Diverticular disease and diverticulitis

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

Colonic diverticula are mucosal outpouchings through the muscle wall, commonly in the sigmoid colon. Their incidental presence on imaging or colonoscopy is diverticulosis and needs neither an illness label nor routine treatment. Diverticular disease refers to chronic intermittent lower-abdominal pain, bowel disturbance and local tenderness attributed to diverticula after relevant alternatives are considered. Acute diverticulitis is a new inflammatory episode, usually with more sustained focal pain, fever and objective inflammation. The distinction matters: chronic bloating in someone who happens to have diverticula may still be IBS, while sudden focal inflammation may be cancer, ischaemic colitis, urinary disease or gynaecological pathology rather than diverticulitis.

Clinical assessment grades physiology and complications before naming severity. Record onset, pain site, bowel and urinary symptoms, vomiting, bleeding, previous attacks, immune status and medicines including NSAIDs, corticosteroids, opioids and anticoagulants. Examine for focal tenderness, mass, distension and peritonism, checking hydration and sepsis observations. Free perforation may present dramatically, but older or immunosuppressed people can have muted signs. Colovesical fistula causes pneumaturia, faecaluria or recurrent polymicrobial urinary infection; stricture produces progressive obstructive symptoms. CT with intravenous contrast is the main test for admitted suspected complicated disease because it distinguishes uncomplicated inflammation from abscess, free gas, obstruction and an alternative tumour.

Management is severity-led. A stable, systemically well person with clinically uncomplicated disease may receive paracetamol, oral fluid and diet advice, no antibiotics and an explicit early review. Antibiotics are reserved for systemic illness, important comorbidity, immunosuppression or complications, with IV treatment for admitted complicated cases and review for oral step-down. Abscess management depends on size, anatomy and response: drainage or surgery is considered above 3 cm, while any septic deterioration demands source-control reassessment. Generalised peritonitis requires urgent operative decision-making. Once recovered, reassess unresolved symptoms and whether luminal evaluation is needed to exclude malignancy based on CT confidence, prior colonoscopy and local protocol. Elective resection is individualised for complications or continuing burden, not triggered by a simple count of attacks.

Key points

Diverticulosis means the presence of diverticula, diverticular disease means attributable chronic symptoms, and diverticulitis is acute inflammation that may be uncomplicated or complicated.
Typical acute diverticulitis produces constant left lower-quadrant pain and tenderness with fever or raised inflammatory markers, but right-sided disease and atypical presentations occur.
Peritonitis, abscess, fistula, obstruction, perforation or sepsis defines complicated disease and changes the setting, imaging urgency and treatment.
For a systemically well person with uncomplicated acute diverticulitis, NICE supports considering no antibiotics, simple analgesia and clear re-presentation advice.
Use antibiotics when the person is systemically unwell, immunosuppressed or significantly comorbid, following current local choices, allergies, renal function and resistance policy.
Contrast CT within twenty-four hours of admission is recommended when complicated diverticulitis is suspected and inflammatory markers are raised; it maps abscess and alternative diagnoses.
A CT-confirmed abscess larger than 3 cm prompts consideration of percutaneous drainage if feasible or surgery; deterioration warrants re-imaging and source-control review.
After recovery, do not promise that seeds, nuts or popcorn must be avoided. Encourage a healthy balanced fibre-containing diet once the acute episode settles, adjusted for tolerance.

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

Age and wall susceptibility

Diverticula become common with age as mucosal pouches herniate through points of weakness where vasa recta cross the colonic muscle.

Motility and lifestyle factors

Altered colonic pressure, low physical activity, obesity, smoking and dietary patterns may modify disease risk, while no single behaviour explains every case.

Medicine and host risk

NSAIDs, corticosteroids, frailty and immunosuppression increase concern for complicated inflammation or muted presentation rather than directly proving diverticulitis.

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

1
Diverticulum formation
Segmental pressure and structural weakness push mucosa and submucosa through the muscular wall, most often in the sigmoid colon.
2
Local inflammation
Obstruction or microperforation at a diverticular neck allows bacterial and faecal inflammation to extend into pericolic fat.
3
Complicated extension
Failure of local containment produces abscess, free perforation, fistula or inflammatory fibrosis with stricture and obstruction.

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

Symptomatic uncomplicated disease

Intermittent left lower-quadrant discomfort, constipation or diarrhoea and local tenderness without fever or systemic inflammation may reflect diverticular disease, but functional bowel and malignant causes remain relevant.

Uncomplicated acute diverticulitis

New constant focal pain and tenderness with low-grade fever or inflammatory-marker rise, but no abscess, free perforation, fistula or obstruction, defines an uncomplicated episode after adequate assessment.

Abscess or contained perforationRed flag

Persistent fever, focal mass, sepsis, failure to improve or CT fluid collection and extraluminal gas suggests a complication requiring hospital antibiotics and radiological or surgical source-control planning.

