The Posterior Trans-Sphincteric Approach to the Anorectum

THE POSTERIOR TRANS-SPHINCTERIC APPROACH TO THE ANORECTUM
- Lessons from Paediatric Surgery

Amrish Vaidya*, Kishore Adyanthaya**, Sudha Gulkhandia***
*Hon. Asst., Paediatric Surgeon; **Hon. Asso., Paediatric Surgeon; **Resident, Department of Peadiatric Surgery, Bombay Hospital, Mumbai 400 020.
INTRODUCTION
Operations on the rectum in adults are generally performed through the abdominal or the perineal route, or by using a combination of the two approaches. While this may be appropriate for the surgical management for malignant disease, the posterior trans-sphincteric approach may be better suited for the management of benign diseases.

This approach, though described by colorectal surgeons in the past, was popularised by Alberto Pena, following his description of its use in the correction of congenital anorectal malformations in 1982. Since then, it has become a commonly performed approach by paediatric surgeons worldwide. Important observations have since been made on the anatomy of the sphincters as seen through the posterior sagittal route, which are of relevance to all interested in this area.

This approach could have applications in adult surgery, particularly when operating on those areas of the pelvis, which are too low to be approached through the abdomen, or too high to access through the perineum.

The posterior sagittal approach

The patient is placed in the prone jackknife position, so as to elevate the pelvis. The legs are gently spread symmetrically on either side. The incision runs from the mid sacrum to the site of the anus, either actual, if the anus is present, or determined, by stimulating the skin using an electrical muscle stimulator. The site of the anus in this case is visible as the area of maximum indrawing of the skin, generally corresponding to an area of hyperpigmentation.

The incision is deepened, through the visible striated muscle, which correspond to the external sphincter fibres, frequently using the muscle stimulator to check for equality of contraction in the tissue on either side, as a guide to keeping the incision strictly in the midline.

The coccyx can be sagittally split to gain access to the plane above the levator attachment. The levator ani can then be divided in the midline.

In case this approach is used to treat anorectal malformations, the blind ending rectum is frequently looked for in the space so exposed.

Since the initial description of this approach, other applications have been added. Thus, if the rectum and anal canal are intact, they can be exposed, dissected or excised, under direct vision.

The space anterior to the anus may be accessed, by splitting the rectum and anus in the midline, and cutting through the muscles anterior to these structures to reach the urethra in the male or the vagina in the female. Though some surgeons prefer to dissect the rectum all around and retract it aside to reach the deep anterior perineal structures, this step could denervate the anus and rectum causing possible loss of sensation and incontinence. Splitting the rectum and anus preserves the nerve supply, which comes from both sides and theoretically maintains a superior degree of continence. This, however, would require a protective colostomy or exceptionally good bowel preparation.

At the end of the procedure, the stimulator is used to identify corresponding muscle fibres from both sides, which are sutured in the midline, or around the structures they are associated with. If the rectum has been split, it is sutured in two layers anteriorly and posteriorly to reconstitute the lumen, following which the posterior muscles are sutured as described.

Surgical anatomy of the continence muscles : lessons from the posterior sagittal approach

Traditionally, the continence muscles are described as having three components, namely the levator ani, the external sphincter and the internal sphincter. Whereas the internal sphincter constitutes a part of the wall of the rectum, the external sphincter and levator ani are striated muscles which are external to but in close contact with the anus and rectum.

The levator ani has been described as a funnel like sling, made of the pubococcygeus, the ischiococcygeus and the puborectalis muscles, each component having a different origin and insertion, and consequently, different actions on the anorectum.

The argument of the proponents of the posterior sagittal approach has been that the subdivisions so described are arbitrary, and based more on anatomic dissections in cadavers rather than what is visible during surgical exposure. Using muscle stimulators during the entire course of the dissection, it is thought that the entire muscle complex constituting the continence mechanism is actually a continuum of muscle extending from the pelvis to the skin. The uppermost part of this complex corresponds to what we are accustomed to calling the levator, and the lowermost part could be termed the external sphincter. However, one part merges with the other, there being no distinct fibres of either group.

