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Section VI • Update in the Evaluation of Outlet Obstruction 245
Fig. VI.35. The 0.5 T superconducting, open-configuration
magnetic resonance system used for dynamic pelvic examinations.A wooden chair that functions as a toilet allows imaging in the sitting position
Dynamic Pelvic MRI in the Supine
Position
Dynamic pelvic MRI may also be performed in the
supine position using nearly all commercially
available closed- or open-configuration MR systems with horizontal access. When dynamic pelvic
MRI is performed in these MR systems, the patient
is placed in the supine position,and a pelvic phased
array coil is used for signal transmission and/or
reception.After filling the rectum, the examination
starts with a localizer sequence similar to that
described above. For dynamic MRI in the different
positions, various MR sequences can be used with
similar results. The basic requirement for the
sequence is the necessity for a fast imaging update.
Some authors have used T2-weighted single-shot
fast-spin-echo sequences (SSFSE) in the midsagittal plane obtained at rest,at squeezing, at straining,
and during defecation [11, 22–24]. Alternatively,
steady state free precession (SSFP) sequences may
be used for this purpose. Our current protocol
includes SSFP sequences obtained at rest, at
squeezing, straining, and after evacuation. For
assessment of defecation, we prefer the use of a T1weighted multiphase GRE sequence since it offers
an imaging window long enough for continuous
imaging the evacuation even in patients with a prolonged evacuation time. The choice of the enema
depends on the MR sequence used for dynamic
imaging of the pelvic floor. For SSFSE and SSFP
sequences, ultrasound gel is most suitable [10, 13,
22, 23]. If the dynamic sequences are performed
with some sort of T1 weighting, the rectum is filled
with an enema consisting of ultrasound gel or
mashed potatoes mixed with a small amount of
gadolinium chelate, as described above.
MR Findings in Patients with Outlet
Obstruction
Rectocele
An anterior rectocele is the most frequent
anatomical abnormality in patients with pelvic
floor disorders and is defined as a rectal wall protrusion or bulging during defecation. The anterior wall is most commonly involved, but a rectocele may also be located in the posterior rectal
wall (Fig. VI.36). Posterior rectoceles are also
referred to as a posterior perineal hernia by some
authors based on the fact that the bulging is
through a puborectalis muscle defect [25, 26].
Rectoceles are more frequently found in
women. The pathogenetic mechanisms include
chronic straining during evacuation and weakness of the rectovaginal septum (congenital or
after an obstetric trauma). Although the clinical
importance of a rectocele is still debatable since
the finding was demonstrated in 20% of asymptomatic women [27], it is recognized that large
rectoceles (i.e., >2 cm in sagittal diameter) may
result in outlet obstruction, and the retention of
stool leads to the need of digital maneuvers in
order to empty the rectum [27, 28].A clinically significant rectocele should be considered based on
the following criteria [29]:
• Patient history
• Size exceeding 2 cm in sagittal diameter
• Retention of contrast medium
• Reproducibility of the patient’s symptoms of
outlet obstruction
• The need for evacuation assistance.
The physical examination detects most of the
rectoceles, but valuable information such as size
and emptying are not provided based on physical
examination. Therefore, dynamic pelvic imaging
is helpful in providing this information. Dynamic
MRI enables an accurate assessment of size, loca-
246 Benign Anorectal Diseases
a b
Fig.VI.36. A 42-year-old woman with clinical symptoms of outlet obstruction. The examination was performed with the patient
in supine position using a closed-configuration conventional magnetic resonance (MR) system. T1-weighted spoiled gradientecho MR image during defecation enables the identification of an anterior (arrow) and posterior (empty arrow) rectocele (a).
Steady state free precession image obtained after evacuation shows a complete evacuation of both rectoceles (b)
tion, and degree of emptying of a rectocele, which
are important factors for treatment strategy.
Using dynamic pelvic MRI, an anterior rectocele
may be classified with regard to its size, expressed
as the depth of wall protrusion beyond the
expected margin of the normal anterior rectal
wall, into small (<2 cm), moderate [2–4 cm
(Fig. VI.37)], and large (>4 cm) [17]. In addition,
rectoceles are classified into those with complete
evacuation (Fig.VI.36) and those with incomplete
evacuation (Fig. VI.37) depending on the contrast
material retention at the end of defecation.
Treatment decision in patients with rectocele
highly depends on associated imaging findings. It
is known that anterior rectocele as a solitary find-
ing is rare [19]. Anismus, internal rectal prolapse,
and enterocele are often associated with the presence of a rectocele [30–32], and therefore, the
treatment should be tailored according to the
imaging findings in order to achieve an optimal
outcome [33,34].
