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30 Benign Anorectal Diseases
References
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Ultrasound elastography of the prostate. A new technique for tumor detection. Ultraschall Med 21:8–15
2. Mueller MP, Stamos MJ, Cavaye DM et al (1992) Threedimensional transrectal ultrasound: preliminary
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4. West RL, Dwarkasing S, Felt-Bersma RJF et al (2005)
Hydrogen-peroxide-enhanced three dimensional
endoanal ultrasonography and endoanal magnetic
resonance imaging in evaluating perianal fistulas:
agreement and patient preference. Eur J Gastroenterol
Hepatol 16:1319–1324
5. Parks AG, Gordon PH, Hardcastle JD (1976) A classification of fistula-in-ano. Br J Surg 63:1–12
6. Buchanan GN, Bartram CI, Williams AB et al (2005)
Value of hydrogen peroxide enhancement of threedimensional endoanal ultrasound in fistula-in-ano.
Dis Colon Rectum 48:141–147
SECTION III
State of the Art in Pelvic Floor
Imaging
Puborectalis
Axis of rectum
Axis of
anal canal
III.1.
Introduction
G.A. Santoro, G. Di Falco
The imaging of pelvic floor structures is presently of great interest. In the last two decades, growing attention has been dedicated to increasing
both understanding on the pelvic floor anatomy
(particularly related to physiology and pathophysiology) and improving technologies for diagnosis. Endoluminal ultrasonography (EUS) and
magnetic resonance imaging (MRI) have become
an important part of the diagnostic workup in
pelvic floor dysfunction [1, 2].Their contributions
should be effectively integrated with other tech-
niques (i.e., endoscopy, anorectal manometry and
electromyography, evacuation proctography) for
a complete assessment of the main pathologic
conditions of the pelvic floor and to plan the best
form of treatment.
The advantage of EUS is that it is inexpensive
and widely available; however, similar to all ultrasound methods, EUS is operator dependent.
Despite the fact that intraobserver and interobserver agreement has been reported in the literature as good or very good [3],measurement of the
different anal structures did not provide homogeneous morphometric results [4, 5]. Many debates
have centered around who should perform EUS
examinations: colorectal surgeons, gastroenterol-
ogists, or radiologists. We are persuaded that the
operator’s experience is the most relevant factor,
irrespective of specialty.
The current 360° rotating endoprobe, specifical-
ly designed for anorectal scanning, has provided
important information to a detailed understanding
of the anatomy of this region [6]. The increasing
interest in endoanal (EAUS) and endorectal
(ERUS) ultrasonography, accomplished with a
wider spread in using these procedures, has
allowed the definition of clinical indications and
the field of applications. The ambitious aim of this
diagnostic tool is to correctly identify very small
and thin structures, with no precise interfaces and
limits with the adjacent structures, which often
cannot be visualized or measured with conventional techniques. The influence of age, gender, parity,
obstetric trauma, body weight, height, and a number of other incompletely understood factors on
variability of anorectal anatomy has for a long time
led to significant confusion and conflicting results.
Both EUS and MRI have contributed to modify
previous knowledge of anorectal anatomy and adequately correlate imaging with pelvic floor dysfunction [7–9]. Significant improvement in reducing investigational problems has been recently
obtained by using more sophisticated devices [i.e.,
three-dimensional (3-D) acquisition systems,
probe pull-through systems, and the newer probe
with integrated 3-D and pull-through devices],
which allow evaluation of the anal canal and rectum in a variety of projections, including the transverse, sagittal, and coronal planes, and all the possible diagonal views. Measurement of linear distance, thickness, and volume are readily available.
However, considering both diagnostic applications and potential pitfalls of EAUS and ERUS, it
is mandatory to standardize as much as possible
the equipment used, technique of examination,
manner of performing measurements, and definitions and subjective interpretations. By minimizing the effect of these confounding variables, different investigators will be able to communicate
and compare results.
The purpose of this section is to describe the
normal anatomy of the anal canal and rectum by
means of EUS and MRI. Attention will be given to
the more recent acquisition in pelvic floor imaging.
