The Anal Canal

This section of the terminal part of the hindgut remains difficult to define; it means different things to different people. To anatomists, the canal has clearly defined borders: from the anal valve line (pectinate line) to the anus. To the surgeon, however, its upper margin is determined by puborectalis contraction - the upper border of the anorectal ring. Seen from a logical point of view, the intrinsic structures of the rectal wall stretch nearly to the skin, the anal canal being that part of it held within the external anal sphincter. This implies that the anal canal is a modified part of the rectum [107]. New tissue introduced from surface structures is only encountered within its wall at and distal to the pectinate (valve) line.

In ARM the anal canal may be either agenetic, where the terminal hindgut is not surrounded by a well-formed external anal sphincter, or dysgenetic, where the terminal hindgut is incompletely formed but surrounded by an external sphincter [30,113].

5.3.1 The Anal Canal in ARM

The following analysis is based on the assumption that the following anatomical considerations are correct:

1. The anal canal stretches from the anorectal ring to the anus; it is defined and its length is determined by the presence of the striated muscle sphincter that surrounds it.

2. That the canal is divided into two component parts: an upper canal formed intrinsically from structures of rectal origin (embryologically of endodermal origin) and a lower canal composed of tissue of somatic (surface) origin (embryologi-cally of ectodermal origin). This is not absolute as endodermally derived structures of rectal origin are contained within its wall. The site of anatomical division between the parts of the canal is the pectinate or anal valve line.

3. Congenital malformations involving the canal are classified as: (1) rectal malformations, where the upper canal and rectum are primarily involved, and anal malformations, where the lower canal, external genital, and cutaneous structures are primarily involved.

4. In rectal malformations, the malformed endoder-mally derived tissues retain the anatomical features of the parent structures to variable degrees [60]. In anal malformations, the ectodermally derived anatomical components do not retain these features.

Given these assumptions, certain conclusions can be drawn. These are now discussed.

5.3.1.1 Agenesis of the Anal Canal

In agenesis of the anal canal, the hindgut terminates above the anorectal ring or at the pelvic floor with a noncommunicative (blind) pouch or a pouch that communicates by fistula with an epithelial surface. The malformation involves the rectum and the upper anal canal. It is therefore a rectal anomaly. Where a fistula is present, it retains, as predicted, anatomical features of its parent structure the rectum. Because of this the fistula is correctly called a terminal channel: a rectal terminal channel in this instance. Indeed, any fistula with its origin in the rectum or upper anal canal should be given this name. This is of importance anatomically as the fistulae that arise in the lower anal canal differ. This terminology is used as follows: (1) the Fistula's site of origin is the rectum/upper anal canal, (2) Its endpoint is the bulbar urethra, (3) the name used is anal canal agenesis with terminal recto-bulbar urethral channel. When this terminal channel traverses, or moves through, the muscular structures of the pelvic floor, it picks up (gains) a coating of striated muscle fibers, which are likened to a forme fruste of an external anal sphincter. Because of this, from a functional point of view, under physiological circumstances, it can act as an anal canal. It is either: (1) a pressure-determined incontinent (refluxing) channel, or (2) a stress-controlled continent channel. The factor that determines this difference is the length of the translevator channal, which is an obvious variable.

5.3.1.2 Dysgenesis of the Anal Canal

In dysgenesis of the anal canal, the hindgut terminates beneath the anorectal ring, at or below the pectinate or anal valve line, as a noncommunicative (blind) pouch, or the pouch communicates by fistula with an epithelial surface. Where the fistula is present it retains none of the features of its parent structure, the lower anal canal, as this structure is anatomically malformed. Because of this the fistula is correctly called a terminal tract. This terminology is used as follows: (1) the Fistula's site of origin is the lower anal canal, (2) Its endpoint is the genital or perineal skin, (3) the name used is anal canal dysgenesis with terminal anocutaneous tract. The anatomical distinctions to be emphasized here are that the tract does not have the mural features of the rectal or normal anal wall and that it loses (sheds) surrounding striated muscle fibers. Functionally, it is not a sphincter-controlled communication.

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