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Patients’ symptoms of incontinence may vary depending on the state of fecal matter. Itis thought that this is the most common factor affecting continence.7 If patients are
continent of solid stool but not liquid or gas, maneuvers to change the consistency
may be enough to ameliorate symptoms and regain fecal control. The rectum acts
as a reservoir where stool accumulates (reservoir continence). Other possible contributing
factors to reservoir function include the adaptive compliance of the rectum,
differences in pressure patterns, and angulations between the rectum and anal canal,
which is due to continuous tonic activity of the puborectalis muscle.
The internal anal sphincter is the major contributor to the high-pressure zone. When
the external sphincter is paralyzed, resting anal pressure changes minimally, suggesting
that the internal sphincter is primarily responsible for resting anal continence.8
Control of the internal anal sphincter is thought to be a complex interaction between
the intrinsic and extrinsic neuronal systems and myogenic neurons.9,10 The external
anal sphincter also has continuous tonic activity at rest and even during sleep.11
Thus, the external sphincter is unique because other striated muscles are electrically
silent at rest. Postural changes and other increases in intra-abdominal pressure such
as sneezing, coughing, and the Valsalva maneuver increase the resting tone of the
external sphincter by an anal reflex. The second sacral spinal segment modulates the
external sphincter, which can be contracted voluntarily for 40- to 60-second periods.12
It was traditionally thought that nerve endings responsible for the determination of
the fecal state exist in the levator ani muscle outside the anal wall; however, Ruhl
and colleagues13 demonstrated that sacral dorsal roots contain some afferents from
low-threshold mechanoreceptors located in the rectal wall and that these afferents
monitor the filling state and the contraction level of the rectum.14 Sensation within
the anal canal is carried out by several types of sensory receptors, including free intraepithelial
nerve endings (pain), Meissner corpuscles (touch), bulbs of Krause (cold),
Pacini corpuscles and Golgi-Mazzoni corpuscles (pressure and tension), and genital
corpuscles (friction).15 Despite an extensive network of nerves within the anal mucosa,
anal continence does not rely heavily on input from these nerve endings. They are
thought to play only a minor role in discrimination between the states of fecal matter.
Thus, when this area is anesthetized, discrimination between solid and gas is
impaired; however, continence is maintained.16
Defecation
At rest, the aforementioned factors keep stool within the rectum. Once this reservoir is
distended, the stimulus for initiating defecation is sent. The resultant process of the left
colon initiating peristaltic waves that result in propulsion of the fecal mass downward
into the rectum occurs once or several times a day.17 Once the rectum is distended,
the internal sphincter relaxes (rectoanal inhibitory reflex) and the external sphincter
contracts maintaining continence. Squatting straightens the angle between the
rectum and the anal canal. Adding the pressure of a Valsalva maneuver overcomes
the resistance of the external sphincter and the pelvic floor descends. If the external
anal sphincter receives inhibiting signals causing relaxation, the fecal bolus passes.
Timing results from the balance of environmental factors acting through cortical
inhibition and basic reflexes of the anorectum.
PHYSIOLOGIC TESTING
Multiple techniques have been developed to assess the physiologic function of the
pelvic floor, rectum, and sphincters. In conjunction with a detailed history and physical
Anatomy and Physiology 9
sphincter and puborectalis striated and voluntary muscles are unique in that they
exhibit electrical activity at rest and even during sleep. It ceases only during defecation.
Traditional concentric EMG uses a probe that is inserted manually either into
the puborectalis or external anal sphincter (Fig. 9). Maneuvers, such as rectal balloon
distention, saline infusion or perianal pinprick, are then performed to elicit reflex
contraction of the sphincter. For a more specific definition of electronic function of
the sphincter, single-fiber EMG can be used. This technique can analyze both innervation
and reinnervation after injury to determine the number of fibers supplying 1
motor unit (fiber density). The latter has shown to be associated with primary
‘‘idiopathic’’ anal incontinence or secondary incontinence from neurologic
disorders.21
Defecography/MR Defecography
This technique uses a contrast agent, usually liquid barium suspension or paste, which
is placed within the rectum, and a series of radiographs or fluoroscopy are obtained.
Defecography can be used to investigate several anorectal abnormalities. It can
measure the anorectal angle, the position of the pelvic floor at rest or during Valsalva
(perineal descent), the presence of a rectocele, rectal intussusception, and function,
including the ability to expel rectal contents.22,23 Balloon proctography can simplify
the procedure of examining the ability to evacuate by providing a quick and clean
test with minimal radiation.24 In the largest series to date, when defecography was
performed for defecation disorders, 67% of patients had one abnormal finding (eg,
rectal intussusception, prolapse, rectocele) and 21% of patients had multiple
disorders.25 MRI technology has been added to the armamentarium of defecographic
techniques.26 It has shown excellent capabilities in diagnosing structural and
Fig. 9. Anorectal electromyelography. (A) Normal. (B) Puborectalis dysfunction. Adapted
from Gordon PH, Nivatvongs S. Principles and practice of surgery for the colon, rectum,
and anus. 3rd edition. Informa Healthcare; 2007; with permission.
functional disturbances, including those diagnosed with traditional defecography, and
the improved characterization of the perirectal soft tissues and surrounding structures.
