Appendiceal abscess/phlegmon is one of the most common complications of complicated (perforated) appendicitis, developing in 2-6% of perforated appendicitis cases. Phlegmon is an unorganized inflammatory mass — an ill-defined inflammatory conglomerate formed by surrounding fat tissue, bowel loops, and omentum adhering together. Abscess is a liquefied necrotic collection — a low-density fluid accumulation surrounded by a thick inflammatory wall (pyogenic membrane) with rim enhancement. Pathophysiologically it progresses as appendiceal perforation → peritoneal contamination → local inflammatory response → phlegmon (early stage) → abscess (late stage, liquefaction). Slightly more common in males (M:F approximately 1.2:1) and while it can occur at any age, perforation risk is increased in children and elderly (late presentation, atypical presentation). Diagnosis is made by CT and treatment approach is determined by abscess size: phlegmon → IV antibiotics + conservative, abscess <3 cm → IV antibiotics, abscess >3-5 cm → percutaneous drainage + antibiotics, followed by interval appendectomy (6-8 weeks later). Rare but serious complications include pylephlebitis (portal vein thrombophlebitis), peritonitis, and sepsis.
Age Range
10-70
Peak Age
30
Gender
Male predominant
Prevalence
Uncommon
Appendiceal abscess/phlegmon is the result of a localized inflammatory process developing after appendiceal perforation. The pathophysiological cascade is as follows: (1) Obstruction of the appendiceal lumen (fecalith, lymphoid hyperplasia, tumor) → intraluminal pressure increase → venous/lymphatic drainage impairment → ischemic necrosis → transmural inflammation → perforation. (2) After perforation, bacteria and infected material spread into the periappendiceal area. As a defense mechanism, the omentum and adjacent bowel loops wrap around the perforation site — this 'walling off' mechanism plays a critical role in preventing diffuse peritonitis. (3) In the early stage, an unorganized inflammatory mass (phlegmon) forms — inflammatory edema, granulation tissue, and fibrin accumulation create an ill-defined mass appearance. On CT, phlegmon appears as a heterogeneous soft tissue density ill-defined mass with dense fat stranding. (4) As the process progresses, central necrosis and liquefaction begin — proteolytic enzymes (collagenase, elastase) cause tissue destruction and purulent material accumulates. At this stage, a well-defined abscess cavity forms — the pyogenic membrane (inflammatory wall) surrounds the cavity. On CT, the abscess appears as a low-density fluid collection and the intense vascularity of the pyogenic membrane is seen as rim enhancement. The physics basis of rim enhancement: the inflammatory wall's dense capillary network rapidly absorbs contrast, while the central liquefied necrotic material is avascular and does not enhance — this center-periphery contrast difference creates the ring appearance. Appendicolith may be seen at the perforation site or within the abscess cavity — this finding is pathognomonic evidence confirming appendiceal origin.
The triad of rim-enhancing low-density fluid collection in the right lower quadrant + appendicolith within/near the collection + dense periappendiceal fat stranding — pathognomonic CT imaging combination of appendiceal abscess developed as a complication of perforated appendicitis. Rim enhancement reflects neovascularization of the pyogenic membrane, appendicolith confirms appendiceal origin, fat stranding reflects the active inflammatory process. This triad differentiates from other right lower quadrant abscesses (ovarian/tubo-ovarian abscess, Crohn's abscess, cecal diverticulitis abscess) with high reliability.
Well-defined or lobulated, low-density (10-30 HU) fluid collection in the periappendiceal area. The wall shows intense rim (ring) enhancement — reflecting active inflammation of the pyogenic membrane. Collection size can vary from a few cm to 10 cm. Gas bubbles may be seen within (gas-forming organisms or bowel gas entering through perforation). Wall thickness is usually 2-5 mm.
Report Sentence
A rim-enhancing low-density fluid collection measuring approximately ___ x ___ cm in the periappendiceal area of the right lower quadrant is identified, consistent with appendiceal abscess developed in the setting of perforated appendicitis.
Dense periappendiceal fat stranding around the collection — fat tissue density higher than normal (>-50 HU), hazy/streaky appearance. Calcified appendicolith may be seen within or immediately adjacent to the collection (typically 100-300 HU). Detection of appendicolith confirms appendiceal origin and excludes alternative diagnoses (ovarian abscess, Crohn's abscess, etc.).
Report Sentence
Dense periappendiceal fat stranding surrounding the collection and calcified appendicolith within/near the collection are identified, consistent with abscess in the setting of perforated appendicitis.
Ill-defined, heterogeneous soft tissue density inflammatory mass in the right lower quadrant — phlegmon. Liquefied collection has not yet formed. The mass appears adherent to cecum, terminal ileum, and omentum. Enhancement is heterogeneous — a mixture of inflammatory tissue and edema. Surrounding fat shows dense stranding. The appendix itself is usually not separately identifiable.
Report Sentence
An ill-defined, heterogeneously enhancing inflammatory mass (phlegmon) adherent to cecum, terminal ileum, and omentum in the right lower quadrant is identified, consistent with complicated appendicitis.
Marked diffusion restriction within the abscess cavity — high signal on DWI, low signal on ADC map. Diffusion restriction is the most reliable MR finding for purulent collections and is critically important in differentiating from simple fluid (reactive peritoneal fluid, seroma). Mild-to-moderate diffusion restriction may also be seen in surrounding inflammatory tissue.
