Keywords

FormalPara Key Concepts

  • The innervation of the anal sphincter complex is a mixed sympathetic and parasympathetic crossed-over system that provides redundant safeguards to continence.

  • Normal continence and defecation require intact sensation and motor control and reflexes to sense, retain, and voluntarily expect the rectal contents at a socially appropriate time and place.

  • The normal physiology of the anus can be disturbed in a variety of ways resulting in lack of control, inability to expel, or chronic pelvic pain.

  • The process of childbirth can contribute significantly to alteration in anorectal anatomy and physiology resulting in a variety of disorders of defecation and/or incontinence.

Introduction

  • The physiology of the anus and its surrounding structures is the physiology of continence and controlled defecation.

  • Normal continence requires a balance between the pressure inside the rectum and the combined tone of the internal and external sphincters.

  • Defecation and the controlled passage of gas or stool at socially appropriate circumstances required very fine sensation and ability to discern the rectal contents. It requires the balance to tip in favor of the rectal pressure and contraction with simultaneous coordinated relaxation of the pelvic floor and internal and external sphincters.

  • Disturbance in any part of this complex balance can result in incontinence either through reduced anal tone, excess rectal contraction, reduced sensation, or the inability to differentiate the consistency of the rectal contents.

  • Disorders tipping in the opposite direction may result in inability to properly or completely empty the rectum.

  • More proximal conditions resulting in chronic diarrhea or constipation may tip the balance. Forces even higher can contribute to the behavioral and psychosocial aspects of ordered and disordered function of the rectum and anal canal.

Normal Anatomy and Physiology

  • The internal sphincter begins as a condensation of the inner circular involuntary smooth muscle of the GI tract at the top of the surgical anal canal, as the top of the anorectal ring. It extends downward to just proximal to the end of the external sphincter.

  • The length of the normal internal sphincter is 2–4 cm. It appears as a hypoechoic band 2–3 mm in diameter on transanal ultrasound imaging.

  • The internal sphincter is chronically contracting and contributes approximately 50–75% of the resting tone of the anus.

  • The external sphincter is a cylinder of striated muscle that extends downward from the levator ani muscle to the distal anoderm.

  • It exists in a chronically contracting state, but when stimulated under voluntary control, it more than doubles the tone of the anus above the resting state.

Innervation of the Anus and Pelvic Floor

  • The parasympathetic fibers to the rectum and anal canal emerge from the sacral foramina at the S2, S3, and S4 levels. They join the sympathetic hypogastric nerves in the pelvic plexus. From there mixed postganglionic fibers extend to the lower rectum and anal canal.

  • The internal sphincter is innervated by L5–S4 mixed autonomic function in crossed fashion so that unilateral injury still results in preserved function.

  • The external sphincter is similarly innervated from branches of S2–S3 via the inferior rectal branch of the pudendal nerve and the perineal branch of S4. This nervous distribution also carries the nerves of sensation and contributes to the functional aspects of continence.

  • The upper anal canal contains a high density of free and organized sensory nerve endings including Meissner’s corpuscles (touch), Krause’s bulbs (cold), Golgi-Mazzoni bodies (pressure), and genital corpuscles (friction).

Normal Continence

Rectal Capacity

  • Normal continence first requires a location to temporarily hold and assess the contents and expel them under control. The empty rectum is a low-pressure vessel with the capacity to receive stool from the sigmoid and to accommodate stool under pressure.

  • Patients with diminished rectal capacity will suffer from fecal frequency and urgency and frequently may contribute to incontinence.

Pressure and Motility

  • Baseline pressure in the rectum is low, about 5 mmHg with frequent low amplitude contractions every 6–12 s. Occasional high-pressure waves up to 100 mmHg have been demonstrated.

  • Pressure in the anal canal ranges 10–14 times that of the rectum.

  • Motor activity is more frequent, and contractile waves are of higher amplitude in the rectum than in the sigmoid producing a reverse gradient that resists the forward motion of stool.

  • Slow waves are observed in the anal canal with increasing frequency distally and help maintain continence by propelling the contents back into the rectum.

Rectoanal Sensation and Sampling

  • The conscious sensation of the need to defecate lives in the levators and the anal canal. Distention of the rectum triggers contraction of the external anal sphincter and significant internal anal sphincter relaxation – the rectoanal inhibitory reflex (RAIR).

