Sigmoid colitis is inflammation of the sigmoid colon, the S-shaped section of your large intestine that sits in the lower left part of your abdomen, just before the rectum. It’s one of the most common locations for colitis because this segment is vulnerable to several types of injury, from autoimmune disease to reduced blood flow. The sigmoid colon is roughly 25 to 40 cm long, and its main job is absorbing the last bits of water and minerals from digested food before compacting stool and pushing it into the rectum for elimination.
Why the Sigmoid Colon Gets Inflamed
Sigmoid colitis isn’t a single disease. It’s a description of where inflammation is happening, and the underlying cause can vary widely. The most common culprits fall into a few categories.
Ulcerative colitis (UC) is the most well-known cause. UC always starts in the rectum and can spread upward. When it reaches the sigmoid colon but goes no further, it’s called proctosigmoiditis. When it extends past the sigmoid into the descending colon, it’s classified as left-sided colitis. Both patterns produce chronic, relapsing inflammation that requires ongoing management.
Ischemic colitis happens when blood flow to the sigmoid colon drops below what the tissue needs to stay healthy. This causes the lining to break down, leading to ulceration and bleeding. Risk factors include high blood pressure, diabetes, heart disease, high cholesterol, and episodes of low blood pressure during or after surgery. The sigmoid colon is particularly susceptible to ischemia because of its position at a vascular “watershed” zone, where the blood supply from two different arterial systems meets.
Infectious colitis can be caused by bacteria like C. difficile, Salmonella, Shigella, E. coli, and Campylobacter, as well as parasites and certain viruses. These infections can inflame any part of the colon but frequently affect the sigmoid and rectum. C. difficile, which often follows antibiotic use, can produce a particularly aggressive form called pseudomembranous colitis.
Less common causes include radiation exposure (typically after pelvic cancer treatment), medication side effects, and microscopic colitis, a condition where the colon looks normal on camera but biopsies reveal inflammation at the cellular level.
Symptoms of Sigmoid Colitis
Because the sigmoid colon sits low in the digestive tract, close to the rectum, its symptoms tend to be distinct from inflammation higher up in the colon. The hallmark signs include cramping or pain in the lower left abdomen, bloody diarrhea, and tenesmus, which is the persistent feeling that you need to have a bowel movement even when your rectum is empty. Tenesmus can be one of the most disruptive symptoms, keeping you near a bathroom even when there’s little stool to pass.
Mucus in the stool is common. Some people also experience urgency so intense that accidents become a real concern. In milder cases, symptoms may be limited to loose stools and occasional cramping. Severe disease, defined as more than six bloody stools per day along with fever, rapid heart rate, or anemia, signals that inflammation has become systemic and needs aggressive treatment. Fulminant disease, the most extreme form, involves more than ten bloody bowel movements a day with signs like abdominal distention and the need for blood transfusions.
How It’s Diagnosed
A colonoscopy or flexible sigmoidoscopy is the primary way doctors confirm sigmoid colitis. During the procedure, the doctor can see the inflamed, reddened, or ulcerated lining of the sigmoid colon directly and take tissue samples. Those biopsies are essential because the visual appearance alone can’t always distinguish between causes. For instance, ulcerative colitis and infectious colitis can look similar through the camera, but under a microscope the patterns of immune cell activity are different.
One marker pathologists look for is the presence of certain immune cells called Paneth cells in the left colon. These cells are normal in the small intestine and right colon, but when they show up in the sigmoid, they signal chronic, ongoing inflammation. In microscopic colitis, biopsies may reveal a thickened collagen band beneath the surface lining or an abnormally high number of immune cells infiltrating the tissue, even though the colon appeared normal during the scope.
Blood tests and stool samples help narrow down the cause before or alongside endoscopy. Stool cultures can identify bacterial or parasitic infections, and inflammatory markers in blood work give a sense of how active the inflammation is.
Treatment Approaches
Treatment depends entirely on the underlying cause. Infectious sigmoid colitis typically resolves with antibiotics or antiparasitic medications. Ischemic colitis often improves on its own with supportive care, though severe cases may need surgery. For ulcerative colitis affecting the sigmoid, the approach is more layered.
Topical Therapies for UC-Related Sigmoid Colitis
Because the sigmoid colon is close to the end of the digestive tract, medications can be delivered directly to the inflamed tissue through enemas or suppositories. This targeted approach is a major advantage. Anti-inflammatory enemas achieve remission in roughly 49% to 58% of patients with mild to moderate left-sided disease within three weeks, compared to just 10% to 18% with placebo. For maintenance, these enemas keep the relapse rate remarkably low, between 18% and 25% over a full year of treatment.
Suppositories work especially well when inflammation is limited to the rectum and lower sigmoid, achieving remission in about 80% of patients. Topical therapy also outperforms oral medication alone for preventing relapses. Combining topical and oral anti-inflammatory treatment together reduces the risk of failing to reach remission by about 35% compared to oral therapy alone, making the combination a common recommendation for distal UC.
Managing Flares With Diet
During an active flare, a low-fiber diet (no more than 10 grams of fiber per day) can reduce the frequency and volume of stools, easing the burden on an inflamed sigmoid colon. This means temporarily cutting back on raw vegetables, whole grains, nuts, and seeds. The goal is to give the colon less physical work to do while it heals. Once the flare subsides, gradually increasing fiber intake back to 20 to 30 grams per day is generally recommended, as fiber supports long-term colon health.
Potential Complications
Left untreated or poorly controlled, sigmoid colitis can lead to serious problems. Repeated bouts of inflammation cause scar tissue to build up, and the sigmoid colon is the most common site for strictures, which are narrowed segments that can partially or fully block stool from passing through. Obstruction from a sigmoid stricture is a surgical emergency.
Toxic megacolon is a rare but life-threatening complication where inflammation penetrates deep into the muscle wall of the colon, destroying the nerve cells that control movement. The colon stops contracting and begins to balloon outward. This can happen in severe ulcerative colitis or untreated C. difficile infection. Other potential complications include perforation (a hole through the colon wall), abscesses in the tissue surrounding the colon, and fistulas, which are abnormal tunnels that form between the colon and nearby organs like the bladder or skin.
The risk of these complications is one reason that even mild sigmoid colitis warrants proper diagnosis and follow-up. Identifying the specific cause and maintaining the right treatment plan dramatically reduces the chance that inflammation will progress to something more dangerous.