Megacolon Pathophysiology and Clinical Implications

Introduction to Megacolon

The rectum is generally preserved from the maximum dilation characteristic of megacolon because the underlying pathophysiology involves transmural neuromuscular dysfunction that predominantly affects the proximal colon, especially the transverse colon, while the rectum maintains its muscular tone and neurological function relatively intact 1
The transverse colon presents the greatest dilation (>5.5-6 cm) because it is the segment most susceptible to loss of muscular tone and neuromuscular dysfunction, while the rectum maintains its structure and function relatively preserved 1

In toxic megacolon, dilation can be total or segmental, but is specifically defined by dilation >5.5-6 cm in the mid-transverse colon, being the area of maximum concern 1
Although the rectum is generally preserved from the maximum dilation characteristic of megacolon, it is affected by the underlying inflammatory process, especially in ulcerative colitis where inflammation typically begins in the rectum 1
The American College of Gastroenterology recommends that patients with megacolon associated with sigmoid volvulus undergo subtotal colectomy, as sigmoid colectomy alone is insufficient because the volvulus tends to recur in the remaining segments of the colon 2
The recurrence rate was 82% in patients with associated megacolon treated with limited resection, compared to 0% in those treated with subtotal colectomy 2
The European Society of Coloproctology suggests that perforation of the transverse colon is associated with a mortality rate of 27-57%, regardless of whether it is contained or free, highlighting why this region is critical 1
The progression of colonic dilation is an indication for urgent surgical intervention, with subtotal colectomy with ileostomy being the procedure of choice 1, 5

It is crucial not to confuse adynamic ileus with toxic megacolon: the presence of systemic toxicity and distension >6 cm defines toxic megacolon, while adynamic ileus represents altered motility without the component of acute systemic toxicity 5
In ulcerative colitis, although the rectum is typically inflamed (the disease begins there), there may be rectal preservation in some cases, allowing for segmental resection instead of total colectomy 6
Persistent fever after 48-72 hours of steroid therapy suggests local perforation or abscess formation, requiring immediate surgical intervention 1, 7