  • It allows the highly innervated sensitive epithelial lining of the upper anal canal to sample the contents of the distal rectum to determine its quality and consistency.

  • Impaired anal sensation has been associated with childbirth, perineal descent, and mucosectomy .

Structural Considerations

  • In addition to the baseline resultant tone provided by the anal sphincter complex and the puborectalis sling, the entire structure is held close by the angulation created by the puborectalis in its chronically contracted unstimulated state.

  • This angle between the axis of the anus and the axis of the rectum is between 80° and 90° and is responsible for the majority of gross fecal continence. It may increase normally above 90 while sitting and will extend beyond 110° during normal defecation.

  • The flap valve theory advocated by Parks suggests the anterior rectal mucosa constitutes a flap that lies over the upper end of the anal canal. Increased inter-abdominal pressure not associated with defecation increased the angulation and closes flap more firmly over the upper anal canal. The flap is opened when the perineum descends and the anorectal angle is straightened.

Role of Hemorrhoids in Normal Continence

  • It has been postulated that the normal function of the hemorrhoids is as an additional important component of normal continence. These vascular cushions have the ability to expand as needed to create a seal above the anus creating the fine-tuning of continence.

Sensation and Innervation

  • The rectum has a mixed sympathetic and parasympathetic innervation derived from the hypogastric nerves and the sacral parasympathetic nerves through the pelvic plexi.

  • Extrapelvic innervation comes to the anus from the pudendal nerve derived from S2 to S4 via the inferior rectal nerve and ultimately spreads around the anus from both sides entering at lateral to slightly anterior positions.

  • There is significant crossover innervation around the anus as a complete disruption of either pudendal nerve does not result in asymmetric sphincter atrophy or fecal incontinence.

  • Sensory innervation within the rectum is sensitive only to stretch, resulting in vague sensation to visceral pelvic pain.

  • Distal rectal stretch or distention can result in significant parasympathetic stimulation of the vagus nerve, thereby resulting in bradycardia and hypotension.

  • Somatic sensory innervation begins in the anal transitional zone proximal to the dentate line for a short variable distance of 0.3–1.5 cm. Within this zone, there is a dense collection of nerve endings for pain, touch, pressure, and temperature. These fibers are derived from the pudendal branches , and complete anesthesia to this area can be provided by bilateral anal nerve blockade.

Normal Defecation

  • Normal defecation is a complicated mechanism that relies on a close interaction between the somatic and autonomic nervous systems and includes the conscious and unconscious control of both sensory input and muscle contraction.

  • Stool arrives in the rectum and is sampled. If it is not an appropriate time for defecation, the anal sphincter will contract, and rectum will start to distend.

  • This process continues with progressive distention of the rectum without a person’s full awareness, but conscious sampling is also present.

  • As the rectum continues to expand, a person becomes aware, and there is an urge to defecate that usually lasts for a few seconds and can be controlled by further contraction of the external anal sphincter (efferent nerve endings end in lumbosacral spine which is under higher control that allows conscious suppression of the urge).

  • When it becomes socially appropriate to proceed, the defecation process again relies on both conscious and unconscious responses.

  • The process starts with contraction of abdominal musculature (Valsalva), which is also associated with contraction of the sigmoid colon to move stool forward.

  • A combination of relaxation of the puborectalis (releases sling around anorectal junction) and levator muscle allows the pelvic floor to descend slightly and straighten the anorectal angle.

  • The rectum itself starts to contract, and both internal and external sphincters relax, and at this point pressure in the rectum exceeds pressure in the anal canal, and defecation will occur.

  • This process can also be aided by assuming the squatting position, which increases the intra-abdominal pressure and straightens the rectum further.

  • Once begun a number of patterns can occur. There may be a single evacuation of the rectal contents accompanied by mass peristalsis of the left and sigmoid colon clearing the bowel in one continuous movement or the passage of smaller volumes of stool individually over a short time requiring recurrent efforts and straining.

  • If a large volume of stool is delivered quickly to the rectum, normal rectal compliance and accommodation may be insufficient. In this case the patient with normal sensation and function will have a sense of acute urgency and can forestall defecation for 40–60 se with the use of voluntary contraction of the external sphincter to allow accommodation or move to a socially appropriate location to evacuate.

