Colonic obstruction, volvulus and cancer recognition

Aetiologies of Large Bowel Obstruction

• Colorectal Carcinoma (60% to 70%): Most common cause. Annular, constricting "apple-core" lesions situated predominantly in the sigmoid colon or rectosigmoid junction.

• Diverticular Disease (10% to 15%): Chronic scarring, fibrosis, and muscular hypertrophy secondary to recurrent diverticulitis creating a tight fibrotic stricture.

• Colonic Volvulus (5% to 10%): Twisting of a mobile colonic segment around its mesentery (sigmoid volvulus ~80%, caecal volvulus ~15%).

• Faecal Impaction / Stercoral Colitis: Massive inspissated faecal mass in the rectum or sigmoid colon of debilitated, elderly, or neuropsychiatric patients.

• Benign Strictures: Inflammatory bowel disease (Crohn's colitis), ischaemic colitis strictures, anastomotic strictures.

• Competent Ileocaecal Valve (~75% of individuals): The one-way valve allows small bowel contents to enter the caecum but prevents retrograde reflux of colonic gas and fluid back into the ileum. When an obstructing lesion occludes the distal colon, the large bowel is converted into a closed-loop system between the competent ileocaecal valve and the tumor.

• Application of Laplace's Law: The law states that wall tension ($T$) in a hollow cylinder or sphere is directly proportional to the intraluminal pressure ($P$) multiplied by the radius ($r$):

• Incompetent Ileocaecal Valve (~25% of individuals): Elevated colonic pressure forces the valve open, permitting retrograde decompression into the distal small bowel. This prevents extreme colonic hyper-pressurization, delaying caecal blowout, but produces prominent small bowel distension and early bilious vomiting that mimics distal SBO.

Colonic Volvulus: Sigmoid vs. Caecal

Volvulus represents torsion of an air-filled loop of bowel around its mesenteric vascular axis, producing closed-loop obstruction and rapid ischaemia:

Sigmoid Volvulus

• Epidemiology: Typically affects elderly, institutionalized, bedridden patients, individuals with severe neuropsychiatric disorders (Parkinson's disease, dementia, schizophrenia on anticholinergic or psychotropic medications), and patients with chronic severe constipation producing a dilated, elongated, redundant sigmoid colon with a narrow mesenteric base.
• Radiological Findings: Plain abdominal radiograph and CT demonstrate the pathognomonic "coffee bean" sign (or bent inner tube): a massive, smoothly curved, dilated loop of colon originating in the pelvis and extending diagonally upward toward the right upper quadrant. The loop has thickened central apposed walls resembling the cleft of a coffee bean, with an absence of gas within the distal rectum.
• Management Algorithm:
  1. Haemodynamically Stable, No Peritonitis: First-line intervention is endoscopic detorsion and decompression via rigid or flexible sigmoidoscopy. The endoscope is advanced gently under direct vision to the twist; a soft, well-lubricated flatus tube (rectal tube) is guided past the obstruction. Detorsion produces an immediate, dramatic expulsion of copious liquid faeces and foul-smelling gas. The flatus tube is secured in place for 24 to 48 hours to maintain decompression.
  2. Elective Definitive Surgery: Endoscopic decompression is temporizing; the recurrence rate exceeds 50% to 70% if managed non-operatively. Therefore, an elective sigmoid colectomy (resection with primary anastomosis) is scheduled during the same admission once bowel inflammation resolves.
  3. Emergency Surgery (Hartmann's Procedure): Indicated immediately if the patient exhibits signs of bowel gangrene, perforation, peritonitis, toxic shock, or if endoscopic detorsion fails or reveals black, ischaemic mucosa. The non-viable sigmoid colon is resected, the rectal stump is closed, and an end sigmoid colostomy is matured.

Caecal Volvulus

• Pathogenesis: Arises from congenital incomplete embryological fixation of the caecum, ascending colon, and mesentery to the posterior parietal peritoneum (present in 10% to 15% of the population), creating an abnormally hypermobile caecum.
• Demographics: Occurs in younger adults (30 to 50 years), pregnant women, and endurance athletes.
• Radiological Findings: Abdominal radiography and CT demonstrate a large, gas-filled, comma-shaped or embryonic kidney-shaped dilated loop extending from the right iliac fossa toward the left upper quadrant or epigastrium.
• Management: Unlike sigmoid volvulus, endoscopic decompression has an exceptionally high failure rate ($> 80\%$) and carries severe perforation risks. The definitive treatment is immediate emergency surgical resection (right hemicolectomy with primary ileocolic anastomosis or diverting stoma). Caecopexy (suturing caecum to lateral wall) carries high recurrence and is reserved for frail patients without necrosis.

Colorectal Carcinoma: Presentation, Staging & Australian Screening

Colorectal cancer (CRC) is the third most commonly diagnosed cancer and the second leading cause of cancer-related death in Australia. Over 90% of cases occur in people aged 50 and older.

Clinical Presentation by Anatomical Location

• Luminal Geometry: Right-Sided (Proximal) Colon (Caecum, Ascending): Large luminal calibre; thin compliant colonic wall; Left-Sided (Distal) Colon (Descending, Sigmoid, Rectum): Narrower luminal diameter; thick muscular wall
• Faecal Consistency: Right-Sided (Proximal) Colon (Caecum, Ascending): Liquid, fluid chyme; Left-Sided (Distal) Colon (Descending, Sigmoid, Rectum): Solid, formed faecal stool
• Tumor Morphology: Right-Sided (Proximal) Colon (Caecum, Ascending): Exophytic, polypoid, fungating masses that ulcerate; Left-Sided (Distal) Colon (Descending, Sigmoid, Rectum): Annular, circumferential, constricting ("apple-core") lesions
• Cardinal Symptoms: Right-Sided (Proximal) Colon (Caecum, Ascending): Unexplained iron deficiency anaemia (microcytic hypochromic anaemia, lethargy, exertional dyspnoea, pallor) due to chronic occult blood loss; palpable RIF mass; vague right-sided abdominal discomfort; Left-Sided (Distal) Colon (Descending, Sigmoid, Rectum): Change in bowel habit (alternating diarrhoea/constipation, narrowing of stool calibre / "pencil-thin" stools); gross haematochezia (fresh red blood mixed with stool); colicky pain
• Obstruction Risk: Right-Sided (Proximal) Colon (Caecum, Ascending): Rare and very late; Left-Sided (Distal) Colon (Descending, Sigmoid, Rectum): High; commonest cause of acute large bowel obstruction
• Rectal Symptoms: Right-Sided (Proximal) Colon (Caecum, Ascending): Absent; Left-Sided (Distal) Colon (Descending, Sigmoid, Rectum): Tenesmus (painful, constant feeling of incomplete evacuation), fresh rectal mucus, bright red PR bleeding

Primary references (checked 7 October 2026): National bowel screening.