Anatomic plane of separation between external anal sphincter and puborectalis muscle: clinical implications

The possible existence of an anatomic and functional separation between the external sphincter and the puborectalis muscle has been reported in the medical literature. In this article we confirm, by means of anatomic and clinical observations, the presence of such a separation, focusing on its importance in understanding the pathway of diffusion for some suppurative anal lesions and to plan advanced sphincter-sparing procedures.

Twenty adult anatomic specimens of the anal region (12 from women) were cut in the sagittal, coronal, and paracoronal planes, stained with hematoxylin and eosin, and examined. The pelvic floor musculature was examined in three patients undergoing postanal repair operations. Thirty primary posterior and posterolateral anal fistulas, preoperatively classified as transsphincteric (22) or suprasphincteric (8) were carefully traced during and after staged fistulotomy in 30 (11 female) patients, and their relationship with puborectalis muscle and external sphincter was evaluated. An attempt was made peranally to separate the external sphincter from the puborectalis muscle in four patients (3 females) aged 56 to 65 years with rectal cancers 4 to 5 cm from the anal verge so as to perform a sphincter-sparing procedure.

A connective plane of separation between puborectalis muscle and external sphincter was clearly identified in 14 (70 percent) anatomic specimens. In three (21 percent) cases the two muscles presented a pronounced overlapping arrangement. An anatomicofunctional separation between puborectalis muscle and external sphincter was easily demonstrated during post-anal repair operations. All fistulous tracks ran between the external sphincter and puborectalis muscle, despite the pronounced upward direction of the ones preoperatively classified as suprasphincteric. A plane of separation between puborectalis muscle and external sphincter was identified and developed in four patients with very low rectal cancers. An abdominoperanal rectolevatorial excision was performed. A coloanal anastomosis was performed on the residual lower anal canal.

An anatomic plane of separation is present between the puborectalis muscle and the external sphincter. The presence of this plane is important to help understand the diffusion of some suppurative anal lesions and to plan advanced sphincter-sparing procedures.