Enterocele
Enterocele is defined as an internal herniation of
the peritoneal sac below the pubococcygeal line
(PCL) into the rectovaginal space. The PCL is
defined as the line that joins the inferior border of
the symphysis pubis to the last coccygeal joint on
midsagittal images (Fig. VI.38) [27, 35].The PCL is
used as the reference line for pelvic floor weakness evaluation [14, 36]. Along this line, which is
independent of the pelvic position, the pelvic
floor muscle and pubovesical ligament attach [37].
The prevalence of enteroceles in patients with
pelvic floor disorders is between 17-37% [38–40],
with women being more frequently affected. In
addition, there is a high correlation between
pelvic surgery (e.g., hysterectomy) and enteroceles formation [41, 42]. Enteroceles most frequently occur at the end of evacuation and can be filled
with omental fat (peritoneocele), small bowel
(enterocele), or sigmoid colon (sigmoidocele).
The extent of an enterocele is measured at 90º to
the PCL from the lowest margin of herniation
content (e.g., small-bowel loop) during evacuation effort on defecography [36]. They are classified as small if they extend less than 3 cm below
the PCL, moderate if they extend from 3 to 6 cm
below this line (Fig. VI.39), and large if they
extend 6 cm or more below this line [14]. Large
enteroceles slipping over the anal canal lead to
outlet obstruction and a feeling of incomplete
evacuation due to rectal compression [43, 44].
Clinical symptoms are poor and nonspecific, and
Section VI • Update in the Evaluation of Outlet Obstruction 247
a b
c
physical examination is insufficient for an accu-
rate assessment. Hence, enteroceles are often
missed at clinical examination [45].
Dynamic pelvic MRI is the method of choice
in the evaluation of enteroceles,being superior to
conventional proctography even when additional
opacification of vagina and bowel is performed
during fluoroscopic examination. At conventional
defecography, separation of the vagina and the
rectum during defecation suggests an enterocele,
but the diagnosis of a small enterocele is difficult
since a distance of 2–3 cm between rectum and
vagina does not allow the differentiation from a
thickened rectal wall [40]. MRI is useful especial-
Fig. VI.37. A 55-year-old woman after hysterectomy with
anterior rectocele and need for evacuation assistance. T1weighted spoiled gradient-echo magnetic resonance (MR)
images were obtained in sitting position in an open-configuration MR system at rest (a),at straining (b), and during evacuation (c). MR image during evacuation (c) shows a moderate
anterior rectocele with incomplete evacuation (arrow)
ly in these cases and can influence the surgical
procedure since an operative closure is necessary
if surgery is performed [14]. An undetected small
enterocele may result in progressive symptoms
and the need for another intervention [39].
MRI, with its excellent soft tissue contrast, also
enables an accurate differentiation between peritoneocele, enterocele, and sigmoidocele without
filling the small or large bowel with contrast
agents. Although sigmoid colon herniation is less
common compared with small-bowel herniation,
sigmoidoceles are more frequently associated
with constipation, and sigmoidectomy may be
performed (Fig. VI.40) [46, 47].
248 Benign Anorectal Diseases
Fig. VI.38. Midsagittal steady state free precession image
obtained in a patient in supine position. The pubococcygeal
line defined as the line that joins the inferior border of the
symphysis pubis to the last coccygeal joint. Pubococcygeal
line is used to measure the extent of the enterocele as well as
to express the position of the anorectal junction (1,posterior
compartment), the position of the vaginal vault (2, middle
pelvic compartment), and the position of the bladder base (3,
anterior pelvic compartment)
Rectal Prolapse
Rectal prolapse is an invagination of the rectal
wall and may be classified as internal or external
[21]. The location may be anterior, posterior, or
circumferential and may involve all rectal wall
layers (full-thickness prolapse) or only the
mucosa (mucosal prolapse) [17]. The internal rectal prolapse, also called intussusception, may be
classified as intrarectal internal prolapse when the
invagination is confined to the rectum or as an
intra-anal internal prolapse when its apex penetrates the anal canal and remains in it during
straining (Fig. VI.41). The external rectal prolapse
is an invagination of the rectal wall beyond the
anal canal (Fig. VI.42). External rectal prolapse is
a clinical diagnosis. The etiology of internal rectal
prolapse is unknown [16], but injury during childbirth and chronic straining in constipated
patients seem to be possible concomitant factors
[48, 49]. Although small internal prolapses are
common findings in asymptomatic patients it has
been demonstrated that internal rectal prolapse is
present twice as often in patients with impaired
defecation in comparison with asymptomatic volunteers [50]. The mean frequency of internal rectal prolapse is between 12% and 27% in patients
a b
Fig. VI.39. A 74-year-old woman with a history of hysterectomy. T1-weighted spoiled gradient-echo magnetic resonance (MR)
images obtained at rest (a) and during evacuation (b) with the patient sitting in an open-configuration MR system. During
evacuation (b),herniation of the peritoneal sac below the pubococcygeal line (PCL) is seen. The sac contains peritoneal fat (peritoneocele) and extends more than 3 cm below the PCL, being classified as a moderate enterocele. In addition, the patient has a
moderate rectal descent
Section VI • Update in the Evaluation of Outlet Obstruction 249
a b
Fig. VI.40. A 28-year-old woman with chronic constipation. T1-weighted spoiled gradient-echo magnetic resonance image
obtained in sitting position and at rest (a). The sigmoid colon is well delineated in normal position (arrow). During evacuation
(b), a moderate enterocele protrudes into the rectovaginal space below the pubococcygeal line. The herniation consists exclusively of sigmoid colon (arrow). In addition, an anterior rectocele with incomplete evacuation is evident (small arrow)
with evacuation disorders [51], and the most frequent clinical manifestation is the feeling of
incomplete evacuation [45]. Dynamic MRI is useful for the diagnosis of internal rectal prolapse.
