A colonoscopy is a procedure for a visual inspection of the large intestine’s inner lining. It uses a long, flexible instrument called a colonoscope, which has a camera on its tip to transmit images. The purpose is to examine the colorectal anatomy for any signs of abnormality, following a specific path through the various segments of the colon.
The Structure of the Large Intestine
The large intestine, or colon, is a continuous tube approximately five feet long that begins at the end of the small intestine and extends to the anus. During a colonoscopy, the instrument is first inserted into the rectum, the final section of the large intestine which is about 15 cm in length. The rectum acts as a temporary holding chamber for stool and its relatively straight structure serves as the entry point for the procedure.
From the rectum, the scope advances into the sigmoid colon, an S-shaped segment that connects to the descending colon. The sigmoid colon is characterized by its significant curves, which can make navigation a technical challenge. Following this section, the scope moves into the descending colon, which travels up the left side of the abdomen and is straighter than the sigmoid.
The scope must then pass through the splenic flexure, a sharp turn where the descending colon meets the transverse colon. After this bend, the colonoscope is advanced through the transverse colon, which drapes across the abdomen. The next turn is the hepatic flexure, which marks the transition from the transverse colon to the ascending colon.
The journey proceeds to the ascending colon, which travels up the right side of the abdomen. The colonoscope’s journey inward concludes at the cecum, a pouch-like structure that marks the beginning of the large intestine. Reaching the cecum is the indicator that a complete examination of the colon can be performed.
A thorough colonoscopy involves advancing the scope to the ileocecal valve, the sphincter muscle separating the large and small intestines. In many cases, the endoscopist will guide the scope through this valve to enter the final few centimeters of the small intestine, an area known as the terminal ileum. This is done to check for inflammation where conditions like Crohn’s disease can manifest.
Anatomical Features and Abnormalities
A primary focus of a colonoscopy is the detection of abnormalities on the inner lining of the colon. A healthy colon has a smooth, pinkish appearance, but several conditions can be identified during the exam:
- Polyps are abnormal growths of tissue that can vary in shape and size, appearing as small, flat bumps (sessile) or growing on a stalk (pedunculated). While most polyps are benign, some can develop into cancer over time, which is why their removal is a preventative measure.
- Diverticulosis is the presence of small pouches, or diverticula, that bulge outward through weak spots in the colon wall, most often found in the sigmoid colon. While the pouches themselves are not harmful, they can become inflamed or infected, a condition known as diverticulitis.
- The lining is also inspected for signs of inflammation. In conditions like ulcerative colitis or Crohn’s disease, the lining may appear red, swollen, and fragile, and may show ulcers. Biopsies can be taken from these areas for a definitive diagnosis.
- Endoscopists also look for any neoplasms or masses, which appear as larger, irregularly shaped growths more concerning for cancer than a typical polyp. If such a lesion is found, biopsies are taken to determine if it is malignant.
The Withdrawal Examination
While advancing the colonoscope to the cecum is necessary, the most detailed inspection of the colon’s anatomy occurs during the withdrawal of the instrument. This phase is performed slowly and methodically, as the primary examination of the mucosal surface is conducted as the scope is pulled back. The goal is to inspect every fold and crevice of the colon wall for any abnormalities.
The endoscopist uses specific techniques to ensure a thorough view, such as rotating the scope to see behind the colonic folds, known as haustra. Air or carbon dioxide is used to keep the colon gently inflated, smoothing out the lining for better visualization. This process allows for the detection of subtle, flat polyps or other small lesions.
Quality standards in endoscopy suggest that the withdrawal time should be at least six minutes. This benchmark helps ensure the physician takes adequate time to perform a comprehensive visual scan. A longer withdrawal time is associated with a higher rate of polyp detection.