omental infarction, are then excluded.
Focus is then turned to the terminal ileum and its subtended mesentery. Gastrointestinal diseases to consider in this anatomic location include acute terminal ileitis, mesenteric lymphadenitis, Crohn's disease and tuberculosis. Genitourinary disease then should be excluded, including acute pyelonephritis, ureteral obstruction, complications of ovarian cysts and masses, and acute postpartum ovarian vein thrombosis. In adult patients, one must also consider acute cholecystitis (which may mimic acute appendicitis if the enlarged gallbladder extends into the right-lower quadrant), pancreatitis, sigmoid diverticulitis, bowel ischemia, and bowel obstruction.
Imaging findings- the normal appendix appears as a tubular or ringlike pericecal structure that is either totally collapsed or partially filled with fluid, contrast material, or air. The normal appendix has a thickness of 3 mms or less and a diameter of 6mms or less,. The periappendiceal fat should appear homogeneous, although a thin mesoappendix may be present. The finding of a normal appendix with no fluid in its lumen, normal periappendiceal fat, and no calcified appendicolith indicates that the appendix is not inflamed.
The main CT criteria for the diagnosis of acute appendicitis include identification of a thickened appendix with a two-wall diameter greater than 6.0-7.0 mm, periappendiceal inflammatory changes, and a calcified appendicolith.[Figure 9] a. Alobaidi et al has recommended the use of bone window settings for detecting appendicoliths when evaluating patients for acute appendicitis, particularly patients in whom evidence of appendicitis is equivocal.
The size criterion to diagnose appendicitis is especially important in the absence of periappendiceal inflammation. Benjaminov et al observed that an upper limit of 6.0 mm for normal appendiceal thickness can be used reliably at CT only if the luminal content is visualized because in the absence of periappendiceal inflammatory changes, it is not possible to differentiate a noncollapsed appendix filled with fluid of the same attenuation as the wall from a thick inflamed appendix if the content is not visualized. They suggested 10.0 mm as the upper limit of normal if the luminal content is not visualized and extraappendiceal inflammatory changes are not present. Patients with an appendiceal thickness of 6.0-10.0 mm should therefore undergo further examination with rectally or intravenously administered contrast material or with US to visualize the wall and thus prevent a false-positive diagnosis of appendicitis.
In early or mild appendicitis the CT findings are very subtle. The appendix may appear minimally distended associated with a hazy, ill-defined increase in CT attenuation in the fat immediately surrounding the appendix. However most patients who undergo CT demonstrate greater degrees of luminal distention and evidence of transmural inflammation. Circumferential and symmetric wall thickening is nearly always present and is best demonstrated on images obtained with intravenous contrast material enhancement. Periappendiceal inflammation [Figure 9]b is present in 98% of patients with acute appendicitis.
Other important findings include focal cecal apical thickening and the arrowhead sign,[Figure 10] which is seen as an arrowhead-shaped collection of contrast medium localized to the upper part of the cecum near the orifice of the appendix,. Inflammatory changes associated with acute appendicitis can cause focal cecal apical thickening, which allows contrast material to assume the configuration of an arrowhead as it funnels at the cecal apex to the point of the obstructed appendiceal orifice. Because the sign is formed by the extension of inflammation from the appendix to the cecum, the arrowhead sign may allow for placement of patients with appendicitis into two surgical groups: those who likely will do well with standard ligation (arrowhead sign not present) and those who may require partial cecectomy (arrowhead sign present).
Complications- Perforated appendicitis is usually accompanied by pericecal phlegmon or abscess formation. Associated findings include extraluminal air, [Figure 11] marked ileocecal thickening, localized lymphadenopathy, peritonitis, and small-bowel obstruction.
If the abnormal appendix is not seen, a specific diagnosis of appendicitis can be made by identifying an appendicolith within a periappendiceal abscess or phlegmon
Although a pericecal phlegmon or abscess is strongly suggestive of appendicitis, these are nonspecific findings that may be seen with other disease entities. If substantial inflammation is present within the right lower quadrant, it may be difficult to differentiate primary appendicitis with secondary inflammation of the cecum and terminal ileum from ileocolitis with secondary inflammation of the appendix.
CT is of considerable value in the treatment of patients who present with a periappendiceal mass and can be used to accurately stage the extent of periappendiceal inflammation and to reliably differentiate periappendiceal abscess from phlegmon, which is of critical importance to the surgeon. Many surgeons believe that there is little value in attempting to drain a nonliquefied phlegmon and prefer initial nonsurgical treatment with antibiotic therapy in such cases. Patients with well-defined and well-localized periappendiceal abscesses typically benefit from CT-directed percutaneous catheter drainage., Patients with extensive and poorly defined collections usually require immediate surgical exploration and abscess drainage.
Magnetic Resonance Imaging
MRI may also be used in the diagnosis of appendicitis in cases where either CT is contraindicated like in pregnancy or in children where it is advisable to avoid radiation. T1-weighted and T2-weighted turbo spin-echo sequences and fat-suppressed inversion recovery turbo spin-echo sequences as well as post contrast T1 weighted sequences can be used. On T2-weighted images, inflamed appendix show markedly hyperintense center and a slightly hyperintense thickened wall with markedly hyperintense periappendiceal tissue.[Figure 12] On post contrast study, intense contrast enhancement of the inflamed appendiceal wall indicates the presence of appendicitis. There is also significant enhancement of surrounding fat on gadolinium-enhanced T1-weighted fat-suppressed spin-echo images. Mild enhancement can however be seen in the normal appendix and gut. Using fat-saturation technique, contrast differences between the inflamed appendix and the surrounding fat is better appreciated. However, MRI has inherent limitation in detecting appendicolith.
Inflammatory diseases of the gut, such as ileal diverticulitis and Crohn's disease may mimic appendicitis and may be cause for false-positive diagnosis of acute appendicitis.
False-negative results usually depend on technique-related limitations, such as inefficient fat saturation causing appendiceal wall enhancement to be obscured by mesenteric fat.
Fat-suppressed gadolinium enhanced MRI images are sensitive (97%) and accurate (95%) in the detection of acute appendicitis25.
Incesu, et al  found MR imaging superior to sonography in revealing appendicitis. Despite some disadvantage, MR imaging can also be used after suboptimal or nondiagnostic sonography in cases of suspected acute appendicitis.
Although rare in infants, appendicitis is common in human population. It is one of the most common cause of acute right lower quadrant abdominal pain and in majority of cases diagnosis of acute appendicitis can largely be made on the basis of history and physical examination.
Though decreasing, still a large number of appendices at surgery are found to be normal, leading to a high rate of negative appendectomies (8- 30%). This is because of similar signs and symptoms of a wide range of acute abdominal clinical disorders and nonspecific laboratory and conventional radiographic findings. In recent years however with the availability of various cross-sectional imaging techniques viz. Ultrasonography, Spiral CT and MRI, false positive diagnosis of acute appendicitis has reduced therefore also reducing rate of negative appendectomies. The overall accuracy of cross-sectional imaging techniques in diagnosing acute appendicitis varies from 87%-98%.
High resolution sonography is an most common imaging technique used in diagnosing appendicitis as it is less expensive, easily available and free from radiation, however it is operator and subject dependent and requires lot of experience. MRI can also be used in the setting of pregnancy, otherwise it expensive, time consuming and cumbersome. Spiral and recently multislice CT has therefore emerged as the most effective tool for diagnosing appendicitis and its complications because of its excellent resolution. It provides exquisite detailed anatomical images for review, and is also fast and operator independent.
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