Radiology Resident Case of the Week
Etiology/Pathophysiology:
Appendicitis is most common in late childhood and early 20Õs, but may occur in any age group. While more common in males in the teenage years, the sex incidence equalizes by the mid 30Õs. It is more common in Western nations as is diverticular disease, probably due to lower fiber intake allowing impaction of inspissated fecal material to occlude the appendiceal lumen. Lifetime risk for developing appendicitis in the U.S. is about 7%.
Pathology:
Following obstruction of the lumen, mucus accumulates causing distention of the mucosa. This predisposes to bacterial infection due to compromised vascularity and stasis. A vicious cycle begins where increasing pressure causes further occlusion leading to still more distention. Eventually the poor blood supply leads to necrosis and eventually perforation. Perforation occurs in roughly 20% of cases and may lead to a focal walled-off abscess or generalized peritonitis.
Miscellaneous:
There is great variability in the position of the appendix. While most often retrocecal with blind end pointing to spleen, it may be located nearly anywhere in the abdomen; hence appendicitis should not be discarded merely because pain does not localize to the RLQ.
Imaging:
The diagnosis of acute appendicitis is a clinical one, and given a typical presentation, imaging is not necessary. Often, however, the clinical picture is less than classic, and then imaging can be extremely helpful. The plain film is not surprisingly non-specific and relatively insensitive, with 20 - 40% interpreted as normal. An exception to this is the detection of an appendicolith by plain film in the setting of acute RLQ pain. While visualized in only approximately 15% of surgically proven cases, the detection of an appendicolith carries a positive predictive value (PPV) of over 95%. Other imaging such as barium enema and abdominal CT are sometimes used, but have limitations. CT is good for the detection of perforation and abscess formation, it is relatively poor for acute uncomplicated appendicitis. Barium enema has a significant high false positive and negative rate. Tc99m has enjoyed limited use.
Ultrasonographic examination of the appendix uses high resolution (5 MHz) probes with a graded compression technique. Scanning is begun over the point of maximal tenderness, gradually increasing pressure to scan deep structures in both longitudinal and axial slices. The remainder of the RLQ is then examined in detail; a quick examination of the abdomen and pelvis is also recommended in all cases. A normal exam is one in which the appendix is not visualized despite adequate technique and compression (anterior abdominal muscles compressed against retroperitoneum), or more rarely, in which a normal appendix is seen. The normal appendix is < 6 mm in diameter and has a wall thickness < 2mm. An indeterminate exam results when inadequate compression is obtained, or an appendix of 6 to 9 mm in diameter is seen. A positive exam is characterized by a non-compressible, blind ending tubular structure (on longitudinal) or target lesion (axial) with wall thickness > 2mm and diameter > 9 mm. The presence of a fecolith is also taken as a positive sign. The absence of mucosal folds or peristalsis is important in order to differentiate from cecum or ileum.
Multiple studies have shown that US is fairly good for diagnosing acute appencitis, with sensitivities = 80%, specificities = 95%, and PPV and NPV both around 90%. It is less sensitive in cases of perforated appendix, however. US signs of perforation include loculated periappendiceal fluid, prominent pericecal fat, and circumferential loss of submucosal layer. Numerous studies have demonstrated that while fecoliths are relatively infrequently found in acute appendicitis (ca 15%), the presence of a stone is associated with a much higher rate of perforation, around 50%. The presence of perforation is important to detect, as such cases are often initially treated by percutaneous drainage followed by delayed surgery.
Five patterns are detected by US. Early acute appendicitis demonstrates increased diameter and wall thickness with no luminal distention. In gangrenous cases there is loss of mucosal visualization with anechoic luminal fluid. Perforated appendicitis shows asymmetric wall thickening, free fluid, intramural air and often decreased tenderness. Phlegmon refers to inflammatory changes causing matting of cecum, appendix and ileum with no fluid usually. Finally, frank abscesses are marked by focal fluid collections.
An algorithm has been proposed for diagnosis of appendicitis by US. Typical cases require no imaging. Atypical and female patients should be scanned by US. Positive cases should go to surgery, and clearly negative cases may be observed for a few days. Inadequate studies require further imaging. The detection of abscess or phlegmon should be noted, as antibiotic tx and or percutaneous drainage followed by operation are standard treatment.
In the case presented, an 18 y.o. female presented with RLQ pain. Clinicians could not exclude gynecologic or vascular etiologies, and so US was performed. Figs. 1 are the supine and upright plain films, initially read as negative. Figs. 2, 3, and 4 are US images and demonstrate a complex, noncompressible mass measuring 2 x 3 x 4 cm with a 1 cm central hyperechoic shadowing structure, proved by surgery to be an appendicolith. While initially interpreted as probable abscess, as appendiceal layers and wall are not definitely demonstrated, pathology showed acute appendicitis with surrounding hemorrhage and fatty necrosis, without definite evidence of perforation. On retrospective analysis, a calcified structure, which moves caudally with upright position, is noted along the right sacroiliac joint on plain films, possibly representing the 1 cm fecolith seen at surgery.
DDx:
The differential is long, particularly so in females. Consideration should always be given to : pelvic inflammatory disease, ovarian torsion, ectopic pregnancy, endometriosis, CrohnÕs disease, gastroenteritis, renal colic, cholecystitis, diverticulitis, typhlitis, mesenteric adenitis, necrotic fibroids, hemorrhagic ovarian cysts, infectious ileitis, urinary tract infection, perforated peptic ulcer, pancreatitis, and obstruction.
Key references:
1. Silen, W. Cope's Early Diagnosis of the Acute Abdomen 18th ed. New York : Oxford University Press, 1991.
2. Gore R, Levine M. Textbook of Gastrointestinal Radiology, Vol 1. Philadelphia : W.B. Saunders, 1994.
3. Abu-Yousef, M et al. High Resolution Sonography of Acute Appendicitis. AJR 149:53-58, 1987.
4. Abu-Yousef, M. Sonography of the Right Iliac Fossa. Ultrasound Quar. 8:73-94, 1990.
5. Felson B, Bernhard C. The Roentgenologic Diagnosis of Appendiceal Calculi. Radiology 49:178-190, 1947.
ACR Code:
751.291
Keywords:
Appendicitis; Appendicolith; Ultrasound
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