Intra-abdominal abscess
An intra-abdominal abscess is a walled-off collection of pus within the abdomen. It represents the body's success at containing an infection that would otherwise have spread throughout the peritoneal cavity — but a successful containment is not the same as a resolved infection. Pus within a fibrous wall is effectively inaccessible to antibiotics and to the immune system, and it will not resolve until it is drained. The governing principle in surgical infection has not changed in two thousand years: ubi pus, ibi evacua — where there is pus, drain it.
How they form
When contamination enters the peritoneal cavity, the body attempts to localise it. The greater omentum migrates toward the area of inflammation and adheres to it; adjacent loops of bowel and mesentery are drawn in; and fibrin is deposited, sealing the area off from the rest of the cavity. Within days a fibrous wall forms.
Inside that wall, the environment changes in ways that make the collection self-perpetuating:
- The blood supply is poor, so systemic antibiotics reach the cavity in only low concentrations
- The pH falls and the environment becomes hypoxic, which impairs the function of neutrophils and inactivates several antibiotics, notably aminoglycosides
- Neutrophils die and release enzymes, liquefying the contents into pus and enlarging the cavity
- Anaerobic organisms flourish in the low-oxygen environment, which is why abscess infections are usually polymicrobial, with both aerobes and anaerobes
The consequence is straightforward and clinically important: an established abscess will not be cured by antibiotics alone, regardless of how appropriate the choice or how long the course. Drainage is the treatment; antibiotics are adjunctive.
Where they form, and why it matters
Peritoneal fluid moves in predictable directions, driven by gravity and by the negative pressure created beneath the diaphragm during respiration. Collections therefore accumulate at recognised sites:
- Subphrenic spaces (right and left, beneath the diaphragm) — fluid is actively drawn upward by diaphragmatic movement; a classical site after upper abdominal surgery or perforated ulcer
- Subhepatic space (Morison's pouch) — the most dependent part of the upper abdomen in a supine patient
- Pelvis — the lowest point of the peritoneal cavity when upright, and the commonest site after appendicitis or diverticulitis
- Paracolic gutters — the channels along which fluid tracks between upper and lower abdomen
- Lesser sac — typically after pancreatitis or gastric perforation
- Interloop — between loops of small bowel; often multiple, often small, and the hardest to detect and to drain
Causes
- perforated appendicitis — the classic cause of a pelvic abscess
- perforated or complicated diverticulitis
- anastomotic leak after bowel surgery — the commonest postoperative cause
- perforated peptic ulcer
- gallbladder perforation, cholangitis, liver abscess
- pancreatitis with infected necrosis (which is a distinct entity managed differently)
- Crohn's disease
- trauma, penetrating injury, or a retained foreign body
- gynaecological infection — tubo-ovarian abscess
Symptoms
An abscess should be suspected in any patient who fails to improve as expected after abdominal surgery or an intra-abdominal infection, typically between the fifth and tenth postoperative day.
- Swinging fever — spiking temperatures with intervening normal periods, the classical pattern
- rigors and sweats
- persistent tachycardia
- localised abdominal pain and tenderness, though often less impressive than expected
- Prolonged ileus — a bowel that will not start working after surgery is a common and under-recognised sign
- anorexia, malaise, and failure to thrive after an operation
- persistently raised inflammatory markers
- Site-specific features: a subphrenic abscess causes shoulder tip pain, hiccups, basal atelectasis and a sympathetic pleural effusion, and may produce few abdominal signs at all — the old adage that "pus somewhere, pus nowhere, pus under the diaphragm" reflects exactly this. A pelvic abscess causes diarrhoea, mucus discharge, urinary frequency and tenesmus, and is tender on rectal examination.
In the elderly and the immunosuppressed, fever may be absent and the presentation limited to confusion, anorexia and failure to recover.
How the diagnosis is made
- CT of the abdomen and pelvis with intravenous and oral contrast — the investigation of choice. It demonstrates a fluid collection with an enhancing rim, often containing gas, identifies its size and position, defines a safe access route for drainage, and identifies the underlying cause such as an anastomotic leak.
- Ultrasound — useful for superficial, subphrenic, subhepatic and pelvic collections, and at the bedside; less reliable where bowel gas or dressings interfere.
- MRI — in pregnancy, in children, and for pelvic collections.
- Blood tests — white count, C-reactive protein and procalcitonin; blood cultures, particularly during a fever spike.
- Culture of the drained pus — essential, and it frequently changes antibiotic therapy. Anaerobic culture must be requested specifically.
Treatment
Drainage
Percutaneous drainage under CT or ultrasound guidance is the first-line treatment and succeeds in the great majority of cases, avoiding surgery. A catheter is placed into the cavity under local anaesthetic and left to drain, with irrigation as required. Its advantages over surgery are substantial: no general anaesthetic, no laparotomy in a patient who has often recently had one, and a recovery measured in days.
