Colonic narrowing, medically termed a stricture or stenosis, describes the abnormal constriction of the large intestine’s inner channel, known as the lumen. This condition impedes the normal movement of waste, causing stool and gas to slow down or become completely blocked. The colon is the final segment of the digestive tract, where it absorbs remaining water and electrolytes, solidifies waste, and propels stool for elimination. When the colon narrows, this essential process is disrupted, often leading to symptoms like pain, bloating, and constipation.
Narrowing Due to Chronic Inflammation
Chronic inflammation is a frequent cause of colonic narrowing, primarily through the mechanism of repeated injury and repair. The cycle of irritation, ulceration, and healing leads to the deposition of scar tissue, or fibrosis, within the bowel wall. This scar tissue is rigid and non-elastic, progressively shrinking the diameter of the colon and creating a stricture.
Inflammatory Bowel Diseases (IBD), particularly Crohn’s Disease, are the most common inflammatory conditions leading to this outcome. Crohn’s Disease is characterized by transmural inflammation, meaning it penetrates the entire thickness of the bowel wall. This deep inflammation triggers the activation of myofibroblasts, which produce the collagen that forms dense, obstructive scar tissue. Approximately 70% of individuals with Crohn’s Disease may develop stricturing complications within ten years of diagnosis.
In contrast, Ulcerative Colitis (UC) typically involves inflammation limited to the innermost lining of the colon, the mucosa. Stricture formation is far less common in UC, affecting less than five percent of patients. When a stricture develops in a person with UC, it raises concern for a potential underlying malignancy, requiring careful medical evaluation. The resulting stricture is generally classified as either inflammatory, which may respond to medication, or fibrotic, which typically requires procedural intervention.
Localized Scarring from Acute Events
Narrowing can also result from intense, localized inflammation or injury that heals with a patch of scar tissue. This mechanism is distinct from the chronic pattern seen in IBD, as the damage is often a one-time event that leaves a residual scar. A severe episode of diverticulitis, particularly when complicated by an abscess, is a common example.
Diverticulitis involves the inflammation or infection of small, pouch-like herniations, known as diverticula, that protrude through the muscular wall of the colon. A severe inflammatory attack leads to surrounding tissue swelling and the formation of scar tissue as the body attempts to repair the damage. This process of inflammation and subsequent fibrosis can narrow the colonic lumen, often creating a single, short stricture. Diverticular disease accounts for up to ten percent of cases of large-bowel obstruction.
Other severe infectious processes can also lead to similar localized scarring and stricture formation. Certain cases of infectious colitis, such as those caused by Clostridioides difficile or other serious bacterial infections, can cause deep ulceration within the colon wall. When these deep ulcers heal, the resulting scar tissue can contract and narrow the passage. A biopsy is often required to rule out malignancy.
Obstruction from Vascular Issues and Tumors
Colonic narrowing can result from physical obstruction or tissue death due to impaired blood flow, rather than inflammation. Colorectal carcinoma (CRC) is a common cause of narrowing, especially in the left colon and rectum, where it physically blocks the channel. As a tumor grows, it occupies space within the lumen, reducing the area available for the passage of stool.
Colorectal cancer is the most common cause of large bowel obstruction in adults, accounting for over sixty percent of cases. Because the left side of the colon has a smaller diameter than the right, tumors located there can cause obstructive symptoms earlier in the disease progression. This physical narrowing can lead to changes in bowel habits, including alternating constipation and diarrhea.
Another non-inflammatory mechanism is Ischemic Colitis, which results from inadequate blood supply to a segment of the colon. When blood flow is reduced, the colon tissue suffers damage, leading to injury and necrosis. While most cases resolve completely, a severe injury affecting the full thickness of the bowel wall can heal by replacing damaged muscle with fibrous scar tissue. This fibrotic repair causes the bowel to contract and form a stricture. This occurs in approximately ten to fifteen percent of patients following an ischemic episode.
Strictures Following Medical Procedures
A final category of colonic narrowing includes strictures that arise as a complication of medical treatment or surgical intervention, known as iatrogenic causes. One common procedural cause is the formation of an anastomotic stricture, which is scarring that develops where two segments of the colon were surgically joined. This is a recognized complication following colorectal surgery, with incidence rates reported to be between three and thirty percent.
Factors that increase the risk of an anastomotic stricture include technical aspects of the surgery, such as the use of stapled connections, and postoperative complications like an anastomotic leak or pelvic infection. The resulting stricture is a band of scar tissue that contracts over time, often presenting months after the initial operation. Many of these strictures are asymptomatic, but those causing obstructive symptoms require intervention, such as endoscopic balloon dilation.
Narrowing can also occur following therapeutic radiation directed at the pelvis to treat cancers like prostate, cervical, or rectal cancer. This complication develops from six months up to several years after treatment. The radiation damages the small blood vessels supplying the colon wall, a process called obliterative endarteritis, which leads to chronic tissue ischemia and subsequent fibrosis. This chronic scarring results in a rigid, narrowed segment of the colon that can lead to persistent obstructive symptoms.