Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_1408_Библиотеки_им_академика_М_И_Перельмана
.pdf
ventral attachment to the two pubic rami and it dorsally loops around the anorectum and
is possibly attached to the coccyx. From a functional point of view the puborectalis has a
unique function, it is thought to be responsible for the formation of “anorectal angle.” The
latter is best seen on the midsagittal image of the pelvis during barium or MR defecography (
Fig. 2.6). With contraction and relaxation of the PRM, the anorectal angle becomes
more acute and obtuse respectively. During defecation the anorectal angle becomes
obtuse and in patients with FI, with damage to the PRM, the anorectal angle stays obtuse
and does not change significantly with squeeze.
3D-US imaging of the pelvic floor provides a better understanding of the morphology
and function of the PRM. The entire PRM is visualized exquisitely by 3D-US imaging
(
Fig. 2.7). The PRM forms the inferior margin of pelvic-floor hiatus through which the ure-
thra, vagina, and anal canal emerge from the pelvis to the exterior in females. Contraction
of PRM reduces the size of the pelvic-floor hiatus and lifts the anal canal ventrally thus
compressing three orifices, i.e., anal canal, vagina, and urethra against each other and
in turn against the pubic symphysis. The existence of a vaginal high-pressure zone has
been known for long time and it is thought to be related to the pelvic-floor muscle. Characteristics of the vaginal high-pressure zone have been described in detail in recent years
using many different types of pressure-measurement techniques, such as infusion
manometry (side hole and sleeve sensor technique),
42,43
3D-US imaging,44high-
definition manometry,
45
and most recently by FLIP46(Fig. 2.8A and B). These studies
prove that the vaginal high-pressure zone is related to the PRM contraction. The above
observation has important clinical implications in that the PRM function may be best
FIG. 2.6 Magnetic resonance image of the mid sagittal section of the pelvis at rest and during contraction: note the
change in anorectal angle with contraction.
16 ANORECTAL DISORDERS
assessed by recording vaginal pressure. The anorectal angle measurement used to assess
PRM function is cumbersome, it requires imaging studies and is somewhat subjective. On
the other hand, vaginal pressure measurement is relatively simple. One can use 3D
manometry to record the vaginal high-pressure zone accurately. In patients with fecal
continence, besides anal, vaginal pressures are also lower compared to controls suggesting that the PRM function is impaired in a significant number of patients with FI
47
(Fig. 2.9). The PRM contraction, in addition to increasing anal and vaginal pressure, also
increases urethral pressure.
48
It is highly likely that the PRM is involved in the urethral
continence mechanism, which requires future studies.
2.6 Neural Innervation of the Puborectalis Muscle
Branches of the sacral nerve roots of S2, S3, and S4 innervate the pelvic-floor muscles.
There is considerable controversy as to whether the pudendal nerves innervate the levator
ani muscles. An electrophysiological study by Percy and colleagues found the electrical
stimulation of the pudendal nerve did not activate the PRM.
49
It is possible, however, that
in their study the electrodes may not have been precisely located in the puborectalis portion of the levator-ani muscle. The author’s opinion is that the PRM (middle layer of
pelvic-floor muscle),
43
similar to EAS, is indeed innervated by the pudendal ner ve
Rest Squeeze
Pelvic floor hiatus at rest and squeeze
FIG. 2.7 Pelvic-floor hiatus as imaged by 3D-US. Note the U-shaped puborectalis muscle at rest and during contraction.
Note the three structures in the pelvic-floor hiatus and the reduction in the dimensions of hiatus with contraction. The
dorso-ventral dimension of the hiatus gets smaller during contraction resulting in compression of all the structures
against the back of symphysis pubis.
Chapter 2 • Anorectal Anatomy and Function 17
(from the inferior surface of the ileococcygeus muscle) and the deep muscles (pubococcygeus, ileococcygeus, and coccygeus) are innervated by the direct branches of sacral
nerve roots S3 and S43, from the superior surface of pelvic floor. The clinical significance
is that pudendal nerve damage may cause dysfunction of the PRM and EAS muscles (both
constrictor muscles) and this in turn may cause FI.
High definition vaginal canal manometry
Squeeze Rest
(A)
Functional luminal imaging probe
Anal canal
Vaginal canal
Rest
Squeeze
Squeeze
Rest
(B)
FIG. 2.8 (A) Vaginal manometry with high-definition manometry. (B) Vaginal high-pressure zone visualized with
functional luminal imaging probe. (A) From Raizada V, et al. Am J Obstet Gynecol 2010.
