Anatomy of the Anal Region
The anal region is made up of a sophisticated muscular complex, committed to fecal continence, and of the anal canal, featuring many different cellular tissues, which a variety of neoplasms can arise from.
The anal region is anatomically complex and this causes a great deal of confusion between gastroenterologists and clinicians alike. Also endoscopists (both gastroenterologists and surgeons) have inadequate knowledge of the anatomic details of this area, which is daily explored for diagnostic assessment.
In this chapter we provide a brief introduction on the anatomy of the anal region, coupled with endoscopic pictures of the different anatomic portions of this structure.
From a practical standpoint, we distinguish the anal verge, whose inspection is directly made by the endoscopist, from the anal canal, which can be accurately evaluated with the modern video-endoscopes, provided that a correct technique is employed.
The extension of the anal canal is controversial among surgeons and anatomists. Such a controversy is not trivial for both the patient and the clinician since the correct classification of the malignancies arising in this segment strongly affect the appropriate treatment.
Anatomically, the anal canal extends from the level of the upper aspect of the pelvic diaphragm to the anus. In surgical usage, however, the anal canal is frequently limited to that part of the intestine below the pectinate line; this part differs from the part above the pectinate line in several respects, including innervation, venous and lymphatic drainage, and possibly lining epithelium (Fig. 1).
Fig. 1: the “anatomical” anal canal (blue arrow) extends from the skin of the anal verge (blue) to the dentate line (green).
Surgeons are inclined to consider the anal canal as a longer structure, extending from the anal verge to the point where the pubo-rectalis muscle is palpable, comprising the transitional epithelium (Fig. 2) or part of the columnar epithelium of the rectum.
Fig. 2: the “surgical” anal canal has a longer extension (white arrow) compared to the anatomical one, and comprises proximally also the transitional epithelium (yellow) adjoining the rectal mucosa (orange).
The sphincter complex
The sphincter complex is made up by two concentric muscular structures: the external muscle complex and the internal sphincter, among which an intermediate layer of vertical fibers is interposed, called complex longitudinal layer (Fig. 3).
The external sphincter is usually described in three parts. The subcutaneous part surrounds the lowermost portion of the canal. The superficial part, situated above the subcutaneous division, is attached to the perineal body and coccyx. The deep part, more or less continuous with the superficial division, surrounds the uppermost portion of the canal and is associated with the pubo-rectalis posteriorly.
Fig.3. The external muscle apparatus is composed by three main units: the subcutaneous, distal to the lower margin of the internal sphincter; the superficial, oval-shaped, circumferentially surrounding the whole internal sphincter; and the deep unit, that includes the pubo-rectalis sling, which in turn is part of the levator ani muscle.
The external sphincter is usually described in three parts. The subcutaneous part surrounds the lowermost portion of the canal. The superficial part, situated above the subcutaneous division, is attached to the perineal body and coccyx.
The deep part, more or less continuous with the superficial division, surrounds the uppermost portion of the canal and is associated with the pubo-rectalis posteriorly.
Macroscopic appearance of the anal canal
Despite its reduced extension (25-35 mm), the anal canal represents one of the most complex areas of the gastrointestinal tract and contains a number of different cellular lines, which can give rise to a wide spectrum of malignancies. The anal canal is separated into two parts, upper and lower (fig. 4), by an irregularly shaped line, called the dentate or pectinate line (or muco-cutaneous junction). The dentate line separates the anal canal into an upper and lower parts, not only in structure but also in neurovascular supply (reflecting the differing embryological origin).
The dentate line is formed by the anal column, which consists of a series of anal sinuses (which drain anal glands) at approximately the midpoint of the anal canal. It is a wavy demarcation formed by the anal valves (transverse folds of mucosa) at the inferior-most ends of the anal columns. Anal glands open above the anal valves into the anal crypts (fig. 5).
Fig. 4: the upper anal canal extends from the dentate line to the rectal mucosa (white arrow); the lower anal canal extends distally to the dentate line up to anal verge.
Fig. 5: the crypts of Morgagni (white arrow) are any of the pouched cavities of the rectal mucosa immediately above the ano-rectal junction, intervening between vertical folds of the rectal mucosa (blue arrow).
