Normal Rectum

The rectum is the most distal colon segment, extending from the anal canal to the rectosigmoid junction. The dentate line marks the boundary between the anal canal and the rectum, and is characterized by a mixture of rectal mucosa (columnar epithelium) and sensitive anal skin (squamous epithelium). The fingerlike proximal ends of the squamous epithelium are somewhat thickened, forming the anal papilla. At the pouchlike distal ends of the columnar epithelium, also fingerlike in form, are the anal crypts, at the base of which are the anal glands. There is no distinct anatomical demarcation between the rectum and the sigmoid colon. As a rule, the boundary between the rectum and the sigmoid colon is considered to be at a height of 16 cm proximal to the anocu-taneous line (measured using a stiff rectoscope), at which point the colon usually angles sharply toward the sigmoid colon. This demarcation is significant for treating tumors: distal to this line, rectal carcinomas in certain stages are treated differently than colon carcinomas located proximal to the line.

The rectal segment between these two demarcations runs fairly straight (hence "rectum") and typically has three large folds protruding into the lumen from the sides, the so-called Kohlrausch folds (Fig. 6.1). The distal segment of the rectum is wider than the proximal segment, forming the rectal ampulla. The proximal rectum is located intraperitoneally, while the distal segment becomes retroperitoneal in the abdominal cavity. The peritoneal reflection, about 7-8 cm from the anal margin, is deeper on the ventral side near the pouch of Douglas than on the dorsal side. This is important for transmural injuries of the rectum.

As in the rest of the colon, the rectal mucosa is smooth and reflective; the blood vessels are clearly visible and easily distinguished from the surrounding surface. As a rule, the vessels in the rectum are more prominent than in the rest of the colon. This is especially true of the venous vessels connected with the vessel branches in the region of the anal canal (hemorrhoidal plexus). The vessels range from pronounced, but still normal, submucosal veins, to pathologically widened veins (e.g., in the case of pronounced hemorrhoidal disease), to rectal varices (e.g., related to portal hypertension, 0 6.1).

Tips for examining the rectum

► When examining the proximal rectum it is important to pay attention to the areas behind the prominent rectal folds: these should be carefully inspected to avoid missing pathologies such as polyps. Sufficient air insufflation is necessary to flatten the rectal folds. This can often be difficult with patients who have a weak anal sphincter tonus and for whom the insufflation of air into the rectum may cause discomfort and the urge to evacuate the bowels. Thus, after inspecting the rectum, excess air should be suctioned.

► A common mistake toward the end of colonoscopy is withdrawing the instrument too rapidly from the distal rectum, thereby neglecting a thorough inspection of the anorectal area. Retroflexion of the instrument can enable inspection of this area and usually the dentate line (Figs. 6.2, 6.5), and can be performed on almost any patient. Chapter 5 describes the technique in detail.

Normal Rectum

|T| 6.1 Normal vascular patterns in the distal rectum

Vascular Pattern Retum Colonoscopy

a Fine, but clearly visible vascular pattern contrasting sharply with smooth, reflective mucosa. b More pronounced venous blood vessels, somewhat prominent.

b d a Fine, but clearly visible vascular pattern contrasting sharply with smooth, reflective mucosa. b More pronounced venous blood vessels, somewhat prominent.

c Spidery, prominent venous blood vessel with clearly visible vessel branches supplying it.

d Very prominent venous blood vessels, borderline pathology, in a patient with inner hemorrhoids.

Rectal Folds

Fig. 6.1 Rectum with crescent-shaped, infolding rectal valves. Despite the straight path of the rectum, the folds obscure visualization. Note the distinct and pronounced vascular pattern with the typically clearly visible submucosal veins.

Rectal Valves Picture

Fig. 6.2 Anorectal area and upper anal canal, with forward-viewing instrument.

Contractions of the sphincter obscure visualization with a forward-viewing instrument, especially of the surrounding dentate line, which is barely visible.

Fig. 6.1 Rectum with crescent-shaped, infolding rectal valves. Despite the straight path of the rectum, the folds obscure visualization. Note the distinct and pronounced vascular pattern with the typically clearly visible submucosal veins.

Fig. 6.2 Anorectal area and upper anal canal, with forward-viewing instrument.

Contractions of the sphincter obscure visualization with a forward-viewing instrument, especially of the surrounding dentate line, which is barely visible.

Rectal Ampulla Retroflex Endoscopy

Fig. 6.3 Anorectal area and upper anal canal viewed with a retroflexed endoscope. Retro-flexing the endoscope allows a better view of the dentate line; the transition between the pale squamous epithelium of the anal canal and the reddish columnar epithelium of the rectum is visible. The anal crypts and the slightly thickened anal papilla are also clearly visible (e.g., at about the 9-o'clock position).

Fig. 6.3 Anorectal area and upper anal canal viewed with a retroflexed endoscope. Retro-flexing the endoscope allows a better view of the dentate line; the transition between the pale squamous epithelium of the anal canal and the reddish columnar epithelium of the rectum is visible. The anal crypts and the slightly thickened anal papilla are also clearly visible (e.g., at about the 9-o'clock position).

Anal Crypt

Fig. 6.4 Close-up view of the dentate line in inversion. The intertwining, fingerlike squamous epithelium and columnar epithelium are clearly visible. Two anal crypts can be seen at about the 2-o'clock and 3-o'clock positions and at about the 12-o'clock position (partially obscured by the instrument) a hy-pertrophied anal papilla.

Fig. 6.4 Close-up view of the dentate line in inversion. The intertwining, fingerlike squamous epithelium and columnar epithelium are clearly visible. Two anal crypts can be seen at about the 2-o'clock and 3-o'clock positions and at about the 12-o'clock position (partially obscured by the instrument) a hy-pertrophied anal papilla.

Normal Rectum

Fig. 6.5 Normal hemorrhoidal plexus viewed in retroflexion. The hemorrhoidal cushions, which help seal the anus, appear as bluish swellings. Vascular branches can be seen coming from the distal rectum.

Fig. 6.5 Normal hemorrhoidal plexus viewed in retroflexion. The hemorrhoidal cushions, which help seal the anus, appear as bluish swellings. Vascular branches can be seen coming from the distal rectum.

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