Physiology of Tibial Nerve and Sacral Nerve Root Stimulation in Fecal Continence (FI)

  • Chronic electrical stimulation of nerves entering the pelvis has effects of visceral function and activity.

  • Unilateral stimulation of the S3 or S4 nerve as it exits the foramen has been used for urinary incontinence for over 30 years, during which time benefits for fecal incontinence were recognized as well – though the mechanism of how sacral nerve stimulation (SNS) creates its effect remains unclear.

  • Similarly, intermittent stimulation of the posterior tibial nerve has a beneficial effect on fecal incontinence through a mechanism that is not fully understood.

  • The following is a summary of findings related to SNS:

    • SNS has no demonstrable effect of rectal compliance or motility.

    • It seems to reduce hypersensitivity in those with reduced capacity and hypersensitivity while increasing sensitivity in those patients with reduced sensitivity.

    • It increases mucosal blood flow when on and returns to baseline when off, and there are higher levels of the neuropeptide substance P identified in rectal biopsies of those undergoing stimulation, which reverses after it is discontinued.

    • Forty studies have examined changes in anal sphincter function through the use of anorectal manometry. Fourteen studies reported a significant increase in voluntary anal squeeze, with eight of these also reporting an increase in resting pressure.

Spinal Cord Injuries and Defecation

  • Patients with spinal cord injuries are a very heterogeneous group of patients with degrees of injury that can vary significantly from patient to patient.

  • High spinal cord injuries (above T7) interrupt higher control and sensation of the abdominal and pelvic floor musculature as well as the colon in the rectum resulting in lower tone in the colon and rectum. Constipation in these patients is multifactorial: (1) slowed transit due to decreased propulsive ability of the colon, (2) inability to generate adequate intra-abdominal pressure or take squatting position to aid defecation, (3) unopposed stimulation of the lower neurons that increase contraction and spasticity of the pelvic floor, and (4) impaired sensation. They often rely on a strict bowel program, which is a combination of laxatives, rectal stimulation, and manual disimpaction. Rectal stimulation can allow some patients to have decreased anal sphincter pressure.

  • Patients with low spinal cord injuries such as cauda equina syndrome often have impaired afferent fibers that results in loss of tone in the internal and external sphincter muscle as well as impaired sensation. This can result in significant incontinence since any generation of intra-abdominal pressure may result in bowel movement.

Obstructed Defecation

  • Obstructed defecation is a poorly understood group of disorders resulting from an alteration in sensation, muscle relaxation, or both. Some causes of abnormal sensation can be fairly evident in patients such as those with significant proctitis (infectious or inflammatory) or those after anorectal injury/surgery.

  • Dysfunction may be associated with conscious/subconscious inhibition of the need to defecate during childhood. According to this theory, repeated delays in defecation result in altered sensation that eventually leads to dyscoordination between the anorectal and pelvic floor musculatures. Changes in sensation cause an increase in stimulation of lower (lumbosacral) neuronal loop (the relaxing effects of the upper parts of the nervous system), which are insufficient to overpower the abnormal stimulation. Once this occurs, and pelvic floor musculature such as puborectalis and sphincter complex fail to relax appropriately, increasingly higher intra-abdominal pressure is needed to overpower the rectal/anal pressure to evacuate.

  • Over time there is damage to the sensory pathways which eventually affect the structural integrity of the pelvic floor. Obstructed defecation disorders include intussusception, rectocele, non-relaxing puborectalis/levator muscle spasm, dyssynergic puborectalis, as well enterocele and rectal prolapse.

  • Intussusception is mucosal descent causing blockage of the lower rectum/anal canal. It is possible that it is a primary process in some patients arising from redundancy of mucosa, poor tone, and pelvic floor descent (either primary structural problems or as a result of childbirth and muscle/nerve damage in women). In most patients it is likely a secondary process resulting from increased pushing and decreased relaxation. Once developed, intussusception itself generates mechanical blockage to defecation and further attempts to generate more pressure to evacuate stool.

  • Rectocele is defined as greater than 2 cm of rectal wall outpouching or bowing anteriorly while straining. Rectoceles are caused by abnormal relaxation of the pelvic floor/sphincter complex or structural defects in the rectal wall created during childbirth. During evacuation generated pressure delivers stool anteriorly toward the weakened portion of the wall that is not contracting appropriately causing a sensation of bulge and incomplete evacuation (Fig. 3.1). Most symptomatic patients likely have a combination of a weaker rectal wall as well as dyssynergy of the sphincters or puborectalis.