Report Sentence
The periappendiceal collection demonstrates marked diffusion restriction (high DWI signal, low ADC), consistent with purulent collection (abscess); simple fluid has been excluded.
Hypoechoic or heterogeneous echogenicity collection in the periappendiceal area. Echogenic debris particles and moving internal echoes on dynamic US may be seen within. The collection is surrounded by irregular, thickened hyperechoic inflammatory tissue. Color Doppler shows increased vascularity in the surrounding wall (inflammation). Appendicolith appears as hyperechoic focus + acoustic shadow on US. Used for percutaneous drainage planning under US guidance.
Report Sentence
A heterogeneous echogenicity collection with internal debris measuring approximately ___ x ___ cm in the periappendiceal area of the right lower quadrant is identified, consistent with appendiceal abscess as a complication of perforated appendicitis.
Reactive cecal wall thickening (>3 mm), increased mural enhancement, and pericecal fat stranding. Reactive thickening of the terminal ileum wall may also be seen. Paralytic ileus in small bowel loops (dilated, aperistaltic loops). Free pelvic fluid may be present. These findings indicate the inflammatory process has spread beyond the appendix and affected adjacent structures.
Report Sentence
Cecal wall thickening, pericecal fat stranding, and paralytic ileus in small bowel loops around the abscess are identified, indicating spread of the periappendiceal inflammatory process to surrounding structures.
Criteria
No liquefied collection. Ill-defined, heterogeneous soft tissue mass. Inflammatory edema, granulation tissue, and fibrin accumulation predominant.
Distinct Features
Generally responds well to conservative treatment (IV antibiotics). Percutaneous drainage not indicated (no fluid to drain). CT shows heterogeneous soft tissue instead of low-density central collection. MR diffusion shows solid inflammatory tissue pattern (better differentiation than CT). Follow-up CT after 1-2 weeks of antibiotics to monitor for abscess development.
Criteria
Well-defined, rim-enhancing fluid collection. Pyogenic membrane (thick inflammatory wall) has formed. Liquefied necrotic material (low-density center).
Distinct Features
Percutaneous drainage indicated if >3-5 cm. Drainage performed under CT or US guidance. Aspirate sent for culture. Drainage applied in addition to antibiotic therapy. Interval appendectomy (6-8 weeks) planned after drainage. Marked diffusion restriction on DWI confirms purulent content.
Criteria
Abscess rupture → peritonitis, or septic thrombophlebitis of the portal venous system (pylephlebitis) develops. Requires urgent intervention.
Distinct Features
In rupture, diffuse free fluid, peritoneal thickening and enhancement are seen. Pylephlebitis: thrombus (filling defect) in portal vein or superior mesenteric vein + surrounding fat stranding + abscess. Liver abscess may develop (portal spread). Pylephlebitis mortality 10-30% — anticoagulation + prolonged antibiotics required. Emergency surgery usually indicated (conservative treatment insufficient).
Distinguishing Feature
In uncomplicated appendicitis, there is no fluid collection or abscess cavity — only dilated appendix, wall thickening, and periappendiceal fat stranding. In appendiceal abscess, rim-enhancing fluid collection and/or phlegmon (unorganized inflammatory mass) distinguishes from uncomplicated appendicitis.
Distinguishing Feature
In cecal diverticulitis abscess, focal cecal wall thickening and inflamed diverticulum are seen; appendicolith is absent and appendix may be normal. In appendiceal abscess, collection is localized around the appendix and appendicolith may be present. Age also helps in differentiation — diverticulitis typically >40 years, appendicitis younger age.
Distinguishing Feature
In Crohn's disease abscess, terminal ileum wall thickening, skip lesions, comb sign (mesenteric vascular engorgement), and sinus tracts accompany. In appendiceal abscess, these findings are absent and pathology remains localized to the appendix. Crohn's abscess may also occur outside the right lower quadrant (pelvic, psoas abscess).
Urgency
urgentManagement
interventionalBiopsy
Not NeededFollow-up
3-monthTreatment of appendiceal abscess/phlegmon requires a stepwise approach based on size and clinical status. Phlegmon: IV antibiotics (ampicillin-sulbactam or piperacillin-tazobactam) + conservative follow-up — 80-90% respond. Abscess <3 cm: IV antibiotics usually sufficient. Abscess >3-5 cm: CT or US-guided percutaneous drainage + IV antibiotics. If drainage fails or peritonitis develops, emergency surgery (appendectomy ± cecal resection). Interval appendectomy (6-8 weeks later) recommended for all patients after abscess/phlegmon resolution — recurrence risk 5-20% and occult appendiceal neoplasm must be excluded in the interval period (especially >40 years). Complications: pylephlebitis (portal vein thrombophlebitis — 0.5-1%, mortality 10-30%), peritonitis, sepsis, enteric fistula. In pylephlebitis diagnosis, portal vein thrombus (filling defect) and surrounding fat stranding are seen on CT — anticoagulation is added. Follow-up CT before interval appendectomy at 6-8 weeks and pathological examination of appendectomy specimen (neoplasm exclusion) are mandatory.
Initial treatment is IV antibiotics. Percutaneous drainage under CT/US guidance indicated for large abscesses (>3-5 cm). Conservative management preferred in phlegmonous form. Interval appendectomy (6-8 weeks later) recommended as acute surgery carries higher complication risk. Pylephlebitis (portal vein thrombophlebitis) is rare but serious complication.