Diverticular abscess and image-guided drainage

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

A diverticular abscess forms when mesentery or adjacent organs contain a perforation and create an infected pericolic or pelvic cavity. It is complicated diverticulitis even when blood pressure is normal. Contrast CT must establish size, loculation, free gas, organ compression and the relationship to bowel, bladder, vessels and ureter before source control is selected.

Initial care includes intravenous antibiotics and treatment of sepsis. NICE uses 3 cm as the decision threshold: larger collections prompt consideration of percutaneous drainage when feasible or surgery, while smaller abscesses can often move to oral antibiotics once stable. Neither catheter placement nor antibiotic duration proves success; improvement in physiology, pain, inflammatory markers and follow-up imaging must support control.

Key points

Suspected diverticular abscess requires sepsis assessment, intravenous antibiotics and contrast CT to define the infected cavity.
Record all collection dimensions, location, loculation, adjacent-organ effects and whether a safe percutaneous route exists.
For an abscess greater than 3 cm, NICE advises considering image-guided drainage when feasible or surgery.
For an abscess smaller than 3 cm, switch to oral antibiotics when possible and follow the clinical response closely.
Send pus from a drained collection for microbiology and narrow antimicrobial treatment when susceptibilities and progress allow.
Re-image deterioration or failure to improve, because source control may require catheter repositioning, another drain or an operation.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Contained diverticular perforation

Mesentery or an adjacent pelvic organ seals a sigmoid leak, trapping enteric bacteria, inflammatory fluid and tissue debris.

Impaired containment

Immune suppression, diabetes, malnutrition or delayed presentation may permit a larger, multiloculated cavity and reduce response to antimicrobial treatment.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Purulent cavity formation
Neutrophils, organisms and necrotic material accumulate behind an inflammatory wall, leaving an avascular centre that antibiotics penetrate poorly.
2
Pelvic mass effect
An enlarging collection can compress colon, bladder or ureter, adding bowel obstruction, urinary symptoms or hydronephrosis to infection.
3
Drain-mediated control
Catheter evacuation reduces bacterial load and pressure; ongoing bowel leakage, viscous contents or separate locules can prevent durable control.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Localised sepsis

A contained perforation allows inflammatory tissue to wall off faecal contamination into a pericolic or pelvic collection.

Size and accessibility

Larger collections are less likely to resolve with antibiotics alone, while intervening structures may make drainage unsafe.

Source-control failure

Persistent fever, pain and inflammatory markers suggest ongoing contamination, loculation or inadequate catheter position.

Future disease

After recovery, fistula, stricture or recurrent symptoms may justify later elective resection discussion.

Red flags requiring action

Diffuse guarding or free intraperitoneal contamination means the collection is no longer safely contained and requires emergency operative assessment.
Persistent fever, pain, tachycardia or rising CRP after drainage may indicate a blocked or displaced catheter, undrained locules or continuing bowel leakage.
Hydronephrosis, bowel obstruction or worsening kidney function can result from pelvic abscess mass effect and changes the urgency of source control.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Contrast CT abdomen pelvisFirst step
Why
confirm abscess size and drainage anatomy.
Interpretation and limitations
The report should state dimensions, location, loculation, free gas and a feasible access route.
02
Blood cultures and sepsis bloods
Why
identify physiological consequence and guide treatment.
Interpretation and limitations
Cultures should not delay antibiotics in a deteriorating patient.
03
Drain fluid culture
Why
identify organisms from the infected source.
Interpretation and limitations
Tailor antibiotics when susceptibilities return and reassess if cultures conflict with progress.
04
Repeat CT for non-response
Why
identify residual or new collection.
Interpretation and limitations
Re-imaging supports catheter adjustment, further drainage or surgery rather than prolonged blind antibiotics.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Perforated colorectal cancer

A necrotic tumour may present as a pericolic collection; atypical CT, incomplete recovery or luminal narrowing requires later cancer assessment.

Crohn-related abscess

Penetrating ileocolonic Crohn disease can produce collections and fistulae, often with discontinuous small-bowel disease that changes long-term treatment.

Non-colonic pelvic infection

Tubo-ovarian, urinary and postoperative collections may touch the sigmoid; CT origin and source cultures guide the responsible specialty and drainage route.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Worked case: drainage of a five-centimetre pelvic abscessCombine measured anatomy with clinical responseFirst stepA 68-year-old with fever and left iliac fossa pain has CT-confirmed sigmoid diverticulitis with a 5.0 cm pelvic abscess. Blood pressure is 126/70 mmHg, lactate 1.6 mmol/L and interventional radiology identifies a safe transgluteal route.

