Megacolon

Megacolon is the dilation of the colon in the absence of a mechanical obstruction (e.g., colonic tumor/stricture). There are three etiological types of megacolon: acute, chronic, and toxic megacolon. Acute megacolon (Ogilvie's syndrome) is the acute dilation of the colon, characteristically seen in severely medically/surgically ill patients, probably secondary to an electrolyte/metabolic imbalance. Chronic megacolon is the permanent dilation of the colon caused by chronic colonic dysmotility due to an underlying neuropathic (Hirschsprung's disease, chronic Chagas disease) or myopathic (Duchenne's muscular dystrophy) disorder. Patients with acute/chronic megacolon typically present with abdominal pain, bloating, and constipation. Toxic megacolon is a life-threatening dilation of the colon associated with systemic toxicity due to infectious colitis (C. difficile pseudomembranous colitis, Salmonella enterocolitis) or inflammatory colitis (inflammatory bowel disease). Patients typically present with signs of sepsis (tachycardia, hypotension) and a history of abdominal pain and bloody diarrhea. Abdominal x-rays demonstrate a colonic dilation, with/without air-fluid levels, and preserved haustrae. Contrast-enhanced CT scans can identify/rule out a mechanical colonic obstruction and possible complications (colonic ischemia/perforation). Patients with acute/chronic megacolon can often be treated conservatively with bowel rest, dietary modifications, prokinetic drugs, and/or neostigmine. Colonoscopic decompression is often successful in patients with acute megacolon. Surgical intervention for acute/chronic megacolon (colectomy and ileorectal anastomosis) is indicated if conservative treatment fails. Conservative management of toxic megacolon includes bowel rest, IV antibiotics (for infectious colitis), IV steroids (for inflammatory bowel disease). There is a high risk of colonic perforation in patients with toxic megacolon. Hence, no improvement to medical therapy within 24–72 hours is an indication to perform surgery (subtotal colectomy and end ileostomy).

• Definition: Loss of intestinal peristalsis and subsequent dilation of the colon in the absence of a mechanical obstruction
• Classification: Megacolon is classified into three etiological types ^[1]
  • Acute megacolon
  • Chronic megacolon
  • Toxic megacolon

• Description: acute dilation of the colon in the absence of a mechanical obstruction, characteristically seen in severely ill or postoperative patients ^[2][3]
• Etiology
  • Occurs in seriously ill patients or those who have undergone a major surgical procedure
  • Idiopathic; possible factors include:
    • Electrolyte imbalance
    • Major surgery (e.g., orthopedic, pelvic, or cardiothoracic surgery)
    • Trauma (e.g, fractures of long bones, spinal cord injury)
    • Hypothyroidism
    • Drugs which are known to decrease intestinal motility (e.g., anticholinergic drugs, opioid analgesics, antipsychotics, calcium channel blockers)
• Pathophysiology: etiological factors → impairment/destruction of the autonomic nervous system → imbalance between sympathetic and parasympathetic control of intestinal motility → accumulation of feces, air, and intestinal secretions in the intestine → colonic dilation
• Clinical features
  • Gradual abdominal distention
  • Abdominal pain; constipation/diarrhea
  • Tympanitic abdominal percussion; decreased frequency of bowel sounds
  • Signs of colonic ischemia or impending perforation: Fever, tachycardia, and peritoneal signs (guarding, rigidity, rebound tenderness)
• Diagnostics
  • Laboratory values
    • May show signs of underlying disease
    • Hypokalemia as a potential cause
  • Abdominal x-ray
    • Dilation of the cecum and right colon (occasionally up to the rectum) with/without multiple air-fluid levels
    • Haustrae are preserved
  • Contrast enhanced CT scan (oral and IV contrast) ^[4]
    • Confirms x-ray findings
    • Rules out mechanical obstruction (e.g., tumor/stricture)
    • Can diagnose complications (ischemic bowel segments or perforation peritonitis)
  • Colonoscopy and endoscopy
    • Indicated in hemodynamically stable patients when CECT cannot be performed
    • Rules out a mechanical obstruction
    • Colonoscopic decompression can be performed in the same sitting.
• Treatment ^[5]
  • Supportive measures
    • Bowel decompression
      • Nasogastric tube to decompress the stomach
      • Rectal tube to decompress the rectum and distal colon
    • IV fluids and bowel rest (NPO)
    • Treat the inciting factor (e.g., stop the offending drug, correct the electrolyte abnormality.
    • Indicated in patients with mild symptoms and cecal dilation < 12 cm
  • Pharmacologic management: Neostigmine
    • Indicated when no improvement > 24–48 hours of bowel rest and decompression and/or cecal dilation > 12 cm with no signs of colonic ischemia/perforation or peritonitis
    • Treatment with methylnaltrexone may be attempted if opiates are suspected as a precipitating factor.
  • Intervention/surgery
    • Indications
      • Signs of impending or actual colonic ischemia/perforation or peritonitis
      • Failure of conservative therapy
    • Colonoscopic bowel decompression: indicated if neostigmine is contraindicated or unsuccessful ^[6]
    • Cecostomy
    • Colectomy with/without colostomy