Blood supply and innervation
The rectum and anal canal are supplied by the superior rectal artery (the continuation of the inferior mesenteric artery), with assistance from the middle (branches of internal iliac artery) and inferior rectal arteries (branches of the internal pudendal artery), and by the median sacral artery. Arteriographic and histologic studies have shown the predominance of the superior rectal artery in the role of blood supplier and the rich anastomotic network in the hemorrhoid plexus. Vertical mucosal folds, the anal columns, are usually visible in the upper half of the canal. Anal cushions are 3 consistently placed submucosal vascular plexuses formed by anastomosis of rectal veins within anal columns. The columns are vascular, and enlargement of their venous plexus results in internal hemorrhoids (Fig. 5). The submucosal venous plexus above the pectinate line drains into the superior rectal veins (portal system), which may become varicose, resulting in internal hemorrhoids or "piles." The submucosal plexus below the pectinate line drains into the inferior rectal veins, which may become varicose, resulting in external hemorrhoids or piles. The unions of the superior with the middle and inferior rectal veins are important portal-systemic anastomoses.
Fig. 6: The internal hemorrhoids (blue arrows) are located in the upper anal canal, above the dentate line (in green). The external hemorrhoids (white arrows) are located below the dentate line, in the lower anal canal.
Somatic innervation: sensory and motor fibers to muscles and mucosa below the dentate line are provided by the inferior rectal nerve and perineal nerve, branches of the pudendal nerve (S2-S4).
Autonomic innervation: parasympathetic fibers, originating from the posterior part of the inferior hypogastric plexus, supply the smooth muscle, including the internal sphincter. Sympathetic fibers from thoraco-lumbar segments via superior hypogastric plexus and hypogastric nerves are mainly vasomotor.
The muco-cutaneous lining of the anal canal and the muscular structures of the sphincter complex circumscribe some virtual spaces (fig. 8), whose knowledge is essential to understand the site of anorectal abscesses.
Fig 7: Perianal spaces:
1) subcutaneous space;
2) Submucous space;
3) Intersphincteric space;
4) Ischiorectal space;
5) Superior pelvi-rectal space.
The subcutaneous perianal space is located below the dentate line; its borders are the skin of the anal verge below and the internal sphincter above. This space contains the external hemorrhoids.
The submucous space is located above the dentate line; it is delimited by the internal sphincter and by the anoderm. It contains the internal hemorrhoids. The intersphincteric or Eisenhammer’s space is located between the internal and external anal sphincter. In this space the deeper suppurations initially collect and originate the abscesses of the longitudinal layer. The ischio-rectal space is a large space outside the anal canal.
It is delimited externally by the perianal skin and the gluteus, internally by the external sphincter and above by the levator ani muscle and internal obturator muscle.
The two contralateral ischiorectal spaces communicate posteriorly around anorectal region via deep postanal space, located anteriorly to the ano-coccygeal ligament. The superior pelvirectal space is located above the anal canal and is delimited by the superior pelvic aponeurosis below, by the peritoneum above and by the rectum internally.
Anatomy of the anal canal
Video-endoscopy is not the diagnostic procedure of choice to study the anal canal; the presence of the sphincter sleeve wrapping up around the anal canal prevents adequate distension of this segment to correctly visualize anatomic structures. The use of a transparent cap on tip of the scope facilitates the endoscopic assessment of the anal canal, but it is not routinely used in colonoscopy practice (fig. 8).
Fig. 9: A cap on the tip of the scope facilitates the visualization of the anal canal, without requiring excessive insufflation.
More detailed information can generally be derived performing a rectal retro-vision maneuver, with an insufflation adequate for visualizing the anal canal (Fig. 9 A-B).
Fig.9 A – Endoscopic view of the rectum through the anal canal. On the dentate line a hypertrophic papilla can be seen (blue arrow). The white arrow points to the first Houston’s valve.
Fig. 10: Retrovision of the anal canal: red arrows indicate the crypts of Morgagni and blue arrows the intervalvular commissures (see also fig 5).
Fig. 12 A-B: Direct vision (A) and retrovision (B) of the anal canal: extension of the transitional epithelium; the arrows indicate the border with the rectal mucosa.