34 Benign Anorectal Diseases
References
1. Stoker J, Halligan S, Bartram CI (2001) Pelvic floor
imaging. Radiology 218:621-641
2. Stoker J, Rociu E, Zwamborn AW et al (1999)
Endoluminal MR imaging of the rectum and anus: technique, applications and pitfalls. Radiographics 19:383–398
3. Gold DM, Halligan S, Kmiot WA, Bartram CI (1999)
Intraobserver and interobserver agreement in anal
endosonography. Br J Surg 86:371–375
4. Enck P, Heyer T, Gantke B, Schmidt WU et al (1997)
How reproducible are measures of the anal sphincter
muscle diameter by endoanal ultrasound? Am J
Gastroenterol 92:293–296
5. Beets-Tan RGH, Morren GL, Betts GL, Kessels AGH et
al (2001) Measurement of anal sphincter muscles:
endoanal US, endoanal MR imaging, or phased-array
MR imaging? A study with healthy volunteers.
Radiology 220:81–89
6. Dalley AF (1987) The riddle of the sphincters. The
morphophysiology of the anorectal mechanism
reviewed. Am Surg 53:298–306
7. Sultan AH, Kamm MA, Hudson CN et al (1993) Analsphincter disruption during vaginal delivery. N Engl J
Med 329:1905–1911
8. Hussain SM, Stoker J,Lameris JS (1995) Anal sphincter
complex: endoanal MR imaging of normal anatomy.
Radiology 197:671–677
9. Schafer A, Enck P, Furst G, Kahn T et al (1994)
Anatomy of the anal sphincters. Comparison of anal
endosonography to magnetic resonance imaging. Dis
Colon Rectum 37:777–781
III.2.
Endosonographic Anatomy
of the Normal Anal Canal
G.A. Santoro, G. Di Falco
The anal canal is 2- to 4-cm long. The dentate line
of the mucosa denotes the squamocolumnar junction. The circular smooth muscle of the rectal wall
continues downward as the internal anal sphincter (IAS) that extends from the anorectal junction
Longitudinal muscle of rectum
Circular muscle of rectum
Levator ani muscle
Anorectal ring
Deep external sphincter
Internal hemorrhoidal vein
to approximately 1 cm below the dentate line
(Fig. III.1) [1]. The outer longitudinal component
of the muscularis propria conjoined with striated
muscle fibers from the levator ani, particularly the
puboanalis (Fig. III.2), and a large fibroelastic ele-
Superficial external
sphincter
Internal sphincter muscle
External hemorrhoidal
venous plexus
Subcutaneous external
sphincter muscle
Fig. III.1. Normal anatomy of the anal
canal. The muscularis propria of the rectal wall consists of both circular and longitudinal smooth muscle fibers. The circular layer is in continuity with the circular internal anal sphincter muscle. The
longitudinal layer extends into the intersphincteric space of the anal canal. The
external sphincter extends further down
than the internal sphincter
36 Benign Anorectal Diseases
LM
PA
CLL
MSA
a
Coccygeus
Iliococcygeus
PA
b
Fig. III.2. The puboanalis (PA) rises from the medial border
of the puborectalis (PR)
ment derived from the endopelvic fascia, extends
caudally as the conjoined longitudinal layer (CLL)
between the external and internal anal sphincters
and terminates at the anorectal junction [2–5]
(Figs. III.3 and 4). Its fibroelastic component permeating through the subcutaneous part of the
external sphincter terminates in the perianal skin.
Konerding et al.[6], however, failed to detect striated muscle fibers within the longitudinal muscle
that was solely composed of smooth muscle cells
S3
S4
S5
Fig. III.3. The puboanalis (PA ) joins the longitudinal muscle
(LM) of the rectum to form the conjoined longitudinal layer
(CCL). Fibers from the LM run through the internal anal
sphincter to form the muscularis submucosae ani (MSA)
and connective tissue. Part of the longitudinal
muscle extends down into the upper part of the
anal canal through the internal sphincter to form
the muscularis submucosae ani [2–4] (Fig. III.3).