18 ANORECTAL DISORDERS
2.7 Relationship Between Anatomy and Function of the
Levator Ani
The name levator ani implies an elevator of the anus, which is what it actually is. However,
pelvic-floor muscles have two important functions, they provide
1
physical support or an
actual floor to the pelvic viscera, and
2
a constrictor mechanism to the anal canal, vagina,
and urethra. Studies strongly suggest that these two functions of the pelvic floor are quite
distinct and likely related to different components of the pelvic-floor muscles. The pubococcygeus, ileococcygeus, and ischiococcygeus likely provide the physical support or act
as a “floor” for the pelvic organs. On the other hand, the PRM provides a constrictor
Anal canal pressure (mmHg)
(A)
(B)
150
300
C
Fl
C
Fl
C
C
Fl
Fl
5 10 15
Probe size (mm)
20
450
Vaginal pressure (mmHg)
125
375
250
C
Fl
C
Fl
C
Fl
10
Probe size
(mm)
20 30
450
FIG. 2.9 Anal (A) and vaginal pressure (B) at rest (diamonds) and voluntary squeeze (triangles) in controls (C, filled
symbols) and fecal incontinent patients (open symbols). Note an increase in pressure with probe size increase and with
squeeze. Pressures are significantly lower in patients compared with controls both at rest and squeeze.
Chapter 2 • Anorectal Anatomy and Function 19
function to the anal canal, vagina, and urethra. The urethra and anal canal each have two
constrictors or sphincters of their own. In the case of anal canal these are the IAS and EAS,
and in the case of urethra they are the smooth muscle sphincter (located at the bladder
neck, internal urethral sphincter, or lisosphincter) and rh abdo-sphincter (external urethral sphincter). Based on the physiologic studies it is clear that the PRM is the third constrictor or the sphincter of the anal canal and urethra. The vagina, on the other hand, has
only one constrictor mechanism, which is mostly related to the puborectalis portion or the
pelvic-floor muscle. The PRM is relevant to several subspecialties of medicine: gastroenterology, colorectal surgery, urology and urogynecology, radiology, neurology, and any
subspecialists involved in taking care of patients with pelvic-floor disorders.
2.8 Length-Tension Function of the External Anal Sphincter
and Puborectalis Muscle
The basic unit of all muscles in the body is sarcomere, which is made of actin and myosin
filaments. The sarcomere length, which determines the actual muscle length, is a major
determinant of the force a muscle generates during contraction. The length-tension relationship, also known as Starling curve in context of myocardium, has been known for long
time.
50
It is a bell-shaped curve and muscle tension increases with an increase in the muscle length up to a certain length, after which it starts to decrease. The degree of overlap
between the actin and myosin filaments determines the force generated by the sarcomere
or any muscle.
51
At optimal length there is maximum overlap between the actin and myosin filaments. The length at which a muscle/sarcomere operates in vivo (operational
length) and the length at which it generates maximal tension (optimal length) can be quite
different. The myocardium under physiological conditions operates at a short sarcomere
length and when it is stretched increases the force of contraction. Different muscles in the
body may operate at different operational lengths. Studies show that similar to the myocardium, the EAS and PRM also operate at a short sarcomere length
52
(Fig. 2.10). Studies
on the rabbit EAS show that the optim al sarcomere length of the EAS is approximately 20%
larger than the operational length. In humans the EAS and PRM when stretched, for example, by placing probes of increasing diameters in the anal canal and vaginal canal, generate
increased tension.
53,54
Similar to normal healthy subjects, patients with FI who have damaged EAS and PRM also operate at a suboptimal length, even though the slope of lengthtension curve is steeper in normal subjects compared with patients with damaged
muscle.
47
The clinical significance of knowing the length-tension relationship is that it
may be possible to change/adjust the sarcomere length to gain muscle function. Plication
of the EAS muscle in rabbits lead to an increase in sarcomere length and anal-closure pressure that was sustained for 6 months (the duration of the study)
55
(Fig. 2.10). Whether
plication of the EAS and PRM can improve anal-closure function and FI in humans
requires careful study.
