Anal Anatomy


Anal Anatomy

  1. Anal Canal
    • anal verge is the junction between the anoderm and perianal skin
    • dentate line is the mucocutaneous junction located 4 cm above the anal verge
    • mucosa distal to the dentate line is a modified squamous epithelium devoid of hair and glands
    • mucosa proximal to the dentate line is columnar epithelium
    • the change between the 2 types of mucosa is not abrupt; there is a transitional zone that extends for a variable length, from ~ 6 mm below to 20 mm above the dentate line
    • anatomic anal canal extends from the anal verge to the dentate line
    • surgical anal canal extends from the anal verge to the anorectal ring
    • anorectal ring is the lower border of the puborectalis muscle that is palpable on rectal exam – located 1.0 to 1.5 cm above the dentate line
    • posteriorly, the anal canal is attached to the coccyx by the anococcygeal ligament

    Anatomy of the Anal Canal
  2. Anal Sphincters
    1. Internal Sphincter
      • continuation of the inner circular smooth muscle of the rectum
      • involuntary muscle
      • normally contracted at rest
      • provides 80% of anal resting pressure
      • major reflex response to rectal distention is relaxation

    2. External Sphincter
      • voluntary muscle
      • commonly divided into 3 parts: subcutaneous, superficial, and deep
      • specialized continuation of the puborectalis muscle
      • normally contracted at rest
      • provides 20% of anal resting pressure and 100% of generated squeeze pressure
      • its major reflex response to stimuli (postural change, rectal distention, increased intra-abdominal pressure) is further contraction
      • maximum voluntary contraction can only be maintained for 60 seconds before fatigue sets in

      Anal Sphincters
  3. Anal Glands
    • body of the anal glands reside in the intersphincteric plane
    • ducts of the glands penetrate the internal sphincter and terminate in the anal crypts
    • anal crypts are located at the distal end of the columns of Morgagni, which consist of 8 to 14 mucosal folds located above the dentate line

Anal Physiology

  1. Maintenance of Continence
    • principal function of the anal canal is the maintenance of continence and the regulation of defecation
    • principal mechanism that maintains continence is the pressure differential between the rectum (10 mm Hg) and the anal canal (90 mm Hg)
    • also, contraction of the puborectalis muscle maintains the anorectal angle at 80°, so that it functions as a flap valve
    • continence also requires an adequate rectal capacity with normal compliance

    Anorectal Angle
  2. Defecation
    • as feces accumulate in the rectum, the rectal wall distends to accommodate the fecal mass
    • as the rectum distends, the internal sphincter relaxes and the external sphincter contracts
    • relaxation of the internal sphincter allows the rectal contents to reach the anal canal
    • once in the anal canal, sensory receptors located within the anal canal or pelvic floor musculature determine the nature of its contents – flatus, liquid, or solid stool
    • if defecation is to proceed, the external sphincter must be voluntarily relaxed, which also straightens the anorectal angle
    • a voluntary increase in intra-abdominal pressure moves the contents of the rectum into the anal canal
    • with selective relaxation of part of the external sphincter, it is possible to selectively pass flatus but not stool

Rectal Prolapse


Rectal Prolapse
  1. Etiology
    • complete prolapse is characterized by full-thickness eversion of the rectal wall through the anus
    • partial prolapse involves prolapse of the mucosa only
    • most commonly occurs in elderly women and institutionalized patients
    • predisposing anatomic factors include a redundant rectosigmoid, deep pouch of Douglas, patulous anus, diastasis of the levator ani, and lack of fixation of the rectum to the sacrum
    • some studies suggest that intussusception is the primary cause, but what initiates the intussusception is not clear

  2. Clinical Manifestations
    • a history of chronic constipation and straining is common
    • most patients complain of the protrusion; in occult prolapse, a feeling of pressure and incomplete evacuation may be the only symptoms
    • many patients are incontinent
    • chronically prolapsed mucosa may become ulcerated and cause significant bleeding
    • digital rectal exam will usually reveal a patulous anus with poor sphincter tone
    • a concomitant pelvic floor disorder (cystocele or prolapsed uterus) may also be present

  3. Diagnosis
    • the best way to visualize the prolapse is to have the patient squat or sit on the toilet and strain
    • seeing the concentric mucosal rings will help to differentiate prolapse from complicated hemorrhoidal problems (radial folds)

