Fig. 1.1
Puborectalis forming the anorectal angle
Lying posteriorly to the rectum are the sacrum, coccyx, and pelvic diaphragm, along with the associated presacral venous plexus and roots of the sacral nerve plexus. Anteriorly lie the rectovesical pouch, prostate and seminal vesicles in men, and the posterior wall of the vagina and uterine cervix in women.
The rectum traverses the pelvic floor, which is the muscular diaphragm separating the pelvis from the perineum and providing support for the abdominal cavity. The puborectalis slings around the rectum at approximately 5 cm from the anal verge creating the sharp anorectal angle and attaches anteriorly to the pubis. On either side of the rectum are the ischiorectal fossae containing loose areolar fat, branches of the inferior rectal vessel, and nerves which cross the fossae to enter the wall of the anal canal. These potential spaces allow the rectum to distend during defecation. The ischial tuberosities form the outer limits of the ischiorectal fossae.
1.1.1 Mesorectum
The “mesorectum” is a fatty cuff of tissue surrounding the rectum containing perirectal lymph nodes and terminal branches of the inferior mesenteric artery, but no functionally significant nerves. The mesorectum extends through the entire length of the rectum; the lower third of the rectum is completely enveloped by the mesorectum, but it begins to narrow caudally from the level of the insertion of the levator ani muscles [3]. It is thickest posteriorly and is enclosed by fascia propria. Its removal is crucial to the treatment of rectal cancer, since it is a site of metastasis. A total mesorectal excision (TME) is the gold standard for surgery of cancer in the middle and lower thirds of the rectum and is defined as the complete excision of the visceral mesorectal tissue to the level of the levators (Fig. 1.2) [4]. Technically the mesorectum is not a mesentery however, as it does not conform to the definition of “two layers of peritoneum that suspend an organ.”
Fig. 1.2
A total mesorectal excision radically removes en bloc the tumor-bearing part of the rectum with the associated mesorectal compartment
Waldeyer’s fascia propria recti (or fascia propria) is a thin membrane enveloping the mesorectum [5]. Posterior dissection of the rectum in TME follows the loose avascular areolar tissue in the retrorectal space between the outer edge of the fascia propria and the underlying presacral fascia. Beneath the presacral fascia lie the presacral veins which, if injured, can cause profuse and troublesome bleeding. The presacral fascia fuses with the fascia propria just above the anorectal junction posteriorly [6]. There is some debate about the presence of lateral ligaments or stalks. They have been variably described as either a thickening of connective tissue between the mesorectum and lateral pelvic wall or instead as a continuation of the pelvic parietal layer of the fascia, onto branches of the middle hemorrhoidal artery and the rectal nerve [6–8]. The fascia between the urogenital organs and the extraperitoneal anterior rectum, Denonvilliers’ fascia , first described in 1836 by Charles Denonvilliers, is anatomically distinct from the fascia propria of the rectum [9]. The rectogenital septum (rectoprostatic or rectovaginal) consists of two layers of peritoneum partly or fully fused; this corresponds to Denonvilliers’ fascia, the existence of which is disputed in the female. The rectogenital septum contains collagenous, elastic fibers as well as bundles of smooth muscle cells and nerve fibers that emerge from the autonomic inferior hypogastric plexus.
1.1.2 Peritoneal Coverage
The peritoneum completely invests the anterolateral aspects of the upper third of the rectum and the anterior surface of the middle third, but does not cover the lower third. The rectum is thus completely extraperitoneal posteriorly. The peritoneum reflects over the rectum to the apex and back of the body of the bladder in men and the uterus and posterior part of the vaginal fornix in women forming the rectovesical and rectouterine pouch (pouch of Douglas), respectively. This pouch is bordered laterally in women by the rectouterine fold containing the uterosacral ligament over which the pelvic splanchnic nerves lie. In men, the rectovesical pouch is bordered by a peritoneal fold that covers the inferior hypogastric plexus.
1.1.3 Rectal Wall
The rectal wall consists of six layers:
1.