The uppermost part of the muscle is, as described earlier, a funnel shaped muscle that surrounds the rectum from all sides. However, the earlier belief that a distinct puborectalis formed a continuous sling of muscle around the rectum is not thought to be true. It is now thought that the muscle fibres are distinct bilateral muscles which converge on the rectum from both sides and decussate with the fibres from the opposite side to exert a symmetrical action, compressing the rectum from behind towards the pubic bone. Cutting through this 'sling' was, at one time, considered dangerous from the point of view of continence preservation. Since we now know that this muscle is actually made of converging fibres from both sides, which reach, but do not swing around the midline, they can be divided and rejoined, with no apparent loss to the action or nerve supply. The external sphincter, once thought to be circular muscle around the lowest part of the rectum, is also traditionally described in three parts, the superficial, subcutaneous and the deep part. As earlier mentioned, these fibres are seen during surgical exposure, to run in continuity with the levator fibres, on either side of rectum, in a paramedian plane. The orientation of these fibres is along the axis of the rectum, in a cephalocaudal direction. A contraction of these fibres, which are collectively and loosely referred to as the striated muscle complex, elevates the anus. Still lower down, some muscle fibres appear to run horizontally, parallel to the skin on either side, and lateral to the cephalocaudal fibres of the striated muscle complex. These have been christened the para sagittal fibres. Stimulating the parianal skin causes these fibres contract around the fixed point of the anus to pucker up the perianal skin and close the orifice.

Other applications of the posterior sagittal approach

Though this approach has been used most often for the surgery of congenital anorectal malformations, other applications have emerged. This approach has been used in the management of benign rectal strictures and for recurrent rectal prolapse. Rectourinary fistulae, either post traumatic or post irradiation, can be treated through this approach. The structures anterior to the rectum can be accessed either by retracting or splitting the rectum. Besides its use in the management of cloacal malformations in females, when the rectum, vagina and urethra open on the perineum through a common channel and a single opening; other uses are the surgery of the urogenital sinus and enlarged prostatic utricles. A possible application is its use in injuries of the posterior urethra.Though this approach has been used most often for the surgery of congenital anorectal malformations, other applications have emerged. This approach has been used in the management of benign rectal strictures and for recurrent rectal prolapse. Rectourinary fistulae, either post traumatic or post irradiation, can be treated through this approach. The structures anterior to the rectum can be accessed either by retracting or splitting the rectum. Besides its use in the management of cloacal malformations in females, when the rectum, vagina and urethra open on the perineum through a common channel and a single opening; other uses are the surgery of the urogenital sinus and enlarged prostatic utricles. A possible application is its use in injuries of the posterior urethra.

Advantages of the posterior approach are the generally clean and untouched surgical field, with outstanding exposure and direct vision of the anatomy of the area. A meticulous dissection and staying strictly in the midline are mandatory for a satisfactory result.

SUMMARY

Since its description in 1982, the posterior trans-sphincteric approach to the rectum has been widely used by paediatric surgeons. It is now known that it is not necessarily dangerous from the point of view of continence preservation, to cut through the sphincters.

At surgery, using muscle stimulators, the sphincter muscles can be precisely cut in the midline and resutured. The traditional descriptions of the components of the continence muscles is now thought to be arbitrary - they now appear to be a continuous funnel shaped muscle running from the pelvis to the perineum. Advantages of this approach are better exposure, allowing more precise surgery. It is an excellent approach for conditions, which are too high to be approached through the perineum, or too low to be reached through the abdomen.

REFERENCES

deVries P, Pena A. Posterior sagittal anorectoplasty. J Paed Surg 1982; 17 : 638-43.

Pena A, deVries P. Posterior sagittal anorectoplasty : important technical considerations and new applications. J Paed Surg 1982; 17 : 7796-811.

Pena A, Bonilla E, et al. The posterior sagittal approach : further paediatric applications. Ped Surg Intl 1992; 7 : 274-8.

Pena A. The posterior sagittal approach : Implications in adult colorectal surgery. Dis Colon and Rectum 1994; 37 (1) : 1-11.

Pena A. Anorectal malformations : New aspects relevant to adult colorectal surgeons. Seminars in Colon and Rectal Surg 1994; 5 (2) : 78-88.

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