Endoscopy 101 (Colonoscopy, Flex Sig)

Endoscopy (colonoscopy and flexible sigmoidoscopy) lets a gastroenterologist see the lining of the large intestine directly, take biopsies, and sometimes treat problems during the same procedure. For people with Crohn’s disease or ulcerative colitis, scope findings are central to diagnosis, measuring how active inflammation is, checking whether medicines are working, and reducing long‑term cancer risk through regular surveillance.

Key takeaways

• Colonoscopy examines the entire colon and often the last part of the small intestine; flexible sigmoidoscopy looks only at the rectum and lower left colon. (asge.org)

• Good bowel preparation and following the diet and medication instructions are critical so the doctor can see the bowel clearly. (asge.org)

• During a scope, doctors can take biopsies, grade inflammation, remove polyps, and treat bleeding, usually with minimal discomfort under sedation. (asge.org)

• Scope results help confirm Crohn’s vs ulcerative colitis, decide how aggressive treatment should be, and guide “treat‑to‑target” goals such as endoscopic healing. (pubmed.ncbi.nlm.nih.gov)

• Long‑standing colonic IBD often needs regular surveillance colonoscopies (about every 1–5 years) to detect precancerous changes early. (gastro.org)

What colonoscopy and flexible sigmoidoscopy are

Colonoscopy

A colonoscopy uses a long, thin, flexible tube with a camera and light at the tip. The tube is passed through the anus and advanced through the rectum and entire colon, and often into the end of the small intestine (terminal ileum). (asge.org)

It is used to:

• Help diagnose IBD and tell Crohn’s disease from ulcerative colitis

• Assess how far inflammation extends and how severe it is

• Remove polyps and sample tissue (biopsy)

• Screen for and monitor colorectal cancer risk

Most colonoscopies are outpatient, last around 30–45 minutes, and are performed with sedation so patients are relaxed or fully asleep. (asge.org)

Flexible sigmoidoscopy

A flexible sigmoidoscopy (often called “flex sig”) uses a shorter scope to look only at the rectum and the lower left part of the colon (descending and sigmoid colon). (mayoclinic.org)

It:

• Involves a smaller bowel prep

• Often uses little or no sedation

• Usually takes about 15–20 minutes (mayoclinic.org)

In IBD, a flex sig is commonly used to:

• Quickly assess disease activity in the rectum and left colon

• Check response to treatment in ulcerative colitis

• Monitor severe colitis in hospital without a full colonoscopy

Key differences

Why scopes matter in IBD

Endoscopy is a core part of IBD care because it shows the bowel lining directly and allows biopsies from multiple areas.

Scopes help:

• Confirm diagnosis and type of IBD. Ulcerative colitis usually appears as continuous inflammation starting in the rectum, while Crohn’s disease tends to cause patchy inflammation, ulcers, or narrowing, sometimes in the small intestine.

• Define extent and severity. The length of colon involved and how deep or severe the ulcers are influence medication choices and future cancer risk.

• Set a baseline and track healing. Modern “treat‑to‑target” strategies use both symptom control and endoscopic healing as long‑term goals. (pubmed.ncbi.nlm.nih.gov)

• Detect dysplasia and cancer early. For people with long‑standing colonic IBD, colonoscopy can find precancerous changes so they can be removed or treated before cancer develops. (pubmed.ncbi.nlm.nih.gov)

Getting ready: before the procedure

Medication and health review

Before scheduling, the gastroenterologist or nurse will review:

• Current medicines, especially blood thinners, diabetes medicines, and iron

• Allergies to medicines or latex

• Other conditions such as heart, lung, kidney, or liver disease (asge.org)

Some medicines may need temporary adjustment. This is individualized and should be guided by the prescribing clinicians.

Diet and bowel prep

For colonoscopy, most patients:

• Follow a low‑fiber diet for several days

• Switch to a clear liquid diet the day before

• Take a prescribed bowel preparation solution in one or two doses to clean out the colon (asge.org)

For flex sigmoidoscopy, prep may be lighter, sometimes using enemas instead of a full oral prep. (mayoclinic.org)

A clean bowel is essential. Residual stool can hide inflammation or polyps and may require repeating the test.

Practical planning

Patients usually:

• Arrange transport home (sedation rules out driving)

• Plan to be at the facility for several hours, including recovery

• Bring a current medication list and key medical history

What happens on the day

Before the scope

At the endoscopy unit:

1. Staff confirm medical history and the last bowel prep dose.

2. An IV line is placed for sedation medicines.

3. The gastroenterologist explains the procedure and answers questions, then obtains consent.

Patients typically lie on the left side with knees slightly bent.

