Comprehensive Notes on Intestinal Obstruction

Classification and Aetiopathology of Intestinal Obstruction

• Intestinal obstruction is a pathological condition where the contents of the intestines cannot be pushed downwards. It is classified into two primary types:

  • Dynamic Obstruction: Peristalsis is working against a mechanical obstruction. It can be acute or chronic.
  • Adynamic Obstruction: There is no mechanical element. Peristalsis is either absent (e.g., Paralytic Ileus) or inadequate (e.g., Pseudo-obstruction).

• Aetiopathology Causes of Dynamic Obstruction:

  • Intraluminal: Faecal impaction, foreign bodies, bezoars (trichobezoars/phytobezoars), gallstones, meconium ileus, and worms ($Ascaris\,lumbricoides$).
  • Intramural: Strictures (Tuberculosis, Crohn’s disease, ischaemia), malignancy, intussusception, and volvulus.
  • Extramural: Bands and adhesions (commonest cause in the Western world), and hernias (commonest cause globally).

• Classification by Chronicity and Site:

  • Acute: Typically common in the small bowel with sudden onset of severe colicky central abdominal pain.
  • Chronic: Usually seen in large bowel obstruction with absolute constipation.
  • Acute on Chronic: Common in large bowel obstruction.
  • Subacute: Incomplete obstruction where the patient can still pass flatus.
  • High Small-Bowel Obstruction (SBO): Involves the duodenum or jejunum; characterized by early profuse vomiting and rapid dehydration.
  • Low SBO: Involves the ileum; characterized by predominant pain, central distension, and delayed vomiting.
  • Large-Bowel Obstruction (LBO): Characterized by early pronounced distension; vomiting and dehydration occur late.

Pathophysiology of Intestinal Obstruction

• Proximal and Distal Changes:

  • The bowel proximal to the obstruction dilates. Initially, peristalsis increases to overcome the block. If unrelieved, the bowel becomes flaccid and paralyzed.
  • The bowel distal to the obstruction exhibits normal peristalsis and absorption until it becomes empty and collapses.

• Distension Factors:

  • Gas: Resulting from aerobic and anaerobic organism overgrowth. Composition: Nitrogen ($90\%$) and hydrogen sulphide. Derived from swallowed air ($70\%$), diffusion from blood ($20\%$), and bacterial action ($10\%$).
  • Fluid: Accumulation of digestive juices including saliva ($500\,mL$), bile ($500\,mL$), pancreatic secretions ($500\,mL$), and gastric secretions ($1\,L$) per $24\,hours$.

• Dehydration and Electrolyte Loss:

  • Caused by reduced oral intake, defective intestinal absorption, vomiting, sequestration in the bowel lumen, and transudation of fluid into the peritoneal cavity.

Strangulation and Ischaemia

• Definition: Strangulation occurs when the blood supply is compromised, threatening bowel viability.
• Mechanism: Venous return is compromised before the arterial supply. Increased capillary pressure leads to impaired local perfusion, haemorrhagic infarction, and translocation of anaerobic organisms and endotoxins.
• Clinical Signs of Strangulation:
  • Constant and severe abdominal pain.
  • Tenderness with rigidity and peritonism (rebound tenderness or Blumberg’s sign).
  • Fever, tachycardia, and shock.
  • Presence of guarding and absent bowel sounds.
• Closed-Loop Obstruction: Occurs when the bowel is obstructed at both the proximal and distal ends (e.g., malignant stricture of the colon with a competent ileocaecal valve). This leads to rapid increase in luminal pressure, often highest at the caecum, risking necrosis and perforation.

Special Types of Mechanical Obstruction

• Internal Hernia: Entrapment of small bowel in retroperitoneal fossae or mesenteric defects. Sites include:
  • Foramen of Winslow.
  • Holes in the transverse mesocolon or small bowel mesentery.
  • Defects in the broad ligament.
  • Paracaecal, duodenal retroperitoneal, and intersigmoid fossae.
• Bolus Obstruction:
  • Gallstone Ileus: Erosion of a large stone from the gallbladder into the duodenum. It often impacts $60\,cm$ proximal to the ileocaecal valve. Characteristic Rigler’s triad on imaging: SBO, pneumobilia (gas in the biliary tree), and an atypical mineral shadow (gallstone).
  • Bezoars: Trichobezoars (hair masses) and phytobezoars (fruit/vegetable fiber).
  • Stercoliths: Often associated with jejunal diverticula or ileal strictures.
• Postoperative Adhesions: Most common cause of SBO in Western nations. Peritoneal irritation leads to fibrin production. Preventive measures include good surgical technique, washing the peritoneal cavity with saline, and minimizing gauze contact.

Intussusception

• Pathophysiology: Invagination of one portion of bowel (intussusceptum) into an adjacent segment (intussuscipiens). It consists of the Entering/Inner tube (intussusceptum), the Returning/Middle tube, and the Sheath/Outer tube (intussuscipiens).
• Paediatric Presentation: Peak incidence between $5$ and $10\,months$ of age. Often idiopathic ($90\%$). Classic signs include episodes of screaming, drawing up of legs, and ‘redcurrant jelly’ stools (blood and mucus).
• Adult Presentation: Almost always associated with a lead point (e.g., polyp, submucosal lipoma, or Meckel’s diverticulum).
• Physical Signs: Sausage-shaped lump (hardens on palpation) and the ‘Sign of Dance’ (emptiness in the right iliac fossa).