Simple columnar mucus-secreting epithelium
2.
Lamina propria (loose connective tissue beneath the epithelium)
3.
Muscularis mucosa (thin muscle layer)
4.
Submucosa (blood vessels, lymph, and Meissner’s plexus)
5.
Muscularis propria (inner circular and outer longitudinal muscle layers between which lies Auerbach’s plexus)
6.
Serosa (upper third)/adventitia (mid- and lower third)
Rectal cancer spreads stepwise outwardly through the rectal wall, and this is reflected in Dukes’ tumor staging. The muscular coat of the rectum consists of a complete longitudinal coat, which when peeled back exposes circular muscle fibers [10]. The circular muscle fibers become more tightly packed around the plicae transversales (valves of Houston), permanent transverse folds of the upper rectum [10]. There are three folds in total, lying convex to the right at 7–8 cm from the anal verge, to the left at 9–11 cm, and to the right again at 12–13 cm. Kohlrausch’s plica (middle fold) is the most consistent of these folds and corresponds to the level of the anterior peritoneal reflection. Valves of Houston are absent in the mobilized rectum. They are optimal sites for rectal biopsy due to accessibility and a lower risk of iatrogenic perforation (Fig. 1.3).
Fig. 1.3
Transverse folds in the rectum and anal canal
1.1.4 Blood Supply
In sphincter-saving resections of the rectum, the major concern of the surgeon is to secure an adequate blood supply to the portions of the colon and rectum remaining for anastomosis [11]. There is a rich and homogenous vascular supply throughout all the rectal wall. The rectum receives its blood supply from the superior, middle, and inferior rectal arteries.
The superior rectal artery (SRA) is the terminal branch of the inferior mesenteric artery (IMA) . The IMA courses downwards, crossing the left common iliac, and becomes the superior rectal artery in the pelvic mesocolon. The SRA reaches the posterior wall of the rectum at the level of the third sacral vertebra where it pierces the rectal wall and branches into two, left and right in approximately 80 % or occasionally multiply, diverging downwards and outwards while descending vertically or obliquely in the submucosa until approximately 8 cm from the anal margin [12, 13]. From here it supplies the lower rectum and anal canal, where branches reach the level of the rectal columns and condense in the hemorrhoidal plexuses. The middle rectal artery (MRA) is of variable origin, anatomy, and contribution to anorectal blood supply. It more commonly arises from the anterior division of the internal iliac artery, but can originate from the pudendal arteries. The MRA reaches the lower third of the rectum, close to the pelvic floor, and breaks up into several terminal branches. The vessel and its branches are susceptible to injury during low anterior resection when dissection of the rectum is performed close to the pelvic floor. The paired inferior rectal arteries arise from the internal pudendal artery (a branch of the internal iliac artery) as it passes through Alcock’s (pudendal) canal. The inferior rectal artery is closely applied to the inferior surface of the levator ani muscles where it courses at the upper part of the ischiorectal fossa to pierce the rectal walls and enter the external anal sphincter (EAS) reaching the submucosa of the anal canal where it ascends in this plane (Fig. 1.4).
Fig. 1.4
Arterial blood supply of the rectum and mesorectum
1.1.5 Venous Drainage
The paired middle and inferior hemorrhoidal veins and single superior hemorrhoidal vein originate from the three anorectal arteriovenous plexuses.
1.
The external hemorrhoidal plexus around the dentate line.
2.
The internal hemorrhoidal plexus above the dentate line.
3.
The perirectal plexus, which drains to the middle and inferior hemorrhoidal veins only. Venous drainage tends to follow arterial supply in that the superior rectal vessels are the main contributors, draining back to the inferior mesenteric vein. The middle rectal veins are smaller tributaries but drain back to the internal iliac vein, while the inferior rectal vein drains via the internal pudendal vein to join the internal iliac vein and from thence the inferior vena cava.