One major advantage of MRI over conventional
proctography is represented by an accurate appreciation of the wall layers. Dynamic MRI enables
differentiation between a mucosal internal prolapse (Fig. VI.43) and a full-thickness internal
prolapse (Fig. VI.41) [16], which is of clinical
importance since the treatment is different [52].
Internal rectal prolapse is often associated with
other anatomical abnormalities, including enterocele and rectocele.
Pelvic Floor Descent
Pelvic floor descent, or descending perineal syndrome, is an excessive caudad movement of the
pelvic floor during evacuation [36]. This abnormality is most likely the result of a pudendal
nerve injury from a combination of obstetric
trauma and chronic straining [53–55]. Pelvic floor
descent is the most common finding in patients
with outlet obstruction and is associated with
pelvic pain and a feeling of incomplete evacuation. Incomplete evacuation is followed by more
straining, which leads to increasing denervation
of anal and puborectalis muscles and over time to
fecal incontinence [56]. Although clinical examination and electrophysiological tests play a significant role in diagnosis, dynamic MRI provides the
most accurate assessment.
Outlet obstruction may be associated with a
descent of any of the three pelvic compartments
(i.e., posterior, middle, or anterior). The landmark for the posterior compartment is the
anorectal junction. There are different quantification or grading systems in the literature, but in
our clinical practice, we prefer to measure the
descent of the anorectal junction with respect to
the PCL [10]. A small rectal descent is considered
when the anorectal junction is less than 3 cm
below the PCL, a moderate descent when the distance between anorectal junction and PCL is
between 3 and 6 cm (Fig. VI.44), and a large rectal descent when the distance between the
anorectal junction and PCL is more than 6 cm
[10]. Even in the case of patients with clinical
manifestations attributable to the posterior compartment, an isolated rectal descent is rarely pre-
sent.Pelvic floor weakness is generalized and frequently involves multiple sites especially in constipated patients (Fig. VI.45) [26, 35]. Cystoceles
are expression of the descent of the anterior compartment, and descent of the vaginal vault (or any
part of the remaining cervix in case of hysterec-
250 Benign Anorectal Diseases
a b
Fig. VI.41. A 47-year-old woman with constipation and dif-
fuse pelvic pain. T1-weighted spoiled gradient-echo magnetic
resonance (MR) images obtained at rest (a), at straining (b),
and during evacuation (c) with the patient sitting in an openconfiguration MR system. During evacuation (c), all layers of
the rectal wall protrude through the anal canal (arrow) with
formation of a full-thickness internal intra-anal prolapse. The
rectum is incompletely evacuated. Associated anterior and
c
posterior rectoceles are evident (small arrows)
Fig. VI.42. A 58-year-old man with chronic constipation and
incomplete evacuation.Sagittal T1-weighted spoiled gradientecho magnetic resonance image with the patient in sitting
position and obtained during defecation shows a full-thickness external rectal prolapse (arrow)
Section VI • Update in the Evaluation of Outlet Obstruction 251
a
c
tomy) represents descent of the middle compartment. The descent of the anterior and middle
compartments are quantified relative to the PCL
in a similar fashion as described for the posterior
compartment [14].
Anismus
Anismus is an outlet obstruction characterized by
difficulties in rectal evacuation due to an abnormal activity of pelvic floor musculature.In the literature, anismus is also known as spastic pelvic
floor syndrome [17, 45], dyskinetic puborectalis
muscle [41], and pelvic floor dyssynergia [57].
