A flexible sigmoidoscopy is a procedure that lets a doctor examine the lower portion of your large intestine using a thin, flexible tube with a camera on its tip. The tube is inserted through the anus and guided through the rectum, the sigmoid colon (the S-shaped curve near the bottom of your colon), and most of the descending colon on the left side of your body. The whole thing typically takes 10 to 20 minutes.
What It’s Used For
Doctors use flexible sigmoidoscopy for two main purposes: screening for colorectal cancer and investigating symptoms. As a screening tool, the U.S. Preventive Services Task Force recommends it every 5 years starting at age 45, or every 10 years when combined with an annual stool test. It catches polyps and early cancers in the lower third of the colon, where a significant share of colorectal problems develop.
As a diagnostic tool, your doctor might recommend it if you’re experiencing rectal bleeding, unexplained changes in bowel habits, chronic diarrhea, or lower abdominal pain. If the doctor spots polyps during the exam, they can remove them on the spot and send the tissue to a lab for testing.
How It Differs From a Colonoscopy
The biggest difference is reach. A colonoscopy examines the entire colon, while a sigmoidoscopy covers only the lower portion. Because of this shorter scope, the preparation is lighter and sedation is often unnecessary. Many people have a sigmoidoscopy fully awake, with only mild discomfort from air used to inflate the colon for better visibility. A colonoscopy, by contrast, almost always involves sedation and a full-day bowel prep.
The tradeoff is obvious: a sigmoidoscopy can miss problems in the upper parts of the colon. If polyps or other concerning findings turn up during the procedure, your doctor will likely recommend a full colonoscopy as a follow-up.
How to Prepare
Preparation for a sigmoidoscopy is simpler than for a colonoscopy, but you’ll still need to clean out the lower part of your colon so the doctor has a clear view. Typical preparation involves a few steps spread across the week before:
- About a week before: Stop eating corn, popcorn, seeds, and nuts, which can linger in the colon and obstruct the view.
- The day before: Switch to a clear liquid diet for part of the day. You may also take a mild laxative.
- The morning of: Administer one or two enemas at home, at least an hour before you leave for your appointment. Don’t eat solid food after midnight the night before, though small sips of clear liquids are fine up to about four hours before the procedure.
Your specific instructions may vary, so follow whatever your doctor’s office provides.
What Happens During the Procedure
You’ll lie on your left side on an exam table with your knees drawn toward your chest. The doctor inserts the scope through your anus and slowly advances it through the rectum and into the sigmoid and descending colon. The scope pumps a small amount of air into the colon to open it up, which can cause a feeling of pressure, bloating, or mild cramping.
The camera transmits a live image to a screen, letting the doctor examine the lining of the colon in real time. If a polyp is found, the doctor can snip it out through the scope using a small wire loop. Tissue samples can also be taken for biopsy. You won’t feel the polyp removal itself, since the colon lining doesn’t have the same pain-sensing nerves as your skin.
Safety and Risks
Flexible sigmoidoscopy is one of the lowest-risk procedures in gastroenterology. A large systematic review published in PLOS ONE found that the risk of bleeding was roughly 8 per 100,000 people screened, far lower than the rates seen with colonoscopy. The risk of perforation (a small tear in the colon wall) was approximately 2 per 100,000, compared to 53 to 88 per 100,000 for various types of colonoscopy screening.
Minor side effects are common and short-lived. You may feel gassy, bloated, or have mild cramping for a few hours after the exam as the air works its way out. These symptoms resolve on their own.
Recovery and Results
If you didn’t receive sedation, you can drive yourself home and return to normal activities right away. Most people feel back to normal within an hour or two once the bloating subsides. If sedation was used, you’ll need someone to drive you and should plan to rest for the remainder of the day.
Your doctor can often share preliminary findings immediately after the procedure, since they saw the colon lining in real time. If polyps were removed or biopsies were taken, pathology results typically come back within a few days to two weeks. A normal result with no polyps means you can wait five years before your next sigmoidoscopy, or longer if it’s paired with annual stool testing.