The external anal sphincter (EAS) is made up of
voluntary muscle from the levator ani and puborectalis muscle to form a cylinder of muscle that
encompasses the internal sphincter. The anatomy
of the EAS remains controversial [7]. It is
described as having three parts (Fig. III.5):
1. The deep part is integral with the puborectalis. Posteriorly, there is some ligamentous
attachment; anteriorly,some fibers are circular
and some decussate into the deep transverse
perineii.
2. The superficial part has a very broad attachment to the underside of the coccyx via the
anococcygeal ligament. Anteriorly, there is a
division into circular fibers and a decussation
to the superficial transverse perineii.
Section III • State of the Art in Pelvic Floor Imaging 37
Deep
Superficial
Subcutaneus
Fig. III.4. Diagram of the perineal muscles. IAS internal anal
sphincter,EAS external anal sphincter, LM longitudinal muscle, LL longitudinal layer (with permission from [3])
3. The subcutaneous part lies below the IAS.
The anterior part of the external sphincter differs between genders. In males, it is symmetri-
Fig. III.5. The external anal sphincter is subdivided into three
parts: deep, superficial, and subcutaneous
cal at all levels; in females, it is shorter anteriorly, and there is no evidence of anterior ring
high in the canal [8] (Fig. III.6).
a
Fig. III.6. Schematic representation of the external anal sphincter in male (a) and female (b). Anteriorly,the sphincter is short-
er in the female
b
38 Benign Anorectal Diseases
a
Obturator
internus
muscle
Ischial
tuberosity
Piriformis
Puborectalis
muscle
muscle
Iliococcygeus
muscle
Sacrotuberous
ligament
Piriformis
muscle
Ischiococcygeus
muscle
Arcus tendineus
lavator ani
Urethra
c
Vagina
Anus
Ischial
tuberosity
Coccygeus
Iliococcygeus
Pubococcygeus
Puborectalis
Anococcygeal raphe
Urogenital
diaphragm
Levator ani
Iliococcygeus
muscle
Pubococcygeus
muscle
b
Fig. III.7. Schematic representation of
the pelvic anatomy (a-c)
Section III • State of the Art in Pelvic Floor Imaging 39
The levator ani, subdivided in the iliococcygeus, pubococcygeus, and puborectalis muscles, is the fundamental structure of the pelvic
floor,arising from the side wall of the pelvis, supporting the pelvic contents, and separating the
ischioanal fossa below from the supralevator
space above [9] (Fig. III.7). The iliococcygeus lies
posterolaterally, arising from the ischial spine to
insert into the coccyx and anococcygeal ligament. The ischiococcygeus is a small, often rudimentary, subdivision of this. The pubococcygeus
arises from the pubic bone, along with the puborectalis, and from a tendinous arch formed by
obturator fascia running posteriorly toward the
ischial spine (Fig. III.7).Pubococcygeal fibers run
posteriorly toward the coccyx in a plane just cranial to the iliococcygeus. Fibers also cross the
midline to form rectal and vaginal hiatus. The
puborectalis arises from the pubis forming a distinct sling around the anorectal junction
(Fig. III.8).
The perineal body (also named the central
perineal tendon) anatomically is a junctional zone
where fibers from the puborectalis, the external
sphincter, the longitudinal muscle, and the internal sphincter decussate and fuse into muscles of
the anterior urogenital triangle, notably, the deep
and superficial transverse perineii and bulbospongiosus muscles (Fig. III.9). Such anatomic
configuration gives to the perineal body a fundamental function to support all musculoligamentous components of the pelvis, anchoring the anal
canal to the ischial and pubic bones [10]
(Fig. III.10). The lack of these connections could
be one of the most important reasons of a perineal descent, determining also damage to the
pelvic nerves (i.e., pudendal nerves) and muscles
(i.e., levator ani) and disconnecting functionally
the anterior from posterior perineum. Perineal
body damage could predispose to anterior or posterior perineal prolapse of the pelvic organs [11].