20 ANORECTAL DISORDERS
2.9 How to Assess Anatomy and Function of Anal-Closure
Mechanism in Humans
The current “gold standard” to assess the anatomical integrity of the anal-closure mechanism is endoanal US imaging. The endoanal US probe is approximately 15mm in diameter and is placed in the lumen of the anal canal. The above methodology has been in use
since the early 1990s. Using mechanical transducers, one can image the entire length of
the anal canal and using computer software display the anatomy in 3D. Many publications
by many authors prove that the endoanal US technique is reliable. The limitations of the
technique though are,
1
the probe is relatively large in size and may not be tolerated well by
all subjects
2
; anal distension caused by the US probe causes artefactual thinning of the
muscles
3
; the caudal most part of the EAS, located below the IAS, is not well visualized
by the endoanal US approach
4
; perineal body, an important part of the EAS, is easily
viewed in the endo-anal US images of the anal canal; and
5
the anal canal descends more
0
100
200
Time (s)
Anal canal pressure (mmHg)
0 16 32 48 64 80
1 mA 2 mA 3 mA 4 mA 5 mA
Plication and sarcomere length
Before
After
Anal canal pressure before and after EAS plication
EAS plicationto treat anal incontinence
n = 6 n = 5 n = 4
FIG. 2.10 The effect of external anal sphincter (EAS) plication on the anal canal pressure. Note an increase in pressure
following plication length of 20% of the EAS circumference. Also note that the increase in anal canal pressure
following plication is sustained for 24 weeks. From Rajasekaran MR, Jiang Y, Bhargava V, et al. Sustained
improvement in the anal sphincter function following surgical plication of rabbit external anal sphincter muscle.
Dis. Colon Rectum 2011;54:1373–1380.
Chapter 2 • Anorectal Anatomy and Function 21
caudally in the ventral than in the dorsal direction. One has to be careful in the interpretation of axial US images in the dorsal part of the anal canal. The US images show that in
most patients the damage to IAS and EAS is located between the 11 o’clock and 2 o’clock
positions of the anal canal, with 12 o’clock being the ventral midline location. Keep in
mind that the above location is where the perineal body is located, the crossi ng site for
the muscle fibers that pass from the two sides. Transperin eal/translab ial 3D-US imaging of the anal sphincter and pelvic-floor muscles, u sing a 5–8 MHz endovagi nal transducer, has been reported by several groups, mostly in the OB/GYN literature. One
places the US trans ducer on the perineum and can capture a US volume that can be
visual ized off line using com puter software in any desired orthogonal plane. Even
though the transducer is endovaginal, one on ly needs to place the transducer on the
skin of the perineum to acquire these images. The transperineal/translabial US technique is patient friendly since it does not require insertion of a US probe into the anal
canal and the US imaging quality is excellent. One can see the caudal p arts of the anal
canal, EAS, and perineal bo dy quite well in these images (
Fig. 2.11). For transperi neal
anal sphincter imaging the transducer is directed in the dorsal directio n. On the other
hand, to visualize pelvic-floor hiatus a nd PRM the transducer is directed toward the
head of the subject. One can capture these images in real time and therefore study
dynamic changes in the pelvic-floor hiatus during squeeze and push. We have us ed
the transperineal approach to detect anatomical disruption of the anal sphincter
and PRMs qu ite reliably. It can be used in the doctor’s office qui te easily. Another
US imaging technique that can provide useful information on the integrity of the anal
sphincter muscle is high-freq uency US imaging,
56
it allows one to visualize individual
muscle fascicles inside the body of the muscle. One can perform dynam ic highfrequency US imaging to study the motion of muscle fascicles during contraction
and relaxation; it has been done in the limb muscles but not yet in the anal sphincter
or pelvic-floor muscles.
MR imaging of the pelvic-floor and anal-sphincter muscles has been successfully
done by several investigators. The coils to capture MR images are usually placed on
the abdomen of the subjects but endoanal and endovaginal coils (probes) have also been
used. One can visualize EAS much better in the MR than the US images. On the other
hand, IAS is better visualized in the US images. MR defecography is a dynamic study
to assess the defecatory process and stool evacuation to determine pelvic-floor dyssynergia, rectocele, and other anatomical abnormalities that may occur during the evacuation
process. The MR ima ging is usua lly performed in the supine position but open magnets
to perform defecography in the sitting position are also available at a few centers. MR
diffusion tensor imaging to study the myoarchitecture of the anal sphincter and
pelvic-floor muscles has been attempted by only a few centers and is currently a research
tool. It provides information on how muscle fibers are organized in the body of the muscle. We have successfully visualized the crossing of the muscle fibers of the EAS in the
perineal body, even though the analytical protocols for such an analysis are not commercially available.
22 ANORECTAL DISORDERS
FIG. 2.11 Ultrasound (US) images of the anal sphincter muscles obtained with transperineal 3D-US transducer. Axial slices 1 mm apart from the caudal (#1) to
the cranial (#25) end of the anal canal.