1Admit, assess for sepsis, take blood cultures when appropriate and begin intravenous co-amoxiclav after checking penicillin allergy and renal function.
2Review the CT jointly with colorectal surgery and interventional radiology; the collection is above 3 cm, lacks diffuse free contamination and has a safe access route.
3Insert an image-guided catheter, aspirating 65 mL of pus, and send the material for microscopy, culture and susceptibility testing.
4Culture grows susceptible Escherichia coli; narrow treatment with microbiology advice, review the intravenous route within 48 hours and complete a ten-day total course because source control is satisfactory.
5By day four the patient is afebrile, CRP has fallen from 218 to 64 mg/L and CT shows near-complete cavity collapse; remove the drain after output ceases. Because this was complicated diverticulitis and no suitable colonic imaging occurred in the preceding two years, arrange optical colonoscopy at about six weeks after recovery to exclude an underlying tumour.

02Small abscess without a safe drain targetTreat medically and verify improvementCT in a stable 59-year-old shows a 2.4 cm pericolic abscess without free gas, fistula, obstruction or organ dysfunction; pain and oral intake improve after admission.

1Give intravenous antibiotics and monitor observations, urine output, pain and renal function because any abscess is complicated disease.
2Recognise that the collection is below 3 cm and does not provide a useful percutaneous target; record why drainage is not planned.
3At 36 hours the patient is afebrile, tolerating food and CRP has fallen from 132 to 71 mg/L, so switch to an appropriate oral antibiotic.
4Complete the clinician-determined course, which may extend up to fourteen days for a CT-confirmed abscess, and provide precise deterioration advice.
5Confirm continued recovery at follow-up; repeat CT rather than simply extending antibiotics if fever, pain or inflammatory markers fail to improve.

03Drain failure with recurrent sepsisRe-image promptly and change source controlA 6.2 cm pelvic abscess was drained 48 hours ago, but the catheter has stopped producing fluid; temperature is 39.1°C, pulse 118/min and lactate has risen to 3.4 mmol/L.

1Treat evolving sepsis immediately with intravenous fluid, antibiotics, oxygen when required and senior colorectal, radiology and anaesthetic involvement.
2Check the catheter externally for kinking while avoiding forceful blind flushing, and review culture results for resistant or unexpected organisms.
3Urgent repeat CT shows that the catheter is displaced and two locules remain, with new extraluminal gas but no diffuse faecal contamination.
4Because percutaneous control has failed and anatomy is no longer suitable for another safe catheter, proceed to operative washout and sigmoid source control.
5Verify success through falling lactate, restored urine output, improving CRP and postoperative imaging if fever or ileus persists.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

Empirical enteric Gram-negative and anaerobic treatment supports the patient while drainage or operative source control is arranged.

Co-amoxiclav intravenous

For suspected or confirmed diverticular abscess, NICE lists 1.2 g intravenously three times daily; review within 48 hours or after scanning if sooner.

Avoid in serious penicillin allergy, adjust for renal impairment and review hepatic history; use abscess culture and local susceptibility data to narrow treatment, with total duration based on source control and response.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Septic shock

Uncontrolled infected material drives vasodilatation, capillary leak and organ hypoperfusion, requiring resuscitation alongside a new source-control plan.

Fistula formation

A persistent cavity can decompress into bladder, vagina, skin or adjacent bowel and create chronic enteric contamination.

Catheter failure

Kinking, displacement, blockage or inaccessible locules can leave infection untreated even when a drain remains visible at the skin.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Chart temperature, pulse, blood pressure, urine output, abdominal findings and pain alongside CRP so worsening sepsis is recognised before drain review becomes routine.
Record catheter output, character and patency; absent output may mean cavity resolution, blockage, displacement or an undrained locule.
Review intravenous antibiotics within 48 hours or after imaging, and tailor treatment to pus culture, renal function, source control and clinical progress.
Repeat CT when the patient deteriorates or does not improve, then choose catheter adjustment, additional drainage or surgery from the new anatomy.
After recovery from complicated diverticulitis, arrange optical or virtual colonoscopy at about six weeks unless suitable colonic imaging was completed within the preceding two years; persistent or suspicious symptoms still require assessment despite an earlier examination, whereas fully recovered uncomplicated disease without cancer concern does not automatically need routine imaging.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Three centimetres changes options

NICE advises considering drainage or surgery above this size, while smaller abscesses often move to oral treatment when stable.

Drainability is anatomical

Size alone cannot create a safe route; interventional radiology must assess intervening bowel, vessels and pelvic structures.

Culture narrows exposure

A source sample can replace broad empirical therapy with a susceptibility-directed regimen.

Clinical response verifies control

A catheter image is not success unless physiology, pain and inflammatory markers improve.

11Common pitfallsFrequent interpretation and management errors.

01
Continuing antibiotics alone for a worsening large accessible abscess delays source control.
02
Draining a collection without sending culture loses an opportunity to target therapy.
03
Assuming an empty drain proves resolution ignores blockage or malposition.
04
Choosing surgery or drainage from size alone overlooks peritonitis, accessibility and physiological status.

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