Volvulus

• Definition: Twisting or axial rotation of a portion of bowel about its mesentery. It causes obstruction to the lumen ($> 180^{\circ}$ torsion) and vascular occlusion ($> 360^{\circ}$ torsion).
• Sigmoid Volvulus: Most common site in adults. Predisposing factors include chronic constipation and a long pelvic mesocolon. Rotation is usually anticlockwise. Imaging shows the ‘coffee bean’ sign or ‘bent inner tube’ sign.
• Caecal Volvulus: Usually a clockwise twist, representing $1.5\%$ of all intestinal obstructions.
• Compound Volvulus (Ileosigmoid Knotting): Rare condition where the ileum twists around the sigmoid colon.

Clinical Features of Intestinal Obstruction

• The Classic Quartet:
  1. Abdominal Pain: Colicky (small bowel centred on umbilicus; large bowel in lower abdomen). Constant pain suggests strangulation.
  2. Vomiting: Early in high SBO; late and possibly faeculent in low LBO.
  3. Distension: Greater in distal obstructions; may be minimal in high SBO.
  4. Constipation: Absolute (no flatus or faeces) is the cardinal feature of complete IO. Absolute constipation does NOT apply to Richter's hernia, gallstone ileus, mesenteric vascular occlusion, or pelvic abscesses.
• Physical Examination Findings:
  • Inspection: Scars, distension, visible peristalsis.
  • Palpation: Tenderness, masses.
  • Percussion: Tympanitic abdomen.
  • Auscultation: High-pitched ‘borborygmi’/bowel sounds, or silent abdomen if gangrene develops.
  • DRE: Rectum may be empty or contain a palpable mass.

Imaging and Investigations

• Plain Abdominal X-ray (AXR): Supine film is the standard diagnostic choice.
  • Jejunum: Valvulae conniventes (regularly spaced lines crossing the entire width, concertina/ladder effect).
  • Ileum: Described by Wangensteen as ‘featureless.’
  • Large Bowel: Haustral folds (spaced irregularly, do not cross the whole diameter).
• Radiological Signs:
  • Step-ladder Pattern: Characteristic of distal SBO.
  • Rigler’s Sign: Bowel perforation; air in peritoneal cavity outlines both sides of the bowel wall.
  • Claw Sign: Characteristic of intussusception on barium/contrast enema.
  • Pneumobilia: Gas in the biliary tree.
• CT Scan: Highly accurate investigation ($93\%$ sensitivity; $94\%$ accuracy; $100\%$ specificity).

Management and Treatment

• Supportive Conservative Management (Drip and Suck):
  • Nasogastric Tube (NGT) insertion (Ryle or Salem tube) for decompression.
  • Intravenous (IV) fluid replacement (Hartmann’s solution or normal saline).
  • Monitoring: Urinary catheterization (fluid balance), 2-hourly vitals, 8-hourly abdominal exam.
  • Antibiotics: Ampicillin, gentamicin, metronidazole.
• Surgical Indicators: ‘The sun should not both rise and set’ on a case of unrelieved obstruction. Early surgery indicated for:
  • Obstructed/tender irreducible external hernia.
  • Suspected strangulation or peritonitis.
  • Obstruction in a ‘virgin’ abdomen.
  • Closed-loop obstruction.
• Viability Assessment of Bowel:
  • Viable: Shiny, firm (intestinal musculature), dark color becomes lighter upon release/oxygen, visible pulsation in mesentery, peristalsis present.
  • Non-viable: Dull/lustreless, flabby/thin/friable, dark color remains, no detectable pulsation, no peristalsis.
• Specific Surgical Procedures:
  • Sigmoid Volvulus: Flatus tube decompression or rigid/flexible sigmoidoscopy. Failing this, laparotomy with derotation. If gangrenous: Hartmann’s procedure or Paul-Mikulicz operation (Mikulicz-Radecki procedure).
  • Large Bowel Malignancy: Right hemicolectomy (for proximal) or Hartmann’s (for distal). Colonic stenting is used as a palliative measure or bridge to surgery.

Questions & Discussion

• Causes of Adynamic Obstruction:
  • Q: Which of the following are causes? A: Paralytic ileus, B: Hernia, C: Mesenteric vascular obstruction, D: Pseudo-obstruction, E: Adhesions.
  • A: A, C, D. Hernia and adhesions are dynamic causes.
• Strangulation Characteristics:
  • Q: True or False: The arterial supply is compromised before the venous return.
  • A: False. Venous return is compromised before the arterial supply.
• Sigmoid Volvulus Direction:
  • Q: What is the usual direction of rotation?
  • A: Anticlockwise.
• Intussusception in Children:
  • Q: How many cases are reduced non-operatively?
  • A: More than $70\%$ via air or barium enema.
• Radiological Scenarios:
  • 1: 70-year-old male with dilated caecum/proximal colon up to splenic flexure. Diagnosis: Primary colonic tumour.
  • 7: 70-year-old post-CABG man with massive distension and absent bowel sounds. Diagnosis: Pseudo-obstruction (Ogilvie’s syndrome).
  • 8: 68-year-old with ‘coffee bean’ sign. Diagnosis: Sigmoid volvulus.