Sigmoidoscopy and Colonoscopy

Both a colonoscopy and a sigmoidoscopy involve a long, narrow tube with a small camera that is inserted into the patient’s rectum to view the inside of the colon or the lower, sigmoid, colon. A colonoscopy or sigmoidoscopy may be used to help diagnose irritable bowel syndrome (IBS).1
When a patient complains of symptoms such as diarrhea, constipation, bloating, abdominal pain and gas, a physician may order a colonoscopy or sigmoidoscopy to determine if the cause is IBS or another condition or abnormality. Colonoscopy is often used in the cases of:

  • Changes in bowel activity, such as constipation or diarrhea
  • Abdominal pain
  • Bleeding from the anus
  • Blood in the stool, or black, tarry stools
  • Unexplained weight loss
  • Abnormal findings from a lower GI series or barium enema
  • Screening for colorectal cancer2,3

The anatomy of the colon

The large intestine, or colon, is approximately five feet long in adults and is the last section of the digestive system. The colon absorbs liquid from waste materials to form stool and is considered to have four main parts:

  • Ascending colon – this is the first part of the colon which attaches to the small intestine and extends upwards on the right side of the body
  • Transverse colon – this part extends from the ascending colon horizontally across the body to the left side
  • Descending colon – this section extends from the transverse colon downward on the left side of the body
  • Sigmoid colon – this section is S-shaped and connects the descending colon with the rectum, which is connected to the anus2,5

The differences: sigmoidoscopy vs colonoscopy

A sigmoidoscopy is an exam that focuses on the sigmoid part of the colon, the last section. During a colonoscopy, the entire length of the colon is screened for abnormalities, and a longer tube, or scope, is used.
A study by the National Institutes of Health determined that flexible sigmoidoscopy is less invasive and has fewer side effects than colonoscopy. It requires less bowel preparation and poses a lower risk of bowel performation, which is an uncommon event but a minor risk with either of these procedures. The study also confirmed that flexible sigmoidoscopy is effective in colorectal cancer screening, reducing deaths and development of colon cancer.4

What to Expect with a Sigmoidoscopy vs Colonoscopy

The colon must be cleansed prior to the procedure. The cleansing process generally requires an enema or laxatives, a clear liquid diet, and refraining from ingesting any food eight hours before the procedure. Additional preparation instructions are given to the patient from their health care professional and may include refraining from taking certain medications prior to the exam.
During the exam itself, medications are usually given intravenously (IV) to sedate the patient. The colonoscope or sigmoidoscope will be placed into the anus, and air is delivered through the scope into the colon to enhance viewing. A biopsy (tissue sample) or polyps (growth on the lining of the colon or rectum) may be removed, and photos using the scope may be taken of the inside of the colon.3,5

Additional tests for the diagnosis of IBS

A diagnosis of IBS is usually done through a variety of tests, which assist the physician in determining if the patient’s symptoms are caused by IBS or another condition. Other tests that may be used include a physical exam with a symptom history, stool analysis, blood tests, breath tests, and a lower GI series.

Written by: Emily Downward | Last reviewed: June 2016.
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