CT Evaluation of Appendicitis
Executive Summary
Acute appendicitis represents the most frequent acute abdominal emergency globally, with a lifetime occurrence risk of approximately 7%. While diagnosis was historically based on clinical evaluation and mandatory immediate surgery, the advent of advanced imaging—specifically Computed Tomography (CT)—has shifted the paradigm toward high-accuracy radiological confirmation and nuanced management. CT is currently the "gold standard" for diagnosis in the United States, boasting a sensitivity of 90–100% and specificity of 91–99%.
Critical takeaways include:
Diagnostic Superiority: CT is significantly more sensitive than ultrasound (US) in identifying complications like perforation and abscesses.
Protocol Efficiency: Recent evidence suggests that oral contrast provides no diagnostic benefit and unnecessarily delays the procedure; intravenous (IV) contrast remains valuable for identifying wall enhancement and resolving complex cases.
Clinical Differentiation: Accurate radiological assessment is vital for distinguishing between uncomplicated appendicitis (potentially treatable with antibiotics) and complicated/perforated cases (requiring surgical intervention).
Risk Demographics: Perforation rates are highest in pediatric (40–57%) and elderly (55–70%) populations, where clinical presentation is often atypical.
1. Background and Pathophysiology
Epidemiology and Anatomy
Prevalence: Approximately 250,000 cases occur annually in the United States.
Demographics: The condition most frequently affects young white males, with a median age of 22 years.
Anatomical Origin: The vermiform appendix arises from the cecum, approximately 3–4 cm below the ileocecal valve.
Positional Variability: The appendix displays high variability in location, including paracolic, retrocolic, retrocecal, and pericecal positions.
Etiology
Obstructive (90%): Caused by fecaliths, lymphoid hyperplasia, or cancer. This leads to venous engorgement, arterial compromise, and ischemia, eventually resulting in wall infarction and perforation.
Non-obstructive (10%): Appendicitis occurring without luminal obstruction.
2. Radiological Modalities and Indications
Radiological diagnosis utilizes Ultrasound (US), Computed Tomography (CT), and Magnetic Resonance Imaging (MRI).
3. Computed Tomography Protocols
Technique and Contrast Use
Oral/Rectal Contrast: Recent studies indicate that oral contrast provides no benefit in diagnostic sensitivity or specificity and increases the time required for the exam.
IV Contrast: Essential for demonstrating appendiceal wall enhancement and resolving complicated cases, such as abscesses or ectopic appendix locations.
Low-Dose Protocols: Utilizing current reduction (mAs) and voltage reduction (kV) alongside iterative reconstruction can maintain high sensitivity/specificity while minimizing radiation exposure.
Imaging Standard: A thin-section protocol (at least 2.5 mm) is mandatory to allow for multiplanar (coronal and sagittal) reconstructions, which are critical for locating the appendix in uncommon positions.
4. Diagnostic CT Signs
Diagnosis is based on a combination of primary and secondary signs. While a diameter threshold of 6 mm is used, it is not absolute, as 40% of healthy adults may meet this criterion. Appendices measuring 6–10 mm are often considered "indeterminate" and require correlation with secondary inflammatory signs.
Primary Signs
Distended Appendix: Diameter ≥ 6 mm.
Wall Thickening: Appendiceal wall ≥ 3 mm.
Appendicolith: Present in approximately 30% of cases.
Wall Enhancement: Visualized via IV contrast; may show a "target" or "bull’s eye" sign due to submucosal edema.
Secondary (Periappendiceal) Signs
Fat Stranding: Excellent sensitivity (100%) and good specificity (80%).
Fluid Collections/Phlegmon: Indicative of spreading inflammation.
Extraluminal Air: A sign of perforation.
Adjacent Bowel Changes: Focal cecal apical thickening or adjacent bowel wall thickening.
Adenopathy: Enlarged local lymph nodes.
5. Complicated Appendicitis and Perforation
Perforation occurs in 30% of cases and is associated with significantly increased morbidity and mortality.
Risk Factors for Perforation: Most common in the very young (40–57%) and the elderly (55–70%) due to delayed or misdiagnosed presentation.
CT Findings of Perforation:
Transverse diameter typically larger than non-perforated cases (average 15 mm vs. 11 mm).
Presence of extraluminal appendicoliths or extraluminal air (though air volume is often minimal, 1–2 mL).
Abscess formation or phlegmon.
Defects in wall impregnation after IV contrast.
Note on Air: The presence of air within the appendix lumen does not rule out appendicitis; it is seen in over 15% of confirmed cases.
6. Differential Diagnosis and Tumors
Common Mimics
CT is instrumental in distinguishing appendicitis from other conditions that cause right iliac fossa pain:
Gastrointestinal: Enterocolitis, mesenteric lymphadenitis, cecal or sigmoid diverticulitis, Crohn’s disease, epiploic appendagitis, and omental infarction.
Gynecological: Ectopic pregnancy, tubo-ovarian abscesses, PID, and ovarian cyst torsion.
Urological: Urolithiasis, pyelonephritis, and cystitis.
Appendiceal Neoplasms
Tumors should be suspected if the appendiceal diameter exceeds 15 mm. Neoplasms are found in 0.9–1.4% of appendectomies.
Carcinoid Tumor: The most common type (0.3–0.9% of findings); typically involves nodular thickening of the distal third of the appendix.
Adenocarcinoma: Rare; characterized by diffuse thickening of the proximal third.
Cecal Carcinoma: In elderly patients, appendicitis symptoms may mask underlying cecal cancer, especially if accompanied by anemia, weight loss, or a palpable mass.
7. Diagnostic Pitfalls and Limitations
False Negatives: Most frequent in very thin patients lacking the abdominal adipose tissue necessary to outline the appendix, leading to confusion with bowel loops.
False Positives: Occur when other structures (terminal ileum, dilated ureters, or gonadal veins) are misidentified as the appendix.
Distinguishing Feature: The terminal ileum is not a dead end, exhibits peristaltic activity, and does not originate from the base of the cecum.
Early Scanning: Sensitivity may be limited if the CT is performed too early, before secondary signs like "fat stranding" have become visible.
