What is a megacolon?
Megacolon is a condition where the colon of the large intestine becomes wider than normal. This distension can also affect the cecum and extend to the rectum. The proper term for this widening is ‘dilatation’. It is not an increase in length but a dilated portion of the colon while other parts usually remain the same width. The term megacolon literally means large colon. It is important to note that megacolon is a term used to denote widening without a mechanical obstruction. Understandably the colon will become dilated if there is some mechanical obstruction blocking the outflow of its contents. Although the colon can stretch, there is a point where it can be considered to be overdistended. Stretching also prevents the muscles in the colon wall from functioning properly and its contents can therefore not be moved. In severe cases the colon can tear although perforation is uncommon.
Normal colon activity
The colon, like the entire gastrointestinal tract, has muscular walls. These muscles contract in a coordinated manner to push food and wastes through the gut. When food, wastes or any bulk enters a portion of the gut it causes slight stretching of the gut wall. This signals the gut that there are contents within that segment that needs to be pushed along. The stretch signals are sent back to the spine which in turn signals the gut walls to contract. This is known as a nerve reflex. These contractions then push the contents further down.
In this way the contents within a segment of the gut cannot accumulate excessively and cause the gut wall to stretch abnormally. It also ensures that movement is properly coordinated rather that parts of the gut contracting and relaxing unnecessarily which may cause backward flow of contents. This simple yet effective mechanism starts from the time of swallowing and continues all the way along the gut until waste and undigested material is passed out as stool.
Reasons for megacolon
The exact mechanism by which a megacolon arises is not completely understood. A megacolon does not arise with a mechanical obstruction that prevents the movement of the colonic contents. Instead it appears that a megacolon may be a consequence of a muscle or nerve problem, or possibly a combination of both. As explained, if the stretching of the colon wall does not send signals to the nervous system which in turn sends signal back to the wall then the muscles in the wall will not contract. Similarly if the muscles are not functioning properly then it cannot contract at the right time to push the contents along.
Therefore the problem may be due to :
- Nerve impulse conduction and reflexes.
- Neurotransmitters which transmits signals across a gap.
- Overstimulation of muscles by regulating nerves thereby preventing it from contracting (sympathetic) or relaxing (parasympathetic).
- Reduced input from regulating nerves (parasympathetic) preventing it from contracting.
- Excessive opioids acting on the opioid receptors which upsets the reflexes.
There are broadly three types of megacolon – acute, chronic and toxic. The causes and treatment of each may differ but ultimately the underlying problem is essentially the same – the colon is wider than normal and its ability to move the colonic contents are impaired.
Acute conditions like food poisoning, colon infections, biochemical disturbances, acute flareups of chronic bowel diseases and the use of certain medication may lead to acute megacolon. It arises as a complication. Megacolon in these case may develop even after the inflammation of the colon eases. The condition can often resolve once these causative factors are removed. Conservative management may be all that is necessary in these cases if a patient is stable. It is mainly the elderly who are affected with an acute megacolon.
This type of megacolon may be due to birth defects (congenital) and can be seen in infants, or acquired where it may develop in childhood, adolescence or adulthood. Chronic megacolon may arise with various diseases including chronic neurological disorders, metabolic conditions and systemic diseases. Usually there is no inflammation of the colon that is present as is initially seen with acute megacolon or continues with toxic megacolon.
The term toxic megacolon refers to inflammation of the colon along with abnormal widening. This is in contrast to acute or chronic megacolon where dilatation may exist after an inflammatory phase or even without any preceding inflammation. The causes of toxic megacolon may therefore overlap with acute megacolon. The more correct term for toxic megacolon is megacolon with toxic colitis or toxic megacolon with toxic colitis.
There are a number of different causes for each type of megacolon. Some of the more frequently seen causes, which may also be common to more than one type of megacolon, are discussed below.
Inflammatory bowel disease
Crohn’s disease and ulcerative colitis, the two types of inflammatory bowel diseases (IBD), are common causes of acute and toxic megacolon. Dilatation of the colon may arise during a severe and prolonged acute flareup of the condition. A megacolon may arise as a complication and sometimes patients mistaken the symptoms of a megacolon for IBD.
Clostridium difficile infection, also known as pseudomembranous colitis, may arise with prolonged antibiotic use. The normal intestinal flora is affected and C.difficile overgrowth may trigger an inflammatory reaction in the bowels. It is also known as antibiotic-associated diarrhea when it arises after long term antibiotic use.
Chagas disease is a bowel infection caused by the parasite, Trypanosoma cruzi. It is the most common cause of megacolon across the globe. The parasite is spread by the triatomine bug which are more active at night. However, consuming food contaminated with the feces of the bug or contact with the blood of an infected person may also spread the infection.
There are a number of different medications that can contribute to megacolon. It is largely dependent on the dose and duration of use. Some of these drugs include antidiarrheals, anticholinergics, antipsychotics and opiates. Iatrogenic causes, meaning medical therapies that lead to megacolon, may also include chemotherapy and radiation therapy which lead to colitis.
The symptoms of megacolon itself and that of the underlying cause can overlap to a certain degree. Therefore patients may not realize that complications like a megacolon has arisen due to confusion with the existing symptoms of the causative disease. These symptoms include :
- Abdominal distension
- Diminished or absent bowel sounds
- Accumulation of abdominal gas which can be detected with percussion of the abdomen (tympany)
- Abdominal discomfort but pain is usually not present
- Palpable abdominal mass
In addition, there may be other symptoms of toxicity in toxic megacolon especially. This includes :
- Rapid heart rate (tachycardia)
- Abdominal pain and tenderness
The clinical features along with a medical history may be indicative of a megacolon. A digital rectal examination (DRE) needs to be done to exclude impacted feces which will also lead to colonic dilatation. However, further investigation is always necessary to confirm the diagnosis. These investigations include :
- Abdominal x-ray
- Computed tomography (CT) scan
- Abdominal ultrasound
Open sores in the colon known as stercoral ulcers are more likely to be seen with chronic megacolon. These ulcers may be the site of a perforation of the colon. The diameter of the large intestine varies. It is therefore important to establish a point where the colon can be said to be overdistended and therefore confirm a diagnosis of megacolon. Measurements to diagnose a megacolon includes :
- cecum > 12 centimeteres
- ascending colon > 8 centimeters
- transverse colon > 6 centimeters
- rectosigmoid region > 6.5 centimeters
The treatment options for acute, toxic and chronic megacolon can vary. Treatment should be directed at the underlying cause, preventing further distention of the colon and avoiding complications such as a perforated colon. Some of the treatment measures include :
- Decompression with a rectal tube or colonoscopic decompression.
- Enemas and digital disimpaction may be considered especially when fecal impaction is arising secondary to a megacolon.
- Medication to stimulate nerve activity and increase colonic motility.
- Bulking agents may be used in chronic megacolon but can exacerbate the condition.
- Bowel habit retraining in chronic megacolon.
- Surgical removal of part of the colon known as a colectomy. Other procedures like a cecostomy or colostomy may also be considered.