During normal defecation, the pelvic floor
b
Fig. VI.43. A 71-year-old woman with feeling of incomplete
evacuation. Steady state free precession (SSFP) sequence was
used at rest (a) and at squeezing (b). During defecation (c),
dynamic T1-weighted spoiled gradient-echo image shows a
thin hypointense anterior infolding through the anal canal
(arrow) consistent with the typical appearance of an intraanal internal mucosal prolapse
descends, the internal and external sphincters
relax, and the anorectal junction opens due to the
puborectalis muscle reflex inhibition.A paradoxical contraction or an insufficient relaxation of the
puborectalis or external sphincter muscles are the
causes of obstruction and, in most cases, these
dysfunctions are not confined to a single muscle
[58]. The etiology of obstruction is unknown, but
a morphological lesion of the nerves or muscles is
unlikely since at least two thirds of patients are
able to relax the pelvic floor muscles during
biofeedback training [59]. However, there seems
to be an association between anismus and pelvic
surgery, previous sexual abuse, anxiety, and/or
psychological stress [57, 60]. Although it was
reported that anismus may be the main cause of
252 Benign Anorectal Diseases
a b
Fig. VI.44. A 55-year-old woman with chronic constipation and pelvic pain.The examination was performed in supine position
and using a closed-configuration conventional magnetic resonance (MR) system. Steady state free precession sequence obtained
at rest shows no abnormality (a).T1-weighted spoiled gradient-echo MR image during defecation demonstrates a moderate rectal descent (b). The anterior and middle pelvic compartments are normal
Fig.VI.45. A 67-year-old constipated woman after hysterecto-
my. T1-weighted spoiled gradient-echo magnetic resonance
image during evacuation with patient in sitting position
reveals a generalized pelvic floor weakness involving all
pelvic compartments: a large posterior compartment descent
with the distance between anorectal junction and pubococcygeal line more than 6 cm (black arrow) and a small cystocele, which is the expression of the anterior compartment
weakness (small arrow). In addition, the rectovaginal space is
enlarged and is occupied by a large enterocele (large arrow)
that contains fat, small bowel, and sigmoid
outlet obstruction in almost half of constipated
patients [61], the real prevalence is difficult to
assess, being known that false positive and false
negative results are common with different tests
including anorectal manometry, balloon expulsion, and electromyography [7, 62]. Similar, electromyographic findings were described in either
normal or constipated patients [63], and anorectal
manometry showed that nearly 80% of patients
suspected of anismus had appropriate sphincter
relaxation during straining [7]. Therefore, the
diagnostic requires a combination of tests, both
physiological and radiological.
Dynamic pelvic MRI provides valuable information in patients with anismus regarding functional as well as morphological abnormalities of
the pelvic floor during defecation. Using dynamic
pelvic MRI, various signs may be seen in patients
with anismus. In our experience, the best single
finding of functional outlet obstruction is represented by a prolonged attempted defecation and
incomplete evacuation (Fig. VI.46). It has been
demonstrated that an evacuation time longer than
30s is highly suggestive for anismus,having a pos-
itive predictive value of 90% [64]. In that study,
Halligan et al. compared patients with proven
anismus with controls, and a prolonged evacua-
Section VI • Update in the Evaluation of Outlet Obstruction 253
a
c
tion time was present in 83% of patients compared with none of the control group.
Another MR sign used for diagnosis of anismus is the behavior of the anorectal angle.
Normal value of the anorectal angle at rest is
between 90° and 110° [36], decreases during the
squeeze, and increases during defecation as a
result of puborectalis relaxation. In patient with
anismus, during evacuation, the anorectal angle
becomes more acute instead of obtuse, and it was
considered an indicator of the lack of puborectalis muscle relaxation [45, 65]. However, there is
data in the literature that showed that the configuration of rectum and anorectal junction is irrel-
b
Fig. VI.46. A 40-year-old woman with clinical suspicion of
anismus. T1-weighted spoiled gradient-echo magnetic resonance (MR) images obtained in sitting position obtained at
rest (a), at squeezing (b),and during evacuation (c). The puborectalis impression is visible on MR image at squeezing as a
result of puborectalis muscle contraction (arrow) (b). MR
image obtained 40 s after several attempts at defecation shows
no rectal evacuation (c). The final diagnostic of anismus was
established based on clinical, radiological, and physiological
findings
evant to the diagnosis,and this finding cannot differentiate patients with functional outlet obstruction from asymptomatic subjects [66].
Conclusion
Dynamic pelvic MRI using either closed-configuration or open-configuration MR systems is a
rapidly evolving technique that is considered an
alternative to conventional evacuation proctography. In patients with outlet obstruction, the technique may reveal various findings associated with
outlet obstruction.
254 Benign Anorectal Diseases
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