eMedicine Specialties > Gastroenterology > Colon

Megacolon, Chronic

Author: David M Manuel, MD, Fellow, Department of Internal Medicine, Section of Gastroenterology, Providence Hospital and Medical Center
Coauthor(s): Michael H Piper, MD, FACG, FACP, Clinical Assistant Professor, Department of Internal Medicine, Division of Gastroenterology, Wayne State University School of Medicine; Consulting Staff, Digestive Health Associates PLC; Roberto M Gamarra, MD, Fellow, Department of Internal Medicine, Section of Gastroenterology and Hepatology, Providence Hospital and Medical Center; Clifford Y Ko, MD, MS, MSHS, Department of Surgery, Assistant Professor, University of California at Los Angeles School of Medicine
Contributor Information and Disclosures

Updated: Aug 3, 2007

Introduction

Background

Megacolon, as well as megarectum, is a descriptive term. It denotes dilatation of the colon that is not caused by mechanical obstruction. While the definition of megacolon has varied in the literature, most researchers use the measurement of greater than 12 cm for the cecum as the standard. Because the diameter of the large intestine varies, the following definitions would also be considered: greater than 6.5 cm in the rectosigmoid region and greater than 8 cm for the ascending colon. 

Megacolon can be divided into the following 3 categories:

• Acute megacolon (pseudo-obstruction)
• Chronic megacolon, which includes congenital, acquired, and idiopathic causes
• Toxic megacolon

This article is devoted to chronic (noncongenital) megacolon.

Pathophysiology

The pathophysiology of chronic megacolon is incompletely understood. It likely represents an amalgam of primary disorders involving muscular and nervous systems of the intestine. Much basic science work has been performed in this area.

For example, with respect to the large bowel reacting to its luminal contents, fatty acids appear to reduce the volume of the proximal large bowel. Opiate narcotics, on the other hand, reduce the propensity of the colon to constrict.

Control of colonic contractility is through a complex interaction of intrinsic colonic nerves, splanchnic nervous control, and central nervous system input. The final common pathway of intrinsic nervous control of colonic motility is via postganglionic nerves: stimulatory cholinergic nerves and inhibitory nitric oxide-releasing nerves. Evidence suggests that excessive production of nitric oxide may be the mechanism for toxic megacolon in ulcerative colitis; as yet, there is no evidence for a possible role of nitric oxide in chronic megacolon unrelated to inflammatory bowel disease.

Studies in mouse models and in children with chronic colonic pseudo-obstruction show abnormalities involving the number and function of the interstitial cells of Cajal (intestinal pacemaker cells).  Inherited disorders likely involve abnormal maturation and function of these cells, whereas acquired disorders demonstrate decreased numbers of them.

Animal studies show that the splanchnic nerves can dramatically affect colonic motility, both to contract and relax the colon. Extrinsic adrenergic nerves seem mainly to act by reducing acetylcholine release from intrinsic postganglionic nerves, although a direct action on smooth muscle cells cannot be excluded. At this time, the respective roles of the intrinsic and splanchnic nerves in inducing megacolon have yet to be clarified.

Some experts believe it is common practice to separate the disorders associated with chronic megacolon into the following: (1) colonic inertia (eg, generalized delayed transit), and (2) rectosphincteric dyssynergy (eg, functional outlet obstruction).

Frequency

United States

No large-scale studies have been conducted to determine prevalence/incidence of acquired megacolon.

International

The most common cause of megacolon worldwide is infection with Trypanosoma cruzi (Chagas disease).

Mortality/Morbidity

No large-scale studies have been conducted to determine prevalence/incidence of acquired megacolon. However, once present, the approximate risk of a spontaneous perforation from nontoxic megacolon is 3%.

Race

Race has not been documented to play a role in megacolon.

Sex

• The frequency of acquired megacolon is equally distributed between the sexes.
• The congenital megacolon, Hirschsprung disease, predominantly occurs in males.

Age

Although clinically chronic megacolon can occur in any age group, inherited types usually present in young patients, and acquired types usually present in older patients.

Clinical

History

• Historically, chronic megacolon has been categorized into 2 groups, according to when symptoms begin.
  • The congenital group experiences onset of constipation before age 1 year.
  • The acquired group develops symptoms after age 10 years until adulthood.

Physical

• Physical examination generally reveals a distended abdomen, which may or may not be tense.
• Tympany is invariably present.
• Digital rectal examination may demonstrate a hard mass of stool just above the anorectal ring.  Digital rectal examination in a patient with Hirschsprung disease may bring about a large gush of retained fecal material.
• Megarectum with a rectum distended with stool, if chronic, tends to cause the anus to gape open secondary to the dysfunction of the internal sphincter mechanism. These patients may present with factitious diarrhea secondary to overflow incontinence.

Causes

• Causes of acquired megacolon
  • Neurologic diseases
    • Chagas disease
    • Parkinson disease
    • Myotonic dystrophy
    • Diabetic neuropathy
    • Spinal cord injury
    • Paraneoplastic neuropathy
    • Amyloidosis
  • Systemic diseases
    • Scleroderma
    • Dermatomyositis/polymyositis
    • Systemic lupus erythematosus
    • Mixed connective tissue disease
  • Metabolic diseases
    • Hypothyroidism
    • Hypokalemia
    • Porphyria
    • Pheochromocytoma
  • Medication-induced conditions
  • Idiopathic
    • Nonfamilial visceral neuropathy (sporadic hollow visceral neuropathy or chronic idiopathic intestinal pseudo-obstruction)
    • Results from damage to the myenteric plexus from drugs or viral infections
  • The most common nonmechanical cause of acquired megacolon is infection with T cruzi (Chagas disease).
    • This infection results in the destruction of the enteric nervous system.
    • While this disease was originally confined to South America, recent estimates indicate that 350,000 people in the United States are seropositive, a third of whom are thought to have chronic Chagas disease.
• Causes of congenital megacolon
  • Enteric neuropathies
    • Hirschsprung disease (congenital aganglionosis)
      • It is caused by a single gene mutation of the RET proto-oncogene on band 10q11.2.
      • The defect occurs in 1 in 5000 live births.
      • Some cases are familial, with an overall incidence of 3.6% among siblings of index cases.
    • Waardenburg-Shah syndrome (piebaldism, neural deafness, megacolon)
    • Multiple endocrine neoplasia type 2A (MEN 2A) or 2B (MEN 2B)
  • Visceral myopathies
    • Mitochondrial neurogastrointestinal encephalopathy (MNGIE) - Only type III involves marked dilatation of the colon
    • Oculogastrointestinal neuropathy (OGIN)
    • Idiopathic
  • In the newborn period, an unrecognized imperforate anus may be the cause of megacolon.

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