Chapter 2 • Anorectal Anatomy and Function 23
2.10 Functional Assessment of the Anal-Closure Mechanism
The Schuster balloon, infusion manometry, and solid-state sensors that were used to
assess the strength of the anal-closure mechanism have been replaced by high-resolution
anorectal manometry (HR-ARM) during last 10 years, and this is currently the “gold
standard” for clinical anorectal motility studies.
57
There are many advantages to
HR-ARM compared with the old pressure-measurement techniques: the sensors have
high fidelity or, in other words, the response rates of these sensors are fast; there are no
concerns regarding the relative movement between the transducers and the anal canal
during the various maneuvers used during anorectal motility testing; and the display is
reader friendly. The major problem, however, is that the HR-ARM catheter is extremely
expensive and the technique does not necessarily provide any novel information, above
and beyond that provided by a careful recording using the old manometry systems.
High-definition anorectal manometry (HD-ARM) is another novel probe system
58
that
provides information on the asymmetry of the anal sphincter pressure profile
59–61
(Fig. 2.12). The HD-ARM anal probe is more than two times larger than the HR-ARM probe
(10 mm vs 4.5 mm) and pressures recorded by the HD-ARM are generally higher than the
HR-ARM probe. The latter is because the anal sphincter muscles are stretched and lengthened with the larger size probe, which generates greater tension, according to the lengthtension principle described earlier. One of the promises of HD-ARM—that it may be able
to detect the location of damage to the anal sphincter muscle—has not been fulfilled
because studies have so far provided conflicting results. FLIP is the latest “kid on the
High definition manometery—anal canal pressure
SqueezeRest
FIG. 2.12 3D-high-definition manometry of anal canal pressure.
24 ANORECTAL DISORDERS
block” to be used to assess anal closure function. Few studies have been reported on the
use of the FLIP device to study anal-canal distensibility as a measure of the strength of the
anal sphincter-closure mechanism.
6,7
There are limited but promising data that the distensibility of the anal canal is greater in patients with FI as compared with controls. One
study reported high sensitivity and specificity in the diagnosis of FI based on the analcanal distensibility at rest; squeeze values were not necessarily better than the rest values
in discriminating normal subjects from patients. Distending the anal canal with FLIP
brings back the length-tension principle of the anal sphincter muscle in the equation
and needs to be considered in the future studies. We have used vaginal manometry to
assess the PRM function in normal controls and patients with FI.
47
The vaginal highpressure zone shows significant circumferential asymmetry, which needs to be kept in
mind. Since the force responsible for the genesis of the vaginal high-pressure zone is
directed in a dorso-ventral direction (i.e., lift of the anal canal by PRM contraction in a
ventral direction) the pressures are higher in the dorso-ventral direction than in the lateral
direction. The ventral or anterior pressure is highest in the vaginal high-pressure zone.
2.11 Summary and Conclusions
Muscles in general are relatively straightforward in their function, they only shorten and
lengthen with contraction and relaxation, respectively. It is the architecture or the arrangement of the muscle fascicles inside the body of the muscle that determines the physical
function of the muscle in vivo. Flexion and extension at the elbow, for example, is achieved
by a simple arrangement of muscle fibers organized in a linear direction, from the origin
(shoulder) to the insertion (elbow, biceps, and triceps). On the other hand, the muscle fibers
of EAS are placed in the configuration of a figure of eight and this can cause circumferential
closure of the anal canal. Future studies need to focus on the architecture of the muscle
fibers inside various pelvic-floor muscles to better understand their actions and function.
Pelvic floor disorders are many and are generally lumped together. However, it may be
possible to broadly subclassify them into disorders of pelvic-floor support (prolapse, descending perineal syndrome) and constrictor function (FI and urinary incontinence). Furthermore, these disorders may be further divided into dysfunctions of pelvic-floor
contraction (FI and urinary incontinence) and relaxation (constipation and urinary retention). As a clearer picture of the functional anatomy of pelvic-floor muscles emerges, it is
likely that different components of the pelvic-floor muscles may be implicated in different
pelvic-floor disorders. With such a functional classification it may be possible to identify
the different pelvic-floor muscles linked to different pelvic-floor disorders and therefore
provide specific targets and more effective therapeutic strategies to treat the various
pelvic-floor disorders. A better understanding of the correct anatomy of the anal sphincter
and pelvic-floor muscles is crucial for our understanding of the precise function. Most
importantly, prevention of damage or surgical restoration of the sphincter and other
pelvic-floor muscles requires understanding of their correct anatomy.
Chapter 2 • Anorectal Anatomy and Function 25
Соседние файлы в папке Библиотека им академика М.И. Перельмана