British Journal of Radiology 75 (2002),482-488 © 2002 The British Institute of Radiology
Endoanal ultrasound
G T Rottenberg, FRCR
1 and
A B Williams, FRCS
2
Departments of 1 Radiology and 2 Surgery, Guy's
and St Thomas' NHS Trust, Lambeth Palace Road, London SE1, UK
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Abstract
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This pictorial review demonstrates the normal anatomy and abnormalities
that are readily demonstrated with endoanal ultrasound of the anal sphincters.
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Indications
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The main indication for endoanal ultrasound is in the investigation of
faecal incontinence [1].
This may either be idiopathic or a result of obstetric or previous surgical
trauma. Endoanal ultrasound is also used to assess patients with anal pain,
anal sepsis and anal malignancy, as well as to assess the completeness of
sphincter repair following surgery.
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Technique
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Imaging is performed using a single crystal probe such as the B&K Medical
scanner (B&K Medical, Sandhoften, Denmark) and an 1850 axial
type endoscopic probe with a 10 MHz transducer. The mechanically rotated
probe produces a 360° cross-sectional view ideally suited to examination
of the sphincters. The ultrasound probe is covered by a rigid plastic cone
of uniform diameter, which prevents the anatomical distortion that occurs
with the use of a balloon system, and flushed with degassed water for acoustic
coupling [2].
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Normal anatomy
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The four anatomical layers demonstrated on endoanal ultrasound (Figures 1–4
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are as follows. The plastic cone produces an inner, well defined ring of high
reflectivity. Next is the subepithelium, composed of connective tissue with
added elements of smooth muscle derived from the longitudinal layer of the
rectum and anal canal. The submucosa is moderately reflective on endoanal
ultrasound. The mucosa is generally not visible on endoanal ultrasound because
it is lost within the intense reflection from the outer border of the probe
cone and is not within the focal range of the ultrasound crystal. Prominent
vascular channels are commonly seen in this layer and are usually related
to haemorrhoids (Figure 5).
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Figure 1. Normal appearance of the male anal canal at the level of the puborectalis
muscle. a, puborectalis; b, circular muscle of the rectum; c, prostate.
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Figure 2. Normal sphincter appearance in an adult male (mid anal canal).
a, internal anal sphincter; b, longitudinal muscle; c, external anal sphincter.
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Figure 3. Normal sphincter appearance in an adult male (distal to the
termination of the internal anal sphincter) demonstrating the normal
subcutaneous portion of the external anal sphincter (arrow).
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Figure 4. Normal sphincter appearance in an adult female (mid anal
canal). a, internal anal sphincter; b, longitudinal muscle; c, external
anal sphincter. Note the poorer delineation between the external anal sphincter
and the longitudinal layer than in the male patient in Figure 3 .
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Figure 5. Prominent internal haemorrhoids. There is a hypoechoic focus in
the subepithelial region (arrow) that should not be mistaken for
a fistulous track.
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The internal anal sphincter (IAS) is consistently seen as a hypoechoic
ring immediately deep to the subepithelium. It is formed from the caudal continuation
of the smooth muscle layer of the circular muscle of the rectum (Figure 2). The thickness of the IAS is measured
at the 3 O'clock or 9 O'clock position where the muscle is symmetrical.
The muscle thickness is age dependent, increasing in thickness whilst becoming
less distinct with age. Average IAS thickness for an adult is 2 mm.
The conjoined longitudinal muscle layer is a thick band of muscle that
passes between the IAS and external anal sphincter (EAS). It is
a direct continuation of the longitudinal layer of the rectum, together with
elements of striated muscle from the levator plate, and is seen as a layer
of similar reflectivity to the subepithelial layer. This layer may act as
an anchor for the sphincter muscles to the pelvic sidewall and the anal skin.
Bundles of smooth muscle are seen as areas of low reflectivity within this
layer (Figure 2).
The EAS is composed of striated muscle and is often the most difficult
structure to identify on endoanal ultrasound. It has three main components;
deep, superficial and subcutaneous portions. The EAS has a different appearance
in men and women. The muscle bulk of the EAS is thicker and better defined
in men, when it is seen as a striated structure of low reflectivity. In women,
the EAS tends to be of a higher reflectivity, which is often indistinguishable
from the conjoined longitudinal layer (Figures 2 and 4).
Both the anal canal and the EAS are significantly shorter in women than men.
Slight forward angulation of the ring of the EAS can give the false impression
of a deficiency of the anterior EAS high in the female anal canal. It is important
to bear this in mind when interpreting results of ultrasound examination.
High in the anal canal the deep EAS is continuous with a sling of muscle,
the puborectalis. This is seen as a hoop of mixed high and low reflectivity
with a marked striated pattern passing posteriorly around the anal canal.
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Pathology
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IAS disorders
The normal IAS increases in thickness with age owing to increased deposition
of collagen. Normal IAS thickness for a young adult is 1–2 mm,
whilst a measurement of up to 3.5 mm can be normal in the elderly.
Occasionally, very marked thickening of the IAS is seen in the rare, hereditary
IAS myopathy, although thickening of the IAS is more commonly seen in patients
with rectal prolapse or solitary rectal ulcer syndrome [3] (Figure 6). Thinning of the sphincter muscles is also seen
in elderly patients without any evidence of a defect. It is likely that this
is a result of primary degeneration of the IAS (Figure 7).
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Figure 6. Marked thickening of the internal anal sphincter to 4.2 mm (arrow)
in a 43-year-old patient with solitary rectal ulcer syndrome.
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Figure 7. Internal anal sphincter (IAS) atrophy in a 55-year-old
man with passive incontinence. The IAS is thin but intact (arrow).
The appearances are consistent with primary sphincter atrophy in the presence
of normal pudendal nerve terminal motor latencies.
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Post surgical assessment
Endoanal ultrasound has demonstrated the damage to the IAS that can occur
following anal stretch procedure [4] (Figure 8).
Endoanal ultrasound is used to examine patients who have developed passive
faecal incontinence following internal sphincterotomy (Figure 9,
10).
In these patients, endoanal ultrasound demonstrates that the sphincterotomy
is more extensive than intended and extends for the entire length of the anal
canal. This occurs more commonly in women who have a shorter anal canal.
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Figure 8. Internal anal sphincter (IAS) disruption post anal stretch.
The IAS is totally disrupted. The external anal sphincter is intact. Anal
stretch procedures can produce single or multiple defects in the IAS.
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Figure 9. Internal anal sphincter (IAS) defect post surgery in
a 27-year-old man with passive incontinence. There is a defect in the IAS
between 6 O'clock and 11 O'clock (arrows) following surgery
for haemorrhoids. The external anal sphincter is intact (arrowheads).
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Figure 10. Internal anal sphincter (IAS) defect post internal sphincterotomy.
There is an IAS defect between 4 O'clock and 8 O'clock (arrow).
The external anal sphincter appears attenuated but intact (arrowheads).
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Obstetric complications
85–90% of women with post-obstetric incontinence will have
a sphincter defect evident on endoanal ultrasound [5]. The most frