Anatomy, Abdomen and Pelvis: Anal Canal (2024)

Introduction

The anal canalis the terminal part of the gastrointestinal tract thatconveys material from the rectum to the anus. While itis typically around2.5 to 4 cm in length, it is more complex than it seems. The anal canal plays a vital role as a defense against organismsthat might enter the body, it differentiates between solid, liquid, and gas, and it contributes to fecal continence. Often not evaluated in the absence of symptoms,pathologies of the anal canalmay be treated with wide-ranging measures asconservative as a sitz bath to as significantas surgical resection or radiation.

Structure and Function

The anal canal serves as a channel connecting the rectum to the end of thegastrointestinalsystem, the anus. It is locatedwithin the anal triangle of the perineum and in between the fat-filled and wedge-shaped ischioanal, or ischiorectal, fossae thataccommodateits expansion for the passage of fecal material. The transition from the rectum to the anus can be identified by the anorectal junction, or anorectal ring, which appears as a bend in the gastrointestinal tract and is supported by the puborectalis muscular sling. The anorectal junction is also the site wheremuscular fibers ofits two sphincters blend superiorly with those of the puborectalis muscle.

The two functionally and anatomically distinct sphincters surrounding the anal canal are the internal anal sphincter and the external anal sphincter.[1][2]The internal anal sphincter (IAS) is involuntary. Itsfunction is dependent on the integrity of the rectoanal inhibitory reflex (RAIR), which involves the reflexive relaxation of theinternal anal sphincter in the presence of rectal distension. It maintains a resting pressure in the absence of rectal distension. Thus, the internal anal sphincter'sresting pressure is widelyused as a direct measure of its functionality and is often assessedwhen evaluating a variety of pathologies. The external anal sphincter (EAS) is under voluntary control. The external anal sphincter maintains its toneto keep the orifice of the anal canal closed and relaxes upon defecation. While therectoanal inhibitory reflexcauses theinternal anal sphincter to relax in the presence of rectal distension, theexternal anal sphincter worksto keep stool in the anal canal until one is ready to defecate.

The anal canal is typically divided into superior and inferior segments by the pectinate, or dentate, line. The pectinate line isa visible zig-zagging line formed from the inferior aspectof longitudinal foldsknown as the anal columns or anal valves. The pectinate line demarcates the superior two-thirds of the anal canal from the inferior one-third. It alsoserves as an embryologic landmark that explains the differing arterial supply, venous drainage, lymphatic drainage, andnervous supplyof the segments of the anal canal.

Embryology

As with its structure and function, the embryology of the anal canal differs superior and inferior to the pectinate line. Above the pectinate line, the anal canal is the terminal portion of the hindgut and is endodermally derived. As with the rest of the hindgut, the superior anal canal is lined by columnar epithelium. Below the pectinate line, the anal canal is ectodermally derived and is lined with stratified squamous epithelium.

At birth, theexternal anal sphincter is not yet fully developed or under complete voluntary control.An individual usually achieves voluntary control of theexternal anal sphincternear the ages of 18to 24 months.[3][4]

Blood Supply and Lymphatics

An excellent approach to understanding thearterialsupply, venous drainage, and lymphatics of the anal canal is to consider the supply above and below the pectinate line and the area of overlap.[5]

Arterial Supply and Lymphatic Drainage

Above the pectinate line, the anal canalis supplied by terminal branches of the superior rectal artery, a branch of the inferior mesenteric artery. Lymphatic fluid fromthis region is conveyed to the inferior mesenteric lymph nodes.

Below the pectinate line, the anal canal is served by the middleandinferior rectal arteries. The middle rectal artery is a branch of the internal iliac artery, while the inferior rectal artery is a branch of the internal pudendal artery that arises from the internal iliac artery. The lymphatic drainage below the pectinate line isprimarilyto the superficial inguinal lymph nodes.

Venous Drainage

The venous drainage of the anal canalsuperior to the pectinate line is to the superior rectal veins, which drain into the inferior mesenteric vein. These veins are part of the hepatic portal venous system, conveying oxygen-poor, nutrient-rich blood to the liver for processing. The venous drainage of the anal canal below the pectinate line is to the internal pudendal veins, whichconvey their contents to the internal iliac vein. These veins are part of the systemic venous system that transports oxygen-poor, nutrient-poor blood to the heart. Anastomoses of the portal and systemic venous systems within the anorectal canal allow for potential collateral circulatory routes. In patients with portal hypertension, anorectal varices may be apparent. Not unique to the anal canal, these portosystemic anastomoses are also present elsewhere in the body.[6]

Nerves

Similar to the blood supply and lymphatics, it is also useful to discuss the nervous system of the anal canal from the perspectivesuperiorandinferiorto the pectinate line. These tworegionsare distinctlyinnervated and respond to different stimuli.[7]

Above the pectinate line, the anal canal receives autonomic innervation from the inferior hypogastric plexus. The parasympathetic innervation inhibits the tone of theinternal anal sphincter and evokes a peristaltic contraction to allow defecation. Sympathetic innervation worksin opposition to maintain the tone of theinternal anal sphincter and preserve continence.Given viscerosensory signals return from this region, thesuperior aspect of the analcanal is most sensitive to stretch or distention.

Below the pectinate line, the anal canal receives somatic innervation derived from branches of the pudendal nerve. As a result, the inferior aspect ofthe anal canal is highly sensitive to pain, temperature, and touch.

Physiologic Variants

Little anatomic variation of the anal canal exists in the literature. Multiple case reports have discussed anal canal duplication in which a neonate is born with an extra anal canal, either superior or posterior to the main canal.Individuals with a duplicated anal canal tend to havea fairly typical accessory anal canal; however, it ends in a blind pouch with no connection to the rectum. Currently,this condition is treated with surgical excisionto avoid the accessory anal canal acting as a potential nidus of inflammation, fistulization, or malignancy.[8]

There issome variation of the anal canal that relates to aging. Theinternal anal sphincter increases in thickness as individuals age. The effect is usually more pronounced in females over 50. Current guidelines consider maximalinternal anal sphincter thickness greater than 4 mm as abnormal in patients under 50 years of age and5 mm in those over 50 years of age. This becomes important when identifying patients with hypertrophy of the internal anal sphincter.[9]

Surgical Considerations

During surgery involving the anal canal, the surgeon has a variety of techniques availableto identify the transition between the upper portion and lower portion of the anal canal. Identifying the region of the pectinate line is vital to understanding thevascular supply and drainage and innervation of the segments of the anal canal for surgical considerations.

The tight space and compact anatomy of the anal canalcontribute to a difficult compartment for surgical procedures. For instance, the anatomy of the anal canal makes sphincter-saving surgery during cases of lower rectal cancer very challenging. These surgerieshave the potential for negative outcomes, such asanal sphincter compromise with incontinence or permanent colostomy. Given the limited window available for surgeons within the anal canal, other approaches have been explored, such as transvaginal, transperineal, trans-sacral, and abdominal.[10]

Clinical Significance

Incontinence

The anal canal and its musclesmaintain fecal continence; consequently, any failure of the anal canal musculature may result in fecal incontinence. The current guideline for patients with fecal incontinence is to suggest anal manometry to assess sphincter tone. Positive manometry revealing a dysfunction of sphincter tone is usually followed by anorectal imaging to evaluate structural integrity.[11]

Hemorrhoids

Hemorrhoids are a common anal disorder that involves the engorgement and potential prolapse of veins in the lower part of the anus and rectum. A variety of factors predispose individuals to develophemorrhoids, such as pregnancy, age, diarrhea, chronic constipation, sitting for too long (especially on the toilet), heavy lifting, anal intercourse, obesity, and genetics.

There are twomain types of hemorrhoids: internal hemorrhoids and external hemorrhoids. Internal hemorrhoids are deep within the anal canal, superior to the pectinate line, and are usually painless because of the visceral sensations from that region. These hemorrhoids often present with painless bleeding and, upon straining, may prolapse out of the anus. External hemorrhoids exist below the pectinate line and are in close proximity tothe anal opening. Given that external hemorrhoids occur in the inferior anal canal, served bysomatosensory signals, they are often painful and itchy.

Hemorrhoids often resolve spontaneously, but patientsmay relieve the discomfort using dietary fiber, stool softeners, and daily sitz baths. It is also important to advise these patients about adequate fiber intake, decreased straining in the washroom, and decreased time spent sitting on the toilet. Large or bothersome hemorrhoids may be treated with banding or surgical hemorrhoidectomy in the office.[12][13]

Anal Cancer

Anal cancer is a disease in which the normal cells of the anus become malignant. Most cases are related to the human papillomavirus virus (HPV). Factors that enhance the risk of anal cancer include smoking and immunodeficiency. The prognosis of anal cancer is mostly dependent on the size of the tumor and whether the cancer has spread to the lymph nodes. Following the identification of anal cancer, it is staged from 0 to 5 based on size and spread, with stage 5 carrying the worst prognosis. Treatment of anal cancer often includes surgery, radiation therapy, chemotherapy, or some combination of these methods.[14]

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References

1.

Barleben A, Mills S. Anorectal anatomy and physiology. Surg Clin North Am. 2010 Feb;90(1):1-15, Table of Contents. [PubMed: 20109629]

2.

Stewart AM, Cook MS, Dyer KY, Alperin M. Structure-function relationship of the human external anal sphincter. Int Urogynecol J. 2018 May;29(5):673-678. [PMC free article: PMC5889349] [PubMed: 28689239]

3.

Arakawa T, Hwang SE, Kim JH, Wilting J, Rodríguez-Vázquez JF, Murakami G, Hwang HP, Cho BH. Fetal growth of the anal sinus and sphincters, especially in relation to anal anomalies. Int J Colorectal Dis. 2016 Mar;31(3):493-502. [PubMed: 26615552]

4.

Nakashima J, Zulfiqar H. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 1, 2023. Embryology, Rectum and Anal Canal. [PubMed: 31869146]

5.

Lund JN, Binch C, McGrath J, Sparrow RA, Scholefield JH. Topographical distribution of blood supply to the anal canal. Br J Surg. 1999 Apr;86(4):496-8. [PubMed: 10215822]

6.

Liao YH, Hsia WT, Yung-Fang C, Tsai PP. Severe Portal Hypertension-Inducted Anorectal Varices Bleeding Treated via a Transjugular Intrahepatic Portosystemic Shunt. Am J Gastroenterol. 2016 Jan;111(1):27. [PubMed: 26785657]

7.

Takeda M, Miyahara K, Akazawa C, Lane GJ, Yamataka A. Sensory innervation of the anal canal and anorectal line in Hirschsprung's disease: histological evidence from mouse models. Pediatr Surg Int. 2017 Aug;33(8):883-886. [PubMed: 28601899]

8.

Palazon P, Julia V, Saura L, de Haro I, Bejarano M, Rovira C, Tarrado X. Anal canal duplication and triplication: a rare entity with different presentations. Pediatr Surg Int. 2017 May;33(5):609-617. [PubMed: 28255623]

9.

Kumar L, Emmanuel A. Internal anal sphincter: Clinical perspective. Surgeon. 2017 Aug;15(4):211-226. [PubMed: 27881288]

10.

Skibber J, Rodriguez-Bigas MA, Gordon PH. Surgical considerations in anal cancer. Surg Oncol Clin N Am. 2004 Apr;13(2):321-38. [PubMed: 15137960]

11.

Faucheron JL. [Anal incontinence]. Presse Med. 2008 Oct;37(10):1447-62. [PubMed: 18555639]

12.

Guttenplan M. The Evaluation and Office Management of Hemorrhoids for the Gastroenterologist. Curr Gastroenterol Rep. 2017 Jul;19(7):30. [PubMed: 28567655]

13.

Lohsiriwat V. Treatment of hemorrhoids: A coloproctologist's view. World J Gastroenterol. 2015 Aug 21;21(31):9245-52. [PMC free article: PMC4541377] [PubMed: 26309351]

14.

Durot C, Dohan A, Boudiaf M, Servois V, Soyer P, Hoeffel C. Cancer of the Anal Canal: Diagnosis, Staging and Follow-Up with MRI. Korean J Radiol. 2017 Nov-Dec;18(6):946-956. [PMC free article: PMC5639160] [PubMed: 29089827]

Disclosure: Abdelwahab Ahmed declares no relevant financial relationships with ineligible companies.

Disclosure: Tafline Arbor declares no relevant financial relationships with ineligible companies.

Disclosure: Waqar Qureshi declares no relevant financial relationships with ineligible companies.