  • Pelvic floor dyssynergy (pelvic outlet obstruction) results from a failure of the puborectalis and/or sphincter complex to relax or abnormal contraction. During attempts to evacuate, the anorectal angle may not increase or may even become sharper. A patient’s natural response is to generate higher pressures in which only further worsens the symptoms. Over time, these changes likely cause more damage to the musculature and nerves. Similar to the rest of the disorders in this group, rectal sensation is also impaired, but whether it is a result of long-term damage or from an inciting event is unclear.

Fig. 3.1
figure 1

Defecography still image of a rectocele

Full size image

Functional Anorectal Pain

  • There is a small group of pain disorders that are related to more functional rather than structural problems.

  • Levator ani syndrome (levator spasm, puborectalis syndrome) is often described as a dull pain high in the rectum that is often made worse with sitting. Some episodes may be triggered by difficult defecation. Alternations in sensation or behavioral factors (deferring defecation, damage with hard stool) contribute to its development. Prolonged muscle contraction may result in compression of vasculature and relative ischemia leading to activation of nociceptors in the muscle (bradykinin, substance P).

  • Proctalgia fugax is a sudden severe anal pain of unknown etiology, lasting seconds to minutes, that disappears completely. It is associated in some patients with a thickened internal sphincter muscle. Some studies suggest smooth muscle contraction is responsible for this pain.

Pathophysiology of Obstetric-Related Problems

  • One of the worrisome potential sequelae of pregnancy and delivery is fecal incontinence which may develop as a result of direct disruption of the anal sphincter, muscle, and connective tissue or pudendal nerve injury.

  • Progesterone released during pregnancy causes decreased gut motility and diminished tonic contraction of anal sphincters.

  • Androgen, progesterone, and estrogen receptors are found in the squamous epithelium of the anal canal, indirectly supporting possible effects of this hormone on the sphincters.

  • Progesterone causes ligamentous laxity that, when combined with increased intra-abdominal pressure, contributes to stretching of the pelvic floor musculature, widening of the levator hiatus, and potentially pudendal nerve injury. Pudendal nerve injury can affect the anal sphincters by de-innervating them and causing muscle atrophy as well as by affecting sensory components and altering RAIR. Evidence of neuropathy in the pelvic floor musculature has been found after delivery as well as in idiopathic FI and constipation.

  • Labor further complicates issues of continence with further muscle stretching or even evulsion and pudendal nerve injury. A longer second stage of labor (pushing) is associated with higher rates of FI later in life.

  • Use of additional devices to aid labor such as forceps and vacuum is associated with increased incidence of FI.

  • Tearing and episiotomy are additional risk factors for FI and related to direct damage to the sphincter complex.

  • Cesarean section is associated with lower incidences of flatus and stool incontinence, but this difference is smaller when comparing emergent cesarean sections and vaginal deliveries.

  • Emergent cesarean is often initiated after failure of labor to progress following significant pushing.

Urogynecological Considerations and Pelvic Pain

  • The pelvic floor is anatomically a very small area that includes the pelvic musculature and its corresponding nerves responsible for maintenance of continence and normal defecation as well as normal urinary gynecologic function. Dysfunction in any single system is common, but more than one system is frequently affected.

  • Physiologic and muscular changes associated with pregnancy and labor which affect the posterior compartment often have similar effects on middle and anterior compartment structures as well.

  • Uterine prolapse and urinary problems are more common in multiparous women than nulliparous ones. The mechanism for these issues is a combination of hormonal effects as well as direct damage to the pelvic floor muscle, nerves, and sphincters.

  • Widening of the levator hiatus has been shown to affect middle and anterior compartments as well as posterior one. This can result in uterine and bladder prolapse in addition to rectal prolapse, intussusception, and rectocele.

  • Pregnancy and delivery effects on anal sphincters can affect urinary sphincters as well, and it is common for women presenting with urinary incontinence to report fecal incontinence as well.

  • Urogynecologists see and treat a number of patients with anorectal problems, and treatments available are similar between specialties (e.g., pelvic floor physical therapy, sacral nerve stimulation).

  • Pelvic floor prolapse problems may contribute to obstructed defecation. Failure to diagnose concomitant middle and anterior compartment problems may compromise success of treatment of posterior compartment dysfunction.