1.1.6 Lymphatic Drainage
Villemin originally described how the lymphatic drainage differs between the upper and lower rectum, accompanying the superior, middle, and inferior rectal vessels. He referred to them as the superior, middle, and inferior trunks, respectively, the superior trunk draining the upper two thirds of the rectum and following the superior rectal nodes to the pararectal nodes, then to those at the inferior mesenteric arterial axis, and from there to the para-aortic nodes. The middle trunk drains the lower third of the rectum and runs mainly to the internal iliac nodes along the middle rectal vessels, while the inferior group drains the anal canal above the dentate line to the inguinal nodes (but can also drain to the internal iliac and superior rectal nodes) [14].
Lymphatic drainage, rather than “upward,” can also to a variable degree follow an “outwards” path [15], although this is variable and inconsistent [16]. Japanese surgeons hence favor wide lateral clearance in rectal cancer, although this risks injury to the pelvic autonomic nerves, with consequent urinary and sexual dysfunction [17].
1.1.7 Innervation
The endoderm-derived rectum and left colon receive autonomic innervation by the parasympathetic and sympathetic nervous systems. Sympathetic fibers originate from L1, L2, and L3 and pass through ganglionated sympathetic chains to form the pre-aortic plexus, which is closely applied to the vessel. The pre-aortic fibers pass downwards below the aortic bifurcation at which point they form a presacral or (superior hypogastric) plexus. This hypogastric plexus branches into left and right along the pelvic sidewalls, running parallel and medial to the ureters, and then merges with parasympathetic nerve fibers originating from S2, S3, and S4. The parasympathetic fibers emerge from the sacral foramen as nervi erigentes or pelvic splanchnic nerves. These fibers pass inward and forward to form a network with the sympathetic fibers, which occurs at the pelvis plexus (inferior hypogastric plexus) lying above the levator ani. The inferior hypogastric plexus gives rise to the periprostatic plexus which is situated anterior to Denonvilliers’ fascia and innervates the prostate, seminal vesicles, corpora cavernosa, vas deferens, urethra, ejaculatory ducts, and bulbourethral glands. In women it gives rise to the uterovaginal plexus supplying the vaginal walls, erectile tissue of the vestibule, clitoris, and uterus. The pelvic nerves also contain afferent sensory fibers, which transmit information from distension receptors in the rectal ampulla.
During an operation on the rectum the nerves supplying the urogenital organs are susceptible to damage at several points: the superior hypogastric plexus at the sacral promontory, the nervi erigentes at the posterolateral aspect of the pelvis, the pelvic plexus at the lateral pelvic wall and levator ani, and the periprostatic or uterovaginal plexus during anterior rectal dissection. Meticulous observance to the nervous anatomy during mobilization of the rectum is essential to minimize the risk of permanent bladder paresis or sexual dysfunction.
1.2 Anal Canal
The anatomical limits of the anal canal vary as to surgical or anatomical definition. The distal limit is at the anal verge, which is the junction between the anoderm and skin; however, proximally it can be described as either beginning at the dentate line (anatomical) or anorectal ring (surgical) [18]. Its length is approximately 2 and 4 cm, respectively. The anal orifice (anus) is closed, forming a tight, ridged seal, at rest.
Anatomical studies measuring the length of the surgical anal canal (anorectal ring to anal verge) demonstrate an average length of 4.2 cm (range 3.0–5.3 cm). There is a significant difference in length between men [4.4 cm (range 3.2–5.3 cm)] and women [4.9 cm (range 3.0–5.0 cm)]. The average length of the anal canal from the dentate line to the anal verge is 2.1 cm (range 1.0–3.8 cm), and there are, again, significant differences between the sexes, with women’s marginally shorter [19] (Fig. 1.5).
Fig. 1.5
Anal canal
1.2.1 Anatomical Relations
Anterior to the anal canal lies the perineal body; posteriorly the anococcygeal ligament separates it from the bony coccyx. Laterally lie the potential spaces that are the ischiorectal fossae, containing loose areolar tissue.
1.2.2 Dentate Line
The dentate or pectinate line represents the boundary between the upper mucosal endoderm and lower cutaneous ectoderm (mucocutaneous junction) and lies approximately 1–2 cm from the anal verge (Fig. 1.5). Above the dentate line, the innervation of the intestine is by sympathetic and parasympathetic routes and blood supply and lymphatic drainage via the iliac vessels. Below, nervous innervation is somatic and the lymphovascular destination is the inferior hemorrhoidal system.
The cranial part of the dentate line is the point at which the anal valves lie. There are 6–12 vertical columns or ridges, called the columns of Morgagni, which have anal valves at their base and contain the internal hemorrhoidal plexus giving this area a characteristic purplish tinge on examination. Anal glands situated in the nearby submucosa discharge mucin via anal ducts to anal crypts at the level of the dentate line. Blockage of these ducts, perhaps by foreign material, is a causative factor in perianal sepsis, abscesses, and fistulae.
1.2.3 Histopathology
The epithelial type transitions from typical intestinal columnar epithelium of the rectum to stratified squamous epithelium at the anal verge (also known as the anocutaneous line of Hilton). Histologically the anal canal can be divided into three regions: the upper zone, from puborectalis to above the dentate line, the middle zone, or “anal transition zone (ATZ),” the morphology and extent of which is controversial, and the lower zone from the ATZ to the anal verge. The upper zone rectal mucosa is of simple columnar epithelium with short irregular crypts and more smooth muscle fibers within the lamina propria than the rectum. Around the dentate line, there is a transitional zone measuring 3–11 mm in length where the columnar mucosa becomes stratified squamous. As reported by Fenger, this is the zone interposed between uninterrupted colorectal-type mucosa above and uninterrupted squamous epithelium below, which has relevance for the treatment of anal canal carcinomas [20]. The anal transitional zone has been reported to be of stratified columnar or cuboidal epithelial type, although the majority consensus is that it consists of transitional-type epithelium. Here lie the anal crypts, sinuses, and ducts of the anal glands. The anorectal line was first described by Robin and Cadiat in 1874 as the junction between the glandular part of the anal canal and the simple columnar epithelium above. The anal glands, which are branched, straight, and tubular, extend into the submucosa and even muscularis mucosa and produce mucin onto the anal surface through anal ducts for lubrication of passing stool. The distal zone from the ATZ to the anal verge comprises non-keratinizing squamous epithelium with no glands or hair follicles (anoderm), and this then merges with the perianal skin which demonstrates features of normal skin such as hair follicles, keratin, and apocrine glands.
The cut-off between intestinal-type mucosa and anal canal is of importance in operations such as proctectomy and ileal pouch-anal anastomosis where it has been suggested that the ideal point of resection is at the anorectal line, thereby preserving all functions of the anal canal without residual intestinal mucosa [21]. It is thought that preservation of the ATZ via restorative proctocolectomy and ileal pouch-anal anastomosis improves clinically relevant and functional outcomes [22].
1.2.4 Continence
Continence to gas, liquid, and solid stool is facilitated by three main groups of contributory muscles: (1) circumferential closure provided by the two anal sphincters internal anal sphincter (IAS) and external anal sphincter (EAS), (2) pubococcygeus providing lateral compression, and (3) the angulation at the anorectal junction created by puborectalis. Continence is a highly complex function, still not yet fully understood, which also relies on the expansion of the anal cushions, the anatomical configuration of the rectum and anal canal, and the voluntary and involuntary reflexes coordinated by the colorectum and anus. The contribution to resting pressure of the anal canal is 55 % IAS, 30 % EAS, and 15 % vascular cushions [23].
1.2.5 Internal Anal Sphincter (IAS)
The internal anal sphincter is the distal 2–4 cm continuation of the inner circular muscle layer of the rectum and consists of smooth muscle (Fig. 1.6). It terminates with a well-defined rounded edge just above the external anal sphincter and anal orifice. It provides continuous tonic maximal contraction and is a natural barrier to the involuntary passage of gas and feces. It is the main contributor to anal resting pressure and thus continence [24]. The IAS also has periods of relaxation triggered by rectal distension (rectoanal inhibitory reflex, RAIR) and rectal sampling.