      • Rectal Prolapse and Prolapsed Hemorrhoids
    • proctoscopy is mandatory and may demonstrate a polyp or cancer serving as the ‘lead point’ of the intussusception
    • a complete colonoscopy or barium enema is necessary to rule out concomitant colon pathology
    • anal manometry can document the degree of anal sphincter damage but does not influence the surgical procedure chosen
    • defecography may help if the diagnosis is in question or if a concomitant pelvic floor disorder is suspected

  4. Surgical Options
    • surgery is the mainstay of treatment
    • can be managed from the perineal or abdominal approach
    • the perineal approach is reserved for older patients who cannot tolerate a laparotomy
    • abdominal procedures are associated with lower recurrence rates
    • abdominal procedures can be done open or laparoscopically
    • no ‘gold-standard’ procedure

    1. Narrowing of the Anus
      • reserved for extremely high risk patients with incontinence
      • prototypical procedure is the Thiersch-type anal encircling procedure using a synthetic mesh
      • fecal impaction is the most common complication and laxatives and enemas are usually necessary postop
      • wound infection and erosion of the mesh are also common
      • procedure is rarely done now because of the high recurrence and complication rates

      Thiersch Procedure
    2. Abdominal Procedures
      • associated with lower recurrence rates than perineal procedures
      • consists of rectal mobilization, ± sigmoidectomy, and rectal fixation

      1. Anterior Resection with Rectopexy
        • major advantage is resection of the redundant sigmoid colon, which will improve constipation
        • also, the rectum becomes fixed and adherent to the sacral hollow, helping to reduce recurrence
        • major risk is anastomotic leak
        • most surgeons will suture the rectum to the presacral fascia as a way of ensuring rectal fixation
        • some surgeons also obliterate the cul-de-sac by suturing the endopelvic fascia anteriorly to the rectum
        • both constipation and incontinence are improved by this procedure

        Low Anterior Resection with Rectopexy
      2. Rectal Fixation without Resection
        • indicated when sigmoid resection is not required (no constipation)
        • may be performed with sutures or mesh

        1. Suture Rectopexy
          • the rectum is fully mobilized down to the levators and secured to the sacral fascia with nonabsorbable sutures
          • must avoid the presacral veins and nerves

        2. Mesh Rectopexy
          • the mesh is secured to the sacrum and then sutured to the rectum
          • multiple options exist for fixing the mesh to the rectum
          • obstruction and mesh erosion are complications

          Posterior Mesh Rectopexy
    3. Perineal Procedures
      • usually reserved for patients who will not tolerate a laparotomy

      1. Perineal Rectosigmoidectomy (Altemeier Procedure)
        • for technical reasons, the length of the prolapse should be greater than 3 cm
        • full-thickness circumferential incision is made in the prolapsed rectum 1 - 2 cm above the dentate line
        • cut edge of the rectum is pulled down and the mesorectum divided
        • redundant rectum is then divided and an anastomosis is made to the anal ring
        • the levator muscles may be plicated anteriorly for additional support
        • recurrence rates appear to be high with this approach, probably because the rectum is not fixed to the sacrum

        Altemeier Procedure
      2. Delorme Procedure
        • reserved for patients with short prolapses
        • not a full-thickness resection, but rather a resection of the redundant mucosa 1 – 2 cm proximal to the dentate line
        • the muscle layer is then plicated
        • an anastomosis is then made between the mucosa at the level of the transection and the incision proximal to the dentate line
        • some surgeons will also plicate the levator muscles

        Delorme Procedure

Hemorrhoids

  1. Pathophysiology
    1. Internal Hemorrhoids
      • arise from specialized vascular and connective tissue cushions located in the left lateral, right anterior, and right posterior positions in the anal canal
      • consist of dilated arteriovenous channels (superior hemorrhoidal plexus) and connective tissue
      • since they originate above the dentate line, internal hemorrhoids are covered by columnar mucosa which is not sensitive to pain or touch
      • by filling with blood during defecation, the cushions protect the anal canal from injury and aid in maintaining continence
      • hemorrhoids refer to the abnormal enlargement of these cushions
      • there are many theories as to what causes hemorrhoids: 1) downward displacement or prolapse caused by straining, 2) destruction of the anchoring connective tissue system, 3) abnormal venous distention, 4) increased anal sphincter tone

    2. External Hemorrhoids
      • originate below the dentate line and are covered by anoderm, which is very sensitive to pain
      • arise from the inferior hemorrhoidal plexus
      • since the superior and inferior hemorrhoidal plexuses communicate with each other, mixed hemorrhoids are common

      Internal and External Hemorrhoids
  2. Clinical Manifestations
    1. Internal Hemorrhoids
      • typically cause painless, bright red bleeding after defecation
      • may also cause anal pruritis
      • may also prolapse and become incarcerated