The anatomy of the perineal body differs between
genders.In males, it is smaller and posterior to the
spongious bodies; in females, it lies within the
anovaginal septum [10] (Fig. III.10).
The area around the anorectum is divided into
spaces (Fig. III.11). The perianal space surrounds
the lower anal canal.Lateral to the sphincter is the
ischioanal fossa, which is bounded laterally by the
obturator internus muscle and superiorly by the
levator ani muscle. The intersphincteric space is
not a recognized anatomical term but is used to
describe the area between the internal and the
external sphincter. The supralevator space lies
above the levator ani muscle and is demarcated
superiorly by the pelvic peritoneum.
Endosonographic Anatomy
Most studies that highlighted a better comprehension of the sonographic anatomy of the anal canal
and pelvic floor have emerged from Professor
Clive Bartram and his group from the Intestinal
Imaging Centre at St. Mark’s Hospital in London.
According to these studies, the anus is fundamentally a four-layer structure (Fig. III.12) [3, 4,12–17].
From inner to outer, these are:
1. Subepithelial tissues: moderately reflective.The
mucosa as well the level of dentate line is not
visualized. The muscularis submucosae ani
can be sonographically identified in the upper
part of the anal canal as a low reflective band
(Fig. III.13).
2. Internal anal sphincter: hypoechoic
(Fig. III.14). The sphincter is not completely
symmetric, either in thickness or termination
(Fig. III.15). In older age groups, the sphincter
becomes thicker and loses its uniform lowlevel echogenicity, which is characteristic of
smooth muscle throughout the gut,to become
more echogenic and inhomogeneous in texture [3, 13] (Fig. III.16). Although it seems normal and without lesions, sometimes the IAS
has differences in echogenicity and thickness.
Recently, an increasing interest in IAS degeneration has occurred. In such a condition (first
described by using EAUS), the IAS appears
intact but thinner than normal and hyperechoic; it has been regarded as a cause of passive fecal incontinence [14].
3. Longitudinal muscle: hyperechoic. This muscle
is moderately echogenic, which is surprising,
as it is mainly smooth muscle. However, an
increased fibrous stroma may account for this
(Fig. III.17). The intersphincteric space, in
which the longitudinal muscle is located, presents a wide variability in thickness and is not
always distinctly visible along the entire anal
canal. However, the ability to identify this
structure in normal subjects, differentiating it
from the IAS and EAS, could be useful in
assessing diseases involving the intersphincteric space. The puboanalis is seen as a lowreflective, triangular-shaped band of muscle
40 Benign Anorectal Diseases
Puborectalis
Axis of rectum
Axis of
anal canal
just medial to the puborectalis [3, 4, 13]
(Fig. III.18). By using three-dimensional
EAUS, it is possible to obtain excellent images
of the conjoined longitudinal layer
(Fig. III.19).
4. External anal sphincter: mixed echogenicity.
Endosonography largely overestimates the
size of the EAS due to its failure to recognize
and separate the CLL. The EAS and the CLL
contain large amounts of fat and fibrous tissue,
Ischiocavernosus
muscle
Vagina
Ischiopubic ramus
Fig. III.8. The puborectalis muscle
swings behind the anal canal at its most
proximal limit to encircle the sphincter
posteriorly
which lead to similar echogenicities of both
structures [18] (Fig. III.20).
Ultrasound imaging of the anus can be divided into three levels:high, mid, and low portions [3,
19] (Fig. III.21). The level refers to the following
anatomical structures:
1. High: the sling of the puborectalis and the
deep part of the external sphincter;
Inferior fascia of
urogenital diaphragm
Superficial transverse
perineal muscle
External sphincter
of anal canal
Anococcygeal ligament
Bulbocavernosus
muscle
Gluteus
maximus
muscle
Levator ani
muscle
Fig. III.9. Bulbospongiosus muscle,
transverse perineal muscles, and
external anal sphincter meet in the
perineal body
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