Free perforationRed flag

Sudden diffuse pain, involuntary guarding or rigidity, shock and free intraperitoneal contamination indicate a surgical emergency. Analgesia and imaging should not delay resuscitation and senior colorectal review.

Fistula or strictureRed flag

Pneumaturia, faecaluria or recurrent unusual urinary infection suggests colovesical fistula, while progressive distension, vomiting and obstipation raises an inflammatory or malignant stricture.

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
Serial observations and sepsis assessmentFirst step
Why
Detect physiological deterioration, dehydration and organ dysfunction that determines admission and urgency.
Interpretation and limitations
Hypotension, rising respiratory rate, altered mentation or oliguria outweighs apparently localised tenderness and requires immediate resuscitation, antimicrobial consideration and senior source-control planning.
02
Full blood count, CRP and renal profile
Why
Measure inflammation, anaemia, dehydration, kidney injury and electrolyte disturbance while preparing safe imaging and treatment.
Interpretation and limitations
Raised inflammatory markers support acute inflammation but are non-specific. Normal markers in suspected complicated disease should prompt reconsideration, while immunosuppression can blunt the response.
03
Contrast CT abdomen and pelvis
Why
Confirm location and severity, detect abscess, perforation, fistula or obstruction and identify alternative diagnoses.
Interpretation and limitations
Report collection size and accessibility, free or local gas, obstruction and suspicious mass. In admitted suspected complicated disease with raised markers, NICE recommends CT within twenty-four hours.
04
Blood cultures and lactate
Why
Assess severe sepsis or impaired perfusion before antibiotics when collection does not delay urgent treatment.
Interpretation and limitations
Positive cultures refine therapy; lactate supports physiological risk but is neither required nor sufficient to diagnose perforation. Trends accompany clinical and source-control assessment.
05
Urine testing
Why
Identify urinary infection, haematuria or polymicrobial clues to a colovesical fistula while considering renal and gynaecological alternatives.
Interpretation and limitations
Recurrent mixed organisms, gas or faecal material in urine strengthens fistula suspicion and prompts cross-sectional imaging and colorectal-urological planning; simple pyuria can be reactive.
06
Interval colonic evaluation
Why
Exclude cancer or another luminal lesion after inflammation settles when CT, symptoms, screening status or local policy leaves concern.
Interpretation and limitations
Timing and modality are individualised because acute inflammation raises procedural risk. A recent high-quality colonoscopy and unequivocal uncomplicated CT may alter the need under local guidance.

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

Colorectal cancer

Progressive change in bowel habit, weight loss, iron-deficiency anaemia, a mass or irregular shouldered narrowing requires malignancy exclusion after acute inflammation settles.

Irritable bowel syndrome

Chronic fluctuating pain related to defaecation without fever, focal inflammatory signs or CT inflammation supports IBS rather than acute diverticulitis.

Inflammatory or ischaemic colitis

Bloody diarrhoea, diffuse or segmental mucosal inflammation and disease-specific histology favour colitis; abrupt pain after a low-flow event suggests ischaemia.

Additional chapter-specific clues

Colorectal cancer mimicRed flag

Anaemia, mass, progressive bowel change, weight loss or an irregular shouldered stenosing lesion must not be assumed to be diverticular. Apply cancer pathways and obtain tissue or interval evaluation appropriately.

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

01Community assessmentManage well uncomplicated diseaseFirst stepFocal symptoms suggest acute diverticulitis but the person is systemically well with no complication signal.

1Check observations, hydration, peritonism, oral intake, immune status and comorbidity, considering urinary, gynaecological and malignant alternatives.
2Offer simple analgesia, consider a no-antibiotic strategy and give practical oral fluid and short-term diet advice without imposing prolonged restriction.
3EscalationProvide explicit same-day escalation triggers including worsening pain, fever, vomiting, inability to drink, distension, bleeding or new systemic symptoms.
4Arrange timely review and reconsider examination, bloods or imaging if improvement does not follow the expected course.

02Complicated episodeImage, treat sepsis and control sourceThere is systemic illness, peritonism, obstruction, fistula, suspected abscess or uncontrolled symptoms.

1Admit or transfer, begin ABCDE care, venous access, analgesia, fluid and locally recommended IV antibiotics when complicated infection or sepsis is suspected.
2Obtain urgent colorectal review and contrast CT, generally within twenty-four hours for raised inflammatory markers, without delaying immediate surgery in an unstable perforation.
3Use collection size, location and physiology to choose antibiotics alone, image-guided drainage or operation; send drained pus for microbiology.
4Review IV antibiotics and imaging response within forty-eight hours or sooner, stepping down when appropriate and re-imaging deterioration to reassess source control.

03After recoveryPrevent harm and resolve uncertaintyAcute inflammation has settled enough for outpatient planning.