This provides assessment of other abnormalities, including pelvic floor abnormalities
and descending perineum syndrome. All of this is completed without exposure to
harmful ionizing radiation. Unfortunately, there is extensive morphologic variability
among normal healthy individuals and interobserver variability.
Defecography can be used with other technologies to obtain more information on
anatomy and function. Simultaneous dynamic proctography and peritoneography
identifies rectal and pelvic floor pathologic conditions, such as hernia sacs, and pelvic
floor dynamics during defecation.27,28 When combined together, they provide a large
amount of information in the patient with obstructed defecation to determine which
patients may benefit from surgical intervention and those that are likely to need nonoperative
measures such as biofeedback.
Nerve Stimulation Techniques
Nerve stimulation can further characterize neuromuscular function, providing even
more precise identification of the anatomic site of the nerve (either proximal or distal)
or muscle lesions. Spinal nerves are evaluated when a stimulus electrode is placed
vertically across the lumbar spine. The induced response of the puborectalis or
external sphincter can be detected. The latency of the response can be measured,
and longer times are associated with anal incontinence. A similar technique can be
performed on the pudendal nerve to evaluate the external sphincter and periurethral
striated sphincter muscles (pudendal nerve motor latency). This device consists of 2
electrodes at the tip of a rubber glove and 2 recording electrodes at the base of the
glove (Fig. 10). The latency is again measured, and an increase can be associated
with multiple different disorders; it has been associated with worse outcomes after
overlapping sphincteroplasty in some series.
Ultrasound
Ultrasonography can evaluate anal sphincter integrity and augment manometry and
assess anorectal angles and puborectalis function. Ultrasonography evaluates
discontinuity in anal sphincters, indicating a prior injury that may be seen in up to
30% of postvaginal deliveries. The internal and external sphincters can be evaluated
separately. Various angles are measured with the patient at rest and during maximal
voluntary contraction of the puborectalis. Significant differences have been noted
between incontinent and normal patients. Ultrasonography does have the advantage
of avoiding exposure to radiation and allows for longer viewing time. Anal ultrasonography
relies on the operator for accuracy, but in experienced hands, it can be the
mainstay for anal anatomic investigations. In addition, it can provide information
regarding the presence and location of anorectal abscess and fistula and staging of
tumors.
Compliance
Rectal compliance refers to the amount of force required to distend the rectal wall.
Rectal compliance is measured by inserting an ultrathin polyethylene bag into the
rectum.29 Once in place, the bag is inflated to different volumes, and the pressures
from the rectal wall are measured. Multiple measurements are taken and are plotted
on a pressure-volume curve. The slope of this curve reflects the compliance of the
rectum. There are 3 phases of the compliance curve. The first phase corresponds
to the initial resistance and compliance of the rectal wall. The second phase is more
compliant as evidenced by the increased volume with pressure changes and
12 Barleben & Mills
represents ‘‘adaptive relaxation’’ of the rectal wall. The last phase represents the
terminal compliance of the rectal wall and is generally less compliant than the other
phases. Urge of defecation occurs during the second phase of compliance. Multiple
studies have analyzed the association between anorectal pathologies and rectal
compliance findings, and there is still controversy regarding its utility.30–32 This
technique is also highly variable because of variations in readings of the equipment,
variations in patient’s physiology, and interobserver variations in readings.33
SUMMARY
The anorectal area consists of a relatively small but complex region where multiple
anatomic and physiologic interactions occur to help aid continence and defecation.
A thorough understanding of the anatomy and the available testing modalities is
imperative to diagnose and treat the wide range of pathologic conditions that may
occur.
REFERENCES
1. Grigorescu BA, Lazarou G, Olson TR, et al. Innervation of the levator ani muscles:
description of the nerve branches to the pubococcygeus, iliococcygeus, and
puborectalis muscles. Int Urogynecol J Pelvic Floor Dysfunct 2008;19:107–16.
2. DiDio LJ, Diaz-Franco C, Schemainda R, et al. Morphology of the middle rectal
arteries. A study of 30 cadaveric dissections. Surg Radiol Anat 1986;8:229–36.
3. Kirschner MH. [Vascular anatomy of the anorectal transition]. Langenbecks Arch
Chir 1989;374:245–50 [in German].
4. Miscusi G, Masoni L, Dell’Anna A, et al. Normal lymphatic drainage of the rectum
and the anal canal revealed by lymphoscintigraphy. Coloproctology 1987;9:
171–4.
5. Bauer JJ, Gelernt IM, Salky B, et al. Sexual dysfunction following proctocolectomy
for benign disease of the colon and rectum. Ann Surg 1983;197:363–7.
Fig. 10. Gloves used in nerve stimulation. Adapted from Gordon PH, Nivatvongs S. Principles
and practice of surgery for the colon, rectum, and anus. 3rd edition. Informa Healthcare;
2007; with permission.
Anatomy and Physiology 13