Normal anorectal musculatures and changes in anorectal malformation

We investigated the anorectal musclulature in normal children and anorectal malformations (ARM) to evaluate its role in bowel control mechanism.

Pelves of 50 neonates died of ARM-unrelated diseases and 16 patients with anorectal malformations (8 high, 5 intermediate, and 3 low ARMs) were dissected and analyzed.

Normal anorectal musculature was divided into three muscular tubes: the internal sphincter tube (IAST), longitudinal muscle tube (LMT) and transverse muscle tube (TMT). The LMT came from the outer longitudinal smooth muscle fiber of the rectum and the striated muscle fiber of the levator ani, and the TMT composed of the puborectalis and the external anal sphincter. However, in ARM, the IAST was absent and the LMT, the center of the sphincter muscle complex, was only from the levator ani and could be divided into the pelvic portion and the perineal portion. The former, from the upper rim of the puborectalis to the bulbar urethral, became narrowed and dislocated anteriorly near to the posterior urethra in high ARM and rectal pouch in intermediate ARM. The latter, below the bulbar urethra to the anal dimple, was fused to a column both in high and intermediate ARM. The columnar perineal LMT run downwards and then split, penetrated the superficial part of EAS and terminated at the deep aspect of the skin, to form the anal dimple, which represents the center of the perineal LMT from the perineal aspect. The length of the LMT was longer in high and intermediate ARM than the normal neonate. The columnar perineal LMT and narrowed pelvic LMT could be possibly identified by laparoscopic and perineal approaches retrospectively and widened to allow the passage of the rectum through.

The anorectal musculature in ARM is composed of agenesic LMT and TMT and the narrowed LMT gives anatomical evidence of the center, where the neorectum should pull through.