Peritoneum, omentum, mesentery and retroperitoneum

Overview

The peritoneum is a thin serous membrane of mesothelium. The parietal layer lines the abdominal wall and is supplied by somatic nerves, so its pain is sharp and well localised. The visceral layer covers the organs and has autonomic supply, so its pain is dull and central.

The peritoneum absorbs and secretes fluid quickly, so it can produce large volumes of exudate (peritonitis) or transudate (ascites). The diaphragmatic peritoneum absorbs fastest through lymphatic stomata, which is why sepsis from peritonitis spreads rapidly and why fluid and tumour cells track to the right subphrenic space.

Anatomy you need

• Greater sac and lesser sac (omental bursa, behind the stomach) connect through the epiploic foramen (of Winslow).

• The transverse mesocolon divides the cavity into supracolic and infracolic compartments.
• Fluid from a perforated duodenal ulcer can track along the right paracolic gutter into the RIF and mimic appendicitis.
• Lowest points: supine, the hepatorenal pouch (Morison's pouch); upright, the pelvis (rectovesical pouch in men, rectouterine pouch of Douglas in women).
• Retroperitoneal organs ("SAD PUCKER"): Suprarenals, Aorta and IVC, Duodenum (2nd to 4th parts), Pancreas (except tail), Ureters, Colon (ascending and descending), Kidneys, oEsophagus (lower thoracic part), Rectum (lower part).

Peritonitis

Peritonitis is inflammation of the peritoneum, localised or generalised.

Causes

Secondary peritonitis is polymicrobial: E. coli, other Gram-negative rods, enterococci and anaerobes such as Bacteroides.

Clinical features

• Pain worse on movement and coughing; the patient lies still.
• Tenderness, guarding, rebound; board-like rigidity in generalised peritonitis.
• Fever, tachycardia, vomiting; bowel sounds fade as paralytic ileus develops.
• Late: distension, shock, cold extremities and the sunken hippocratic facies.

Investigations

• Bloods: FBC (leucocytosis), U&E, amylase or lipase (exclude pancreatitis), CRP, lactate, cultures.
• Erect chest X-ray: gas under the diaphragm in perforation (not always present).
• CT abdomen is the most useful test to find the cause.
• Ultrasound for gallbladder and pelvic causes.

Management

1. Resuscitation: IV fluids, oxygen, urinary catheter, nasogastric tube, analgesia.
2. Broad-spectrum IV antibiotics covering Gram-negative and anaerobic organisms.
3. Source control: repair the perforation, remove the organ or drain; thorough peritoneal lavage with warm saline.
4. Supportive care in HDU or ICU. Consider a laparostomy or planned relook for severe faecal peritonitis or bowel ischaemia.

Complications: septic shock and multiorgan failure, residual abscesses, adhesions and later obstruction, wound infection, incisional hernia. Diffuse peritonitis still carries a mortality of about 10%.

Intra-abdominal abscess

Clue: the patient improves after the first operation, then develops swinging fever, sweating, raised white count and malaise days to weeks later. An old saying: unexplained sepsis after surgery means pus under the diaphragm until proved otherwise.

Subphrenic abscess

Features: swinging fever, shoulder-tip pain (phrenic nerve, C3-5), persistent hiccups, basal lung signs.

Chest X-ray: raised, immobile hemidiaphragm, basal collapse or pleural effusion, sometimes a gas-fluid level under the diaphragm.

Diagnosis and treatment: ultrasound or CT, then percutaneous drainage with antibiotics. Open drainage if percutaneous drainage fails or the collection is multiloculated.

Pelvic abscess

• Follows appendicitis, gynaecological infection or any generalised peritonitis.
• Diarrhoea with mucus, tenesmus, urinary frequency, swinging fever.
• Rectal examination: tender, boggy mass bulging into the anterior rectal wall.
• Many settle with antibiotics. If not, drain through the rectum (or posterior vaginal fornix), or under radiological guidance. Laparotomy is seldom needed.

Ascites

Ascites is excess serous fluid in the peritoneal cavity; clinically detectable from about 1.5 L (shifting dullness, fluid thrill when tense).

The serum-ascites albumin gradient (SAAG) is now used more than total protein: 1.1 g/dL or more means portal hypertension.

• Chylous ascites: milky fluid rich in triglycerides; lymphoma, trauma or surgery to lymphatics, filariasis.
• Treatment: treat the cause; salt restriction and spironolactone in cirrhosis; large-volume paracentesis with albumin; TIPS for refractory cases. Peritoneovenous (LeVeen) shunts are now rarely used.

Spontaneous bacterial peritonitis (SBP)

• Infection of ascites with no surgical source; almost always in cirrhosis.
• Usually a single organism (E. coli, Klebsiella, streptococci).
• Diagnosis: ascitic neutrophil count above 250/mm3; culture is often negative.
• Treatment: a third-generation cephalosporin (e.g. cefotaxime) started at once; alternatives are co-amoxiclav or a quinolone.