Access routes are chosen to avoid bowel, pleura and major vessels. Where the route is difficult:
- Transrectal or transvaginal drainage of a pelvic collection, which is safe and effective
- Endoscopic ultrasound-guided drainage, placing a stent between the collection and the stomach or duodenum — particularly useful for lesser sac and peripancreatic collections
Drains are left until output falls below a low threshold and imaging confirms resolution; premature removal is a common cause of recurrence.
Surgery
Reserved for situations where percutaneous drainage is not feasible or has failed:
- multiple or multiloculated collections
- collections with no safe percutaneous access route
- thick, particulate pus or necrotic material that will not pass through a catheter
- an underlying problem requiring surgical correction — a persistent anastomotic leak, ischaemic bowel, a retained foreign body
- a deteriorating patient
Laparoscopic drainage is possible in selected cases.
Antibiotics
Broad-spectrum cover against Gram-negative and anaerobic organisms, rationalised on culture results. Antibiotics alone are appropriate only for small collections, generally under 3–4cm, in a stable patient, and such patients must be monitored closely with a low threshold for drainage if they fail to improve.
Once the abscess is adequately drained, a short course of antibiotics — around four days — is as effective as a prolonged one. Persisting fever despite antibiotics almost always means inadequate drainage rather than the wrong antibiotic, and the correct response is to re-image rather than to change the prescription.
Supportive care
Nutritional support, correction of anaemia and electrolyte disturbance, thromboprophylaxis, and treatment of the underlying condition.
Recovery
After percutaneous drainage, fever usually settles within twenty-four to forty-eight hours — a failure to do so should prompt reassessment. The drain typically stays in for several days to a couple of weeks. Patients are often discharged with the drain in place and reviewed as outpatients.
Recovery from the underlying condition takes longer, and patients who have had an abscess after major surgery are frequently depleted and benefit from active nutritional support and rehabilitation.
When to seek help urgently
Fever, rigors, increasing pain or simply feeling unwell in the days or weeks after abdominal surgery or an abdominal infection requires assessment rather than reassurance. A patient who is not recovering as expected after abdominal surgery has a complication until proved otherwise, and the commonest one is a collection.
Common questions
Why won't antibiotics alone clear it?
Because pus inside a fibrous wall has a very poor blood supply, so antibiotics reach it only in low concentrations, and the acidic, oxygen-poor environment inside inactivates several of them and weakens the immune cells that would otherwise clear it. Draining the pus is the treatment; antibiotics support it.
Do I need another operation?
Usually not. Most collections are drained by placing a small tube through the skin under local anaesthetic, guided by a scan. This avoids another general anaesthetic and another abdominal wound, and it works in the large majority. Surgery is reserved for collections that cannot be reached safely, that are multiple, or where something underlying needs correcting.
How was this drained through my rectum?
A collection deep in the pelvis often sits immediately behind the rectum, and reaching it through the abdominal wall would mean crossing loops of bowel. Draining it through the rectal wall is a short, safe route and is well established. It sounds unappealing but it is usually straightforward and effective.
How long will the drain stay in?
Usually several days to a couple of weeks, until the amount coming out falls to a very low level and a scan confirms the cavity has collapsed. Removing it too early is one of the main reasons collections come back, so it is worth being patient with it. Many people go home with the drain in place.
I am still feverish on antibiotics. Is the antibiotic wrong?
Possibly, but far more often it means the collection is not being drained adequately — the tube has blocked or moved, or there is another collection that has not been found. The correct response is usually a repeat scan rather than simply changing the antibiotic.
Why did this happen after my operation?
Usually because a small amount of contamination occurred during or after the operation, sometimes from a join in the bowel that has leaked a little. The body has walled it off, which is why you have a localised collection rather than generalised infection. It is a recognised complication rather than evidence that something was done wrongly.
Could this come back?
It can, particularly if the drain is removed before the cavity has fully collapsed, or if the underlying cause — a leak, inflamed bowel, a retained stone — has not been dealt with. A follow-up scan before the drain comes out reduces that risk considerably.
Why do I have shoulder pain and hiccups?
These suggest a collection sitting under the diaphragm, which shares its nerve supply with the shoulder tip and irritates the diaphragm mechanically. Collections in this position often cause surprisingly few abdominal symptoms, so shoulder pain and hiccups after abdominal surgery are worth mentioning.
Related conditions
Other conditions of the peritoneal disease covered on this site:
- Peritoneal metastases
- Pseudomyxoma peritonei
- Peritoneal mesothelioma
- Primary peritoneal cancer
- Peritonitis
- Adhesions and adhesive bowel obstruction
This page provides general information and does not replace an individual medical consultation. Fever or feeling unwell after abdominal surgery requires prompt medical assessment.