Abdominal Wall Abscess

An abdominal wall abscess is a localised collection of pus within the layers of the anterior abdominal wall, lying in the subcutaneous tissue, the rectus sheath, or between muscle layers.

It is distinct from intra abdominal abscess, but the two can coexist, and a deep abdominal wall collection may communicate with an intra abdominal source through a fistula or defect.

Common settings:

• Postoperative surgical site infection (SSI), usually in the first 2 weeks, the most frequent cause. Contaminated and dirty wounds, colorectal surgery and emergency surgery carry the highest rates

• Port site and stoma site infection after laparoscopy

• Infected hernia mesh, which may present months to years later with chronic sinus or recurrent abscess

• Rectus sheath abscess, often following trauma, injections, anticoagulant related haematoma that is secondarily infected, or spread from a distant focus

• Umbilical abscess, from omphalitis or an infected urachal remnant

• Fistula from Crohn disease, diverticular disease, appendicitis, perforated tumour or retained foreign body (gallstones, surgical swab)

• Primary skin and soft tissue infection with deeper extension, especially in diabetics, injecting drug users and the immunosuppressed

• Actinomycosis and tuberculosis, which are rare and may form a chronic indurated mass with sinuses

Typical organisms in postoperative and skin origin collections are Staphylococcus aureus (including MRSA), streptococci, and, when bowel flora is the source, Escherichia coli, Bacteroides fragilis and other anaerobes and Enterococcus.

Polymicrobial growth suggests an enteric source.

Risk factors include diabetes, obesity, smoking, malnutrition, steroids and immunosuppression, prolonged operating time, wound contamination, haematoma or seroma, and poor perioperative glycaemic control.

• Local features are pain, erythema, warmth, induration and fluctuance over the abdominal wall. Wound discharge of pus, wound dehiscence, or a tender palpable mass is typical. Systemic features include fever, rigors, tachycardia and malaise, though a deep collection may produce few skin signs.

• Rectus sheath abscess presents with unilateral lower abdominal pain and a firm mass that does not cross the midline and becomes less palpable with the rectus muscle tensed (Fothergill sign, which also applies to rectus sheath haematoma). It is commonly confused with appendicitis, diverticulitis or an incarcerated hernia.

• Features suggesting an underlying intra abdominal cause include faeculent or bilious discharge, discharge of gas, a tract that persists after drainage, recurrent abscess at the same site, and associated abdominal symptoms (change in bowel habit, weight loss, bleeding).

• In mesh infection, look for chronic discharge from a sinus, recurrent infection, a palpable mesh edge with erythema, and failed healing of a prior wound.

Diagnosis is clinical when fluctuance is present, but imaging is advised for suspected deep collections, unclear diagnosis, sepsis, or recurrence.

• Ultrasound: first line for superficial and muscle layer collections; shows anechoic or complex fluid with debris and surrounding hyperaemia
• CT with IV contrast: best for rectus sheath and deep collections, mesh infection, fistula, and intra abdominal source. It shows a rim enhancing collection with gas and a connection to bowel or a defect
• MRI or fistulography: for complex tracts and suspected Crohn disease
• Blood tests: full blood count, C-reactive protein, creatinine, blood glucose or HbA1c, and lactate if septic; blood cultures if febrile
• Cultures: send pus (not a surface swab) for Gram stain, aerobic and anaerobic culture and sensitivity; mycobacterial and fungal cultures if chronic or immunosuppressed; histology of excised tissue if a chronic sinus, to exclude actinomycosis, tuberculosis and malignancy
• Colonoscopy or contrast study if an enteric fistula or malignancy is suspected

Source control is the primary treatment. Antibiotics alone do not cure an abscess.

Drainage. Incise and drain under local or general anaesthetic.

Open the wound widely along the line of the skin, break loculations, irrigate with saline, and leave the cavity open to heal by secondary intention or pack lightly with a wick.

Deep and rectus sheath collections: ultrasound or CT-guided percutaneous drainage with a pigtail catheter (8 to 12 Fr) is acceptable for well defined, unilocular collections without necrotic tissue; otherwise surgical drainage.

Remove suture material and any foreign body. In SSI, open the wound over the whole infected length.

Debridement. Remove necrotic fat, fascia and slough. Any suggestion of necrotising infection needs urgent radical debridement, broad spectrum antibiotics and intensive care.

Antibiotics. Not required for a simple, small, superficial abscess in an immunocompetent patient after adequate drainage. Give antibiotics when there is cellulitis extending more than 5 cm, systemic signs of infection, immunosuppression, diabetes, multiple abscesses, or an associated device or mesh.

• Gram positive coverage: cefalexin 500 mg orally every 6 hours or flucloxacillin 500 mg to 1 g orally every 6 hours; IV cefazolin 2 g every 8 hours when severe
• Possible MRSA: vancomycin IV (15 to 20 mg/kg every 8 to 12 hours, guided by trough or AUC levels) or linezolid 600 mg every 12 hours; oral options include doxycycline 100 mg twice daily or co-trimoxazole (trimethoprim-sulfamethoxazole) 160/800 mg twice daily
• Enteric flora or deep source: piperacillin tazobactam 4.5 g IV every 6 to 8 hours, or ceftriaxone 2 g daily with metronidazole 500 mg every 8 hours; carbapenem (meropenem 1 g every 8 hours) for resistant or severe disease
• Duration: 5 to 7 days for soft tissue infection after drainage; 4 days after adequate source control for an intra abdominal source; longer for osteitis, mesh infection or bacteraemia

Tailor to cultures. Control blood glucose (target 140 to 180 mg/dL or 7.8 to 10 mmol/L in hospital).

Wound care. Daily packing and dressing, or negative pressure wound therapy (125 mmHg continuous, changed every 48 to 72 hours) once the cavity is clean and infection controlled. Delayed primary or secondary closure for large wounds. Nutritional optimisation.

Mesh infection. Superficial infection can sometimes be salvaged with drainage and antibiotics. Persistent infection needs mesh excision. Partial or complete removal is required with macroporous polypropylene mesh that is infected; synthetic multifilament and e-PTFE meshes almost always require removal. Rebuild the abdominal wall at a later stage, using biologic or lightweight mesh in a clean field.

Underlying cause. Treat fistula related collections by addressing the source (resection, diversion, anti-TNF therapy in Crohn disease with sepsis control first). Recurrent abscesses at the same site demand investigation for a retained foreign body, sinus or fistula.

Follow up. Review at 1 week and until the wound has healed. Monitor for incisional hernia at 6 and 12 months.