      1. Classification
        • first degree: visible on anoscopy but do not prolapse below the dentate line
        • second degree: prolapse out of the anal canal during defecation but spontaneously reduce
        • third degree: require manual reduction
        • fourth degree: incarcerated

        Hemorrhoid Classification
    2. External Hemorrhoids
      • may cause symptoms by swelling
      • thrombosis may cause severe pain
      • skin tags may form, which represent prior thrombosed hemorrhoids which have become organized into fibrous appendages

      Thrombosed External Hemorrhoid
  3. Diagnosis
    • anoscopy is the definitive test
    • consider screening colonoscopy if the patient is anemic, has risk factors for colon cancer or is due for routine screening, or the hemorrhoidal disease is unimpressive

  4. Treatment
    1. Medical Therapy
      • most patients with first- and second-degree hemorrhoids are adequately treated with bulking agents, stool softeners, and increasing fluid intake

    2. Elastic Band Ligation
      • effective treatment for most cases of second- and third-degree hemorrhoids
      • can only be used on internal hemorrhoids – placement of elastic bands on anoderm or transitional epithelium is extremely painful
      • a single ligation may be done in the office every 2 weeks
      • rarely, severe pelvic sepsis (pain, fever, difficulty voiding) has been reported after this procedure

      Elastic Band Ligation
    3. Sclerotherapy
      • involves injection of a sclerosant (3% normal saline) into the internal hemorrhoid
      • useful in patients who are on anticoagulants or who are immunocompromised or coagulopathic
      • can treat all the hemorrhoids at one setting
      • no need to stop the anticoagulants
      • good short-term efficacy, but the recurrence rate is high

    4. Hemorrhoidectomy
      • indications include large third-degree hemorrhoids, fourth-degree hemorrhoids, acutely thrombosed hemorrhoids, gangrenous hemorrhoids
      • may be done as an open or closed technique
      • dissection must be superficial to the internal sphincter muscles
      • a bridge of intact skin and mucosa should be left between excised hemorrhoid sites to avoid anal stenosis
      • acutely thrombosed external hemorrhoids are best treated with excision not incision
      • complications include urinary retention (30%), fecal incontinence (2%), infection (1%), delayed hemorrhage (1%), and stricture (1%)

      Closed Hemorrhoidectomy
    5. Stapled Hemorrhoidopexy
      • involves a circular resection and anastomosis of a 1- to 2-cm ring of anorectal mucosa and submucosa
      • operation divides the vascular pedicles, resuspends the prolapsing tissue at the anorectal ring, preserves the vascular cushions (important for continence), and avoids painful perianal wounds
      • exact purse-string suture placement is crucial
      • if the suture is placed too deep, then the vaginal wall may be incorporated anteriorly; if the suture is placed too close to the dentate line, then severe intractable pain may result
      • procedure is associated with higher patient satisfaction than conventional hemorrhoidectomy, but the recurrence rates are also higher
      • cannot be used for external hemorrhoids

      Stapled Hemorrhoidectomy
  5. Special Problems
    1. Strangulated Hemorrhoids
      • injection of dilute epinephrine together with 150 to 200 IU of hyaluronidase aids in reducing edema and greatly facilitates the procedure
      • must take care to preserve viable anoderm

      Strangulated Hemorrhoids
    2. Pregnancy
      • hemorrhoids often exacerbated in the 3rd trimester
      • usual approach is conservative: Sitz baths, fiber, stool softeners
      • often resolve spontaneously after delivery
      • acutely thrombosed hemorrhoids can be operated on in the left lateral position

    3. Portal Hypertension
      • anorectal varices are seen in 80% of patients with portal hypertension, but only account for 1% of massive GI bleeding
      • direct suture ligation may be tried
      • TIPS procedure or portosystemic shunts may be required







References

  1. Sabiston 20th ed., pgs 1394 - 1402
  2. Cameron, 10th ed., pgs 190 - 194, 255 - 261
  3. UpToDate. Overview of Rectal Procidentia (Rectal Prolapse). Madhulika Varma, MD, Scott R. Steele, MD. Mar 05, 2019. Pgs 1 – 22.
  4. UpToDate. Surgical Approach to Rectal Procidentia (Rectal Prolapse). Madhulika Varma, MD, Scott R. Steele, MD. Sep 30, 2019. Pgs 1 – 38.
  5. UpToDate. Surgical Treatment of Hemorrhoidal Disease. David Rivadeneira, MD, Scott Steele, MD. Dec 03, 2019. Pgs 1 – 28.