• Description: Permanent dilation of the colon caused by congenital/acquired colonic dysmotility in the absence of a mechanical obstruction ^[7]
• Etiology
  • Congenital (e.g., Hirschsprung disease)
  • Acquired
    • Neuropathies (e.g., diabetic neuropathy, spinal cord injury, Parkinson's disease) ^[8]
    • Myopathies (e.g., Duchenne's muscular dystrophy, myotonic muscular dystrophy)
    • Connective tissue disorders (e.g., scleroderma, systemic lupus erythematosus, dermatomyositis)
    • Chronic Chagas disease ^[9]
  • Idiopathic - reason unknown ^[10]
• Pathophysiology: Etiological factors cause a neural and/or motor dysfunction of the bowel → bowel dysmotility → progressive colonic dilation ^[7]
• Clinical features
  • Recurrent episodes of:
    • Constipation
    • Abdominal pain and distention (bloating)
    • Anorexia, early satiety, and nausea
  • Examination findings: Abdominal distention; mild abdominal tenderness
  • Signs and symptoms of the underlying disorder
• Diagnostics
  • Laboratory values: may show signs of underlying disease
  • Abdominal x-ray may show:
    • Dilation of the cecum and right colon (occasionally up to the rectum) with/without multiple air-fluid levels
    • Haustrae are preserved
  • Contrast-enhanced CT scan (CECT; oral and IV contrast)
    • Confirms x-ray findings
    • Rules out mechanical obstruction (e.g., tumor/stricture)
    • Can diagnose complications (ischemic bowel segments or perforation peritonitis) ^[4]
  • Colonoscopy and endoscopy: indicated in hemodynamically stable patients when CECT cannot be performed; rules out a mechanical obstruction
  • Colon transit studies: to assess colonic motility ^[11][12][13]
    • Colon transit scintigraphy: Investigation of choice to diagnose and determine the extent of delayed colonic transit ^[12][13]
    • Colon transit test using radio-opaque markers ^[11]
    • Wireless motility capsule test ^[11][14]
  • Manometry
  • Colonic biopsy
• Treatment ^[1][15]
  • Conservative management
    • Dietary modifications
    • Osmotic laxatives and enemas to empty the colon and prevent fecal impaction
    • Prokinetic drugs: Erythromycin, metoclopramide, neostigmine
    • Treatment of the underlying disorder
  • Surgery: indicated in patients who do not improve/worsen on conservative therapy
    • Total abdominal colectomy with ileorectal anastomosis
    • See "Surgical treatment" of Hirschsprung's disease

• Description: a life-threatening, acute dilation of the colon associated with systemic toxicity
• Etiology
  • Infectious colitis
    • Bacterial: C. difficile (pseudomembranous colitis), Salmonella, Shigella, Campylobacter infections
    • Parasitic: Trypanosoma cruzi (Chagas disease), E. histolytica (amebic dysentery) infections
  • Inflammatory colitis: Ulcerative colitis, Crohn disease
• Pathophysiology: colitis (inflammation)
  • Colonic accumulation of inflammatory mediators and bacteria → nitric oxide synthesis → colonic dilation ^[16]
  • Edema and inflammation of the colonic smooth muscle → colonic dysmotility → colonic dilation
• Clinical features
  • (Bloody) diarrhea and vomiting
  • Abdominal distention and pain
  • Signs of sepsis (fever, tachycardia, hypotension) and dehydration
  • Signs of underlying disease (See Ulcerative colitis, Crohn disease)
• Diagnostics
  • Laboratory findings
    • Neutrophilic leukocytosis
    • Anemia
    • ↑ ESR/CRP
    • Hypokalemia
  • Abdominal x-ray findings
    • Dilation of the colon (transverse colon diameter > 6 cm)
    • Loss of haustration
    • Multiple air-fluid levels
  • Contrast enhanced CT scan (oral and IV contrast)
    • Confirms x-ray findings
    • Rules out mechanical obstruction (e.g., tumor/stricture)
    • Can diagnose complications (ischemic bowel segments or perforation peritonitis) ^[4]
  • Stool tests for C. difficile toxin: in patients with preceding diarrhea (see "Diagnosis" of clostridium difficile infection)
  • Colonoscopy and endoscopy: contraindicated in suspected toxic megacolon due to high risk of perforating the colon
• Diagnostic criteria
  • Radiographic evidence of colonic dilatation
  • Three of the following: fever (> 101.5°F), tachycardia (> 120 beats/min), leukocytosis (> 10,500/μL), or anemia
  • One of the following: dehydration, hypotension, altered mental status, or electrolyte disturbance
• Treatment
  • Conservative treatment ^[17][18]
    • Admission to intensive care unit
    • Complete bowel rest: NPO
    • Nasogastric tube insertion
    • IV fluids
    • Correction of fluid and electrolyte imbalances
    • Broad-spectrum IV antibiotics (e.g., ampicillin, gentamicin, and metronidazole)
      • Oral vancomycin or fidaxomicin for C. difficile colitis
    • Rotation techniques: change position frequently to facilitate the evacuation of bowel gas (prone or knee-elbow position)
    • Avoid all anticholinergic or narcotic medications
    • IV steroids (hydrocortisone, dexamethasone) for inflammatory bowel disease
  • Surgery
    • Indications: no response to medical management within 24–72 hours; development of complications
    • Procedure: subtotal colectomy and end ileostomy
• Complications
  • Colonic ischemia → colonic perforation→ peritonitis
  • Sepsis and multiorgan dysfunction

Colonoscopy should be avoided in patients with suspected toxic megacolon since it increases the risk of colonic perforation.

References:^{[4][15][16][19][20][21]}