1Review the CT diagnosis, cancer risk, previous colonoscopy and any unresolved anaemia, weight loss or bowel change to decide on interval luminal assessment.
2Reintroduce a balanced diet and increase fibre gradually if tolerated, with adequate fluid; explain that routine avoidance of seeds, nuts and popcorn is unnecessary.
3Minimise regular NSAIDs and opioids where alternatives exist, support activity, weight and smoking goals, and avoid prophylactic antibiotics or aminosalicylates for recurrence.
4Refer for colorectal discussion when fistula, stricture, recurrent abscess or continuing quality-of-life burden raises elective surgery, individualising benefits and stoma or recurrence risks.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Provides first-line pain relief for uncomplicated diverticular disease or diverticulitis without the gastrointestinal and renal risks of NSAIDs.

Paracetamol

Use the standard adult oral dose within the current BNF maximum, reduced when low body weight, liver disease or other risk makes that necessary.

Check combination products, alcohol, nutrition and liver disease to prevent accidental excess. Inadequate pain control or increasing requirement should trigger reassessment, not automatic opioid escalation.

Treats likely colonic infection in higher-risk uncomplicated presentations where a no-antibiotic strategy is inappropriate.

Oral antibiotic for selected diverticulitis

Choose the current NICE and local oral regimen only for systemic illness, immunosuppression or significant comorbidity, adjusting for allergy, renal function and interactions.

Antibiotics are not routine for a systemically well episode. Review diarrhoea and Clostridioides difficile risk, fluoroquinolone restrictions where relevant, clinical response and whether complicated disease was missed.

Covers likely colonic organisms during sepsis management while drainage or operative source control is assessed and delivered.

Intravenous broad-spectrum antibiotic

Give the locally approved IV regimen promptly for admitted suspected complicated diverticulitis, then review within forty-eight hours or after imaging for step-down and duration.

Take cultures when feasible without delaying treatment, adjust for renal function and allergy, and narrow to microbiology. Antibiotics cannot compensate for an undrained abscess or perforation.

Supports bowel regularity when dietary fibre alone is insufficient or poorly tolerated in stable diverticular disease.

Bulk-forming laxative

Introduce a locally formulary-approved fibre preparation gradually with adequate fluid after acute inflammation has settled, titrating to comfortable formed stools.

Avoid during suspected obstruction, severe acute pain or inadequate fluid intake. Bloating can worsen initially; persistent symptoms require diagnostic review rather than unlimited dose escalation.

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

Abscess, perforation and sepsis

A microperforation may form a contained collection, while free rupture causes purulent or faecal peritonitis, shock and urgent source-control need.

Fistula and stricture

Chronic inflammation can connect colon to bladder, vagina, skin or bowel, or heal with fibrosis that causes large-bowel obstruction.

Diverticular haemorrhage

A vasa recta can rupture into a diverticulum, causing abrupt painless lower-GI bleeding that is mechanistically separate from diverticulitis.

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

During acute care, trend pain distribution, temperature, pulse, blood pressure, respiratory rate, urine output, oral tolerance and peritoneal signs.
Recheck CRP, white count, renal function and electrolytes according to severity, interpreting the trajectory alongside examination rather than as a discharge target alone.
For an abscess, document size, drainage feasibility, culture results, drain output and the named trigger for repeat imaging or surgery.
After discharge, confirm symptom resolution, nutritional recovery and completion or early cessation of antibiotics as planned, with clear access for deterioration.
Track unresolved anaemia, bowel change, CT uncertainty and interval endoscopy results so a cancer mimic is not lost after the acute episode.

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

Diverticula may be incidental

Their prevalence rises with age, so finding diverticula does not prove they caused chronic pain. Match anatomy, inflammation and symptom pattern before attributing disease.

Antibiotic restraint is active care

For a systemically well uncomplicated episode, observation, analgesia and safety-netting can be the guideline-supported plan. It still requires reliable review and diagnostic vigilance.

Abscess size changes options

The 3 cm threshold in NICE guidance helps identify collections where percutaneous drainage or surgery should be considered, but anatomy and physiology still determine feasibility.

Attack count does not operate

Elective resection is based on complications, persistent burden, operative risk and patient preference rather than an automatic number of uncomplicated episodes.

Seeds are not culprits

Routine avoidance of nuts, seeds, fruit skins or popcorn is unsupported. A gradual fibre-rich diet after recovery is more useful, adjusted to individual tolerance.

11Common pitfallsFrequent interpretation and management errors.

01
Using diverticulosis on a scan as the explanation for every episode of abdominal pain.
02
Giving antibiotics automatically to a systemically well uncomplicated presentation without review or safety-netting.
03
Managing an immunosuppressed patient at home because fever and CRP are only modest.
04
Treating a diverticular abscess with repeated antibiotics while failing to reassess source control.
05
Performing acute colonoscopy through severe active inflammation without a compelling specialist indication.
06
Discharging after apparent recovery without resolving suspicious CT thickening, anaemia or progressive bowel change.