Fig. 1.6
Normal anatomy of the anal canal
The length of the IAS is associated with the functional anal canal length and is measured by anorectal manometry or endoanal ultrasound as being shorter in women than in men (2.0–3.0 cm, 2.5–3.5 cm, respectively) [25]. This is important when considering sphincterotomy in women, whereby a similar length of division to that in men will sacrifice considerably more muscle in the shorter sphincter. There is also evidence to show morphological changes in its thickness with advancing age, ranging from 2 to 4 mm, an increase which is likely to be due to a process of smooth muscle fibrosis [26, 27].
The IAS receives innervation from the autonomic nervous system (sympathetic and parasympathetic), mainly from the inferior hypogastric plexus and from thence via the inferior rectal nerves. The parasympathetic system has an inhibitory effect on the tone of the IAS, causing relaxation [28].
1.2.6 External Anal Sphincter (EAS)
The external anal sphincter forms a clearly demarcated muscular tube around the anal canal and is made of striated, skeletal muscle. Originally, it was proposed that the EAS consisted of two parts [29], or was simply one whole muscle unit [30]. However, the general consensus is now that it is comprised of three parts: the subcutaneous, superficial and deep anorectal elements [31, 32].
The EAS is continuous with the puborectalis component of levator ani at its deepest part, forming the anorectal ring. Macroscopically the EAS is attached to the anococcygeal raphe posteriorly, whereas puborectalis is not. The morphology of the EAS differs between sexes, ages and after childbirth or instrumentation. The external sphincter is approximately 2.7 cm long, but is anteriorly shorter in women (approximately 1.5 cm) [26] and has a thickness of 4 mm on endoluminal imaging. A decrease in the thickness of the external sphincter over time is likely part of a normal ageing process for both women and men [26].
At its distal end, the EAS lies below the IAS, turning inward toward it. The edges of both sphincters are palpable at the anal verge, on either side of the intersphincteric groove that separates them. Laterally, inferior rectal branches of the internal pudendal nerve (S2, S3) and internal pudendal arteries cross the ischioanal fossa to supply the external sphincter and anal mucosa on either side. The EAS also receives some innervation from the perineal branch of S4.
The pudendal nerve is a mixed motor and sensory nerve which arises from the sacral plexus S2–S4, leaves the pelvis through the greater sciatic foramen, and crosses this ischial spine. It then continues in Alcock’s canal (pudendal canal) with the pudendal vessels, to enter the lateral aspect of the ischioanal fossae beneath the levator ani, toward the ischial tuberosity where it branches to give motor fibers to the EAS (inferior rectal nerve) and sensory fibers to the perineum (perineal nerve) as well as the dorsal nerve of the penis or clitoris.
1.2.7 Longitudinal Muscle
The outer longitudinal muscle layer of the rectum continues to the anorectal ring where it merges with fibers of the levator ani and becomes the conjoined longitudinal muscle. It descends between the IAS and EAS in the intersphincteric groove and breaks up at the level of the lower border of IAS into fan-like septa which interdigitate with the EAS and insert into the perianal skin as the corrugator cutis ani [33, 34]. The area enclosed by these septa is the perianal space. The longitudinal muscle is 2.5 mm thick and this decreases with age [26]. The longitudinal muscle consists of smooth muscle with some contributory striated muscle fibers from levator ani. It is thought that it contracts during defecation which leads to shortening of the anal canal; as such the muscle was called the “evertor ani muscle” by Shafik [35].
1.2.8 Levator Ani Muscles (LAM)
The pelvic floor consists of a complex interrelated structure of muscles, ligaments, and fascia. Its layers cranio-caudally include the endopelvic fascia, the muscular pelvic diaphragm (commonly referred to as the levator ani), the perineal membrane (urogenital diaphragm), and a superficial layer of muscles. The pelvic diaphragm is a striated, broad, thin, and funnel-shaped muscular shelf that supports the abdominal contents and plays an important role in continence and evacuation.