Tuberculous peritonitis

Common in Pakistan and other high-burden countries.

• Spread from the ileocaecal region via mesenteric nodes, or by blood from miliary TB.
• Chronic form: weight loss, evening fever, night sweats, abdominal distension; "doughy" abdomen; loculated ascites; matted bowel loops; rolled-up omentum felt as a transverse mass.
• Ascitic fluid: straw-coloured exudate, lymphocytes predominate, raised adenosine deaminase (ADA). Smear for acid-fast bacilli is rarely positive.
• CT: ascites, thickened peritoneum and omentum, enlarged nodes.
• Laparoscopy with peritoneal biopsy (caseating granulomas, tubercles on both layers) is the most reliable diagnostic test.
• Treatment: antituberculous therapy per the national TB programme (check current guidance); surgery only for complications such as obstruction or perforation. Obstruction from strictures often settles on drugs.

Peritoneal tumours

• Secondary carcinomatosis (commonest): stomach, ovary, colon, pancreas. Malignant ascites, omental cake, nodules in the pouch of Douglas (Blumer's shelf on rectal examination).
• Pseudomyxoma peritonei: abdomen fills with mucinous jelly; usually from a ruptured mucinous appendiceal tumour, with secondary deposits on the ovaries; more common in women; treated by complete cytoreduction with heated intraperitoneal chemotherapy (HIPEC) in specialist centres.
• Primary peritoneal mesothelioma: rare; linked to asbestos.

Omentum

• Torsion of the omentum: usually an obese, middle-aged man; mimics appendicitis (right-sided pain, tender mass); excise the twisted segment.
• Omental infarction and appendices epiploicae torsion cause localised pain with a characteristic fat-density lesion on CT.
• Omentum is involved in TB (rolled omentum) and carcinomatosis (omental cake).

Mesentery

Retroperitoneum

Retroperitoneal fibrosis

• A grey-white fibrous plaque starts over the lower lumbar spine and spreads to encase the aorta, IVC and ureters.
• Idiopathic (Ormond's disease) in most; others: drugs (methysergide, beta-blockers), inflammatory aortic aneurysm, radiotherapy, malignancy (lymphoma, carcinoid, metastases), leaked urine or blood. Some cases are IgG4-related disease.
• Back or loin pain, raised ESR, leg oedema, hypertension and renal failure from bilateral ureteric obstruction.
• IVU or CT urogram: hydronephrosis with ureters pulled medially (medial deviation).
• Treatment: stop any causative drug; steroids (or tamoxifen); relieve obstruction with ureteric stents or nephrostomy; ureterolysis with the ureters wrapped in omentum or placed intraperitoneally.

Psoas (retroperitoneal) abscess

• Primary: blood-borne, Staphylococcus aureus; immunosuppression, diabetes, IV drug use.
• Secondary: spread from the spine (TB, Pott's disease, still common in Pakistan), Crohn's disease, appendicitis, diverticulitis, kidney.
• Fever, back or flank pain, flexion of the hip with pain on passive extension (psoas sign); swelling may point into the groin below the inguinal ligament.
• CT diagnosis; CT-guided percutaneous drainage plus antibiotics (and ATT if tuberculous).

Retroperitoneal tumours

• Most retroperitoneal masses are secondary (lymphoma, nodal metastases from testis, kidney and adrenal tumours).
• Primary retroperitoneal sarcomas are rare; commonest types: liposarcoma, then leiomyosarcoma. They present late as a large mass with vague pain.
• CT and MRI for staging; refer to a sarcoma centre before biopsy. Treatment is complete en bloc surgical excision; local recurrence is the main problem.
• A retroperitoneal "lipoma" should be treated as a possible liposarcoma.

Quick revision

• Secondary peritonitis = polymicrobial; resuscitate, antibiotics, source control.
• Subphrenic abscess: commonest in Morison's pouch; swinging fever, shoulder pain, hiccups, effusion; percutaneous drainage.
• Pelvic abscess: mucous diarrhoea, boggy rectal mass, drain per rectum.
• SBP: neutrophils above 250/mm3, single organism, cefotaxime.
• TB ascites: exudate, lymphocytes, high ADA; diagnose by laparoscopic biopsy.
• Pseudomyxoma: mucinous appendix tumour; cytoreduction plus HIPEC.
• Mesenteric cyst: Tillaux sign. Mesenteric adenitis: child after sore throat.
• Retroperitoneal fibrosis: medial ureters, steroids and stents. Commonest retroperitoneal sarcoma: liposarcoma.

Written by The House Job from standard references (Bailey & Love).

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