Define Intestinal Obstruction. Describe its causes, investigations and management.

The Concept & Classification

Intestinal obstruction is a blockage to the onward passage of bowel contents, and it is classified along three useful axes: by level (small vs large bowel), by mechanism (mechanical obstruction vs functional/paralytic ileus), and, most importantly for safety, by whether it is simple or strangulated — the latter meaning the blood supply is compromised, threatening bowel viability. This classification directs both diagnosis and treatment.

Causes

A helpful anatomical framework divides mechanical causes into those in the lumen, in the wall, or outside the wall:

The Four Cardinal Features

Four symptoms define obstruction, and their relative prominence localises the level:

• Colicky abdominal pain — from vigorous peristalsis against the obstruction.
• Vomiting — early and profuse in high/small-bowel obstruction, late and feculent in large-bowel obstruction.
• Distension — greater and more central/peripheral the more distal the obstruction (marked in large-bowel obstruction).
• Absolute constipation (no faeces or flatus) — an early feature of large-bowel obstruction, a late feature of small-bowel obstruction.

Recognising Strangulation

The critical judgement is spotting strangulation, because it demands emergency surgery. Its warning signs are a change from colicky to continuous, severe pain, localised tenderness and peritonism, fever and tachycardia, and a rising lactate — all indicating ischaemic, potentially necrotic bowel.

Investigation

An abdominal X-ray shows dilated loops and air-fluid levels: small bowel lies centrally with valvulae conniventes crossing the whole lumen, whereas large bowel lies peripherally with haustra that only partly cross; a volvulus gives a 'coffee-bean' loop. CT is the key investigation — it identifies the level, the cause, and signs of ischaemia. Bloods and lactate assess the patient's state.

Management

Initial management of most obstruction is conservative — the 'drip and suck' approach: nil by mouth, a nasogastric tube to decompress the stomach, intravenous fluids with correction of electrolytes, and monitoring of urine output. Adhesive small-bowel obstruction without strangulation often settles with this. Surgery is indicated for strangulation, an obstructed/irreducible hernia, a complete or malignant large-bowel obstruction, and failure of conservative treatment.

Fluid Shifts & the Sick Patient

A key point often underestimated is the massive fluid loss of obstruction. Litres of fluid are sequestered in the distended, non-absorbing bowel and lost through vomiting, producing dehydration, hypovolaemia and electrolyte disturbance (notably hypokalaemia). This 'third-space' loss is why aggressive intravenous fluid replacement and careful electrolyte correction — guided by urine output — are as important as decompression, and why the patient can be far sicker than the abdomen alone suggests.

Closed-loop Obstruction

A particularly dangerous form is closed-loop obstruction, where the bowel is blocked at two points (as in a volvulus or an obstructing colonic cancer with a competent ileocaecal valve). The trapped segment cannot decompress in either direction, so pressure rises rapidly, threatening ischaemia and perforation — classically of the caecum in large-bowel obstruction (the caecum has the largest diameter and, by Laplace's law, the highest wall tension). This is a surgical emergency even without other signs of strangulation.

Paralytic Ileus — the Contrast

It is important to contrast mechanical obstruction with paralytic ileus, a functional failure of peristalsis (after surgery, with peritonitis, or from electrolyte disturbance such as hypokalaemia). Here there is distension and vomiting but the pain is not colicky and, crucially, bowel sounds are absent rather than the hyperactive 'tinkling' sounds of mechanical obstruction. Ileus is managed by treating the cause and supportive measures, not surgery — so distinguishing it from mechanical obstruction avoids an unnecessary operation.

• Commonest causes: adhesions (small bowel), colorectal carcinoma (large bowel).
• Initial management = 'drip and suck' (NBM, NG tube, IV fluids, correct electrolytes).
• Surgery for strangulation, obstructed hernia, complete/malignant LBO, or failed conservative treatment.

Bowel sounds distinguish the two at the bedside.

• Classify by level (small/large bowel), mechanism (mechanical vs paralytic ileus), and simple vs strangulated (blood supply compromised).
• Commonest causes: adhesions (SBO), colorectal cancer (LBO); framework — in lumen / in wall / outside wall.
• Four cardinal features: colicky pain, vomiting (early in SBO, feculent late in LBO), distension (marked in LBO), absolute constipation (early in LBO).
• AXR: central small bowel (valvulae conniventes) vs peripheral large bowel (haustra); CT for level/cause/ischaemia.
• 'Drip and suck' initially; operate for strangulation (continuous pain, peritonism, ↑lactate), obstructed hernia, complete LBO or failed conservative care.