The levator ani muscles make up the pelvic diaphragm and consist of pubococcygeus, iliococcygeus, ischiococcygeus, and puborectalis muscles, although the latter’s inclusion in the definition is disputed [36]. It has been suggested that the LAM are divided into a transverse and vertical portion and that puborectalis sits separately [37]. The LAM stretch from the peripheries of the bony pelvis, and then the bilateral LAM meet posteriorly in the midline to insert into the coccyx; the thick white fibers at insertion together are named the anococcygeal raphe [38].
Puborectalis muscle is a U-shaped loop muscular sling hooked around the rectum, forming the anorectal angle. It is not in itself connected to the rectum. Puborectalis originates from the posterior aspect of the pubis and the anterior obturator fascia, wrapping around the rectum to join corresponding fibers of the opposite side. Pubococcygeus runs from the anterior part of the pelvic fascia to the coccyx and lies directly above puborectalis. Iliococcygeus originates from the lateral part of the obturator fascia and inserts into the coccyx. Innervation of the levator ani is by the pudendal nerve on its perineal aspect and the nerve to levator ani (S3, S4) on its pelvic surface [36].
1.2.9 Perineal Body
The perineal body , a complex fibromuscular structure separating the urogenital organs and the anal sphincter, lies between the anal canal and the posterior fourchette in women and the penile bulb in men (Fig. 1.7). It is a vital constituent of both urinary and fecal continence mechanisms, supporting the perineum against increased abdominal pressure and is vulnerable to injury during childbirth. The perineal muscles (superficial and deep transverse perineal muscles) form the bulk of the perineal body as they cross the midline, but it also receives contributions from the external anal sphincter, bulbospongiosus muscle, and levator ani [39].
Fig. 1.7
The perineal body is a complex fibromuscular structure separating the urogenital organs and the anal sphincter
1.2.10 Blood Supply
Arterial supply to the anal canal is via the superior, middle, and inferior rectal arteries. Venous drainage is via the internal hemorrhoidal plexus to the superior rectal vein, to the inferior mesenteric vein, and to the portal system. The external hemorrhoidal plexus drains via the middle rectal vein to the pudendal, internal iliac vein and finally the inferior vena cava and via the inferior rectal vein to the internal pudendal vein.
1.2.11 Lymphatic Drainage
The lymphatic drainage of the rectum has been described above. The drainage of the upper anal canal, above the dentate line, is similar to the rectum, following the venous drainage path to the inferior mesenteric and internal iliac nodes. Below the dentate line, drainage follows the inferior rectal vein to the superficial inguinal nodes.
1.2.12 Perianal Skin
The perianal skin around the anal verge consists of keratinized stratified squamous epithelium with eccrine glands, pilosebaceous units, and apocrine glands. The skin is pigmented and corrugated over the underlying EAS and allows for a large degree of stretch. It is exquisitely sensitive, with somatic innervation via the pudendal nerves S2, S3, and S4. Lymphatic drainage is to the superficial inguinal lymph nodes. The perianal skin is susceptible to any normal dermatological diseases.
1.3 Radiological Evaluation
Ultrasound is readily available, inexpensive, valuable in assessing rectal malignancy, sphincter complex anatomy, and benign anorectal disease. Its operator dependency does however, limit its consistency.
1.3.1 Endorectal Ultrasound (ERUS)
Rectal imaging requires an ultrasound transducer probe covered by a water-filled balloon. The patient is given an enema and examined in the left lateral decubitus position. ERUS images are able to demonstrate five main layers: (1) balloon mucosa interface, (2) mucosa and muscularis mucosa, (3) submucosa, (4) muscularis propria, and (5) interface with fat (Fig. 1.8a, b).
Fig. 1.8
(a) ERUS showing T1 rectal cancer confined to the mucosa and superficial submucosa. SM submucosa, MP muscularis propria, SR serosa. (b) Corresponding ERUS and histological layers (from Siddiqui et al. [40], with permission)
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