During colonoscopy

• Sedation or anesthesia is given so the person feels drowsy or sleeps comfortably. (asge.org)

• The scope is advanced slowly through the rectum, colon, and often into the terminal ileum.

• Air or carbon dioxide is introduced to open the colon; this can cause temporary pressure or cramping. (mayoclinic.org)

• The doctor inspects the lining, takes biopsies, and may remove polyps or treat bleeding if present.

During flexible sigmoidoscopy

For flex sig, the same basic steps occur, but the scope only travels through the rectum and left colon. Sedation may be lighter or not used, and the procedure is shorter.

Most people recall little of the procedure when sedation is used.

After the scope: recovery and risks

Normal recovery

In recovery, staff monitor vital signs until sedation wears off. Common, short‑lived effects include:

• Bloating or gas

• Mild cramping

• A small amount of blood with the first bowel movement if biopsies were taken (mayoclinic.org)

Most people return to normal eating the same day and routine activities by the next day, unless instructed otherwise.

Warning signs

Everyone should be told which symptoms need urgent medical review, such as:

• Heavy or persistent rectal bleeding or blood clots

• Severe or worsening abdominal pain or bloating

• Fever or chills

These can rarely signal complications like perforation or significant bleeding.

How common are complications?

Colonoscopy and flex sigmoidoscopy are considered very safe. Serious problems such as bleeding and perforation occur in only a small fraction of procedures, on the order of tens of cases per 100,000 colonoscopies in screening settings. (pubmed.ncbi.nlm.nih.gov)

The risk is somewhat higher when large polyps are removed, in older patients, or in people with other serious medical conditions.

How scope findings guide treatment

Diagnosing Crohn’s disease vs ulcerative colitis

Scope appearance and biopsies help distinguish:

• Ulcerative colitis: continuous inflammation starting at the rectum, limited to the colon

• Crohn’s disease: patchy lesions, possibly deep ulcers, strictures, fistulas, or small‑bowel involvement

This distinction affects medication choices, use of rectal therapies, and surgical planning.

Grading inflammation and healing

Endoscopists often use scoring systems that grade:

• How red, fragile, or swollen the lining is

• Presence and depth of ulcers

• How much of the bowel is involved

In ulcerative colitis, lower endoscopic scores and a normal‑looking lining correlate with better long‑term outcomes, so many treatment plans now aim for endoscopic remission, not just symptom relief. (businessdevelopment.mayoclinic.org)

Matching treatment to what the scope shows

Scope and biopsy findings help clinicians decide whether to:

• Use topical therapies (suppositories, foams, enemas) for disease limited to the rectum or left colon

• Add or escalate oral 5‑ASA, corticosteroids, immunomodulators, biologics, JAK inhibitors, or S1P modulators for more extensive or severe inflammation

• Investigate and manage strictures, fistulas, or abscesses, sometimes with surgery or interventional endoscopy

• Continue, de‑escalate, or change therapy if inflammation has healed or remains active despite treatment

Cancer surveillance planning

For patients with colonic Crohn’s disease or ulcerative colitis of more than 8–10 years, guidelines recommend regular surveillance colonoscopy every 1–5 years based on risk factors such as disease extent, family history of colorectal cancer, and co‑existing primary sclerosing cholangitis. (pubmed.ncbi.nlm.nih.gov)

Findings such as dysplasia (precancerous changes) can lead to closer follow‑up, removal of visible lesions, or in some cases discussion of colectomy.

FAQs

Why might a doctor choose flex sig instead of full colonoscopy?

Flexible sigmoidoscopy is quicker, often needs less prep and sedation, and still provides useful information about rectal and left‑sided disease, especially in ulcerative colitis. It is commonly used for rapid assessment of flares or to follow response to treatment when the main concern is the lower colon.

How often do people with IBD need colonoscopy?

Frequency depends on disease type, extent, duration, symptoms, and cancer risk factors. Many people have a full colonoscopy at diagnosis, at key treatment decision points, and then every 1–5 years once disease has been present for 8–10 years and involves the colon, mainly for cancer surveillance.

Do biopsies mean the doctor suspects cancer?

Not necessarily. In IBD, biopsies are taken routinely from many segments to confirm diagnosis, grade inflammation, look for chronic changes, and check for microscopic dysplasia. Most biopsies in IBD do not show cancer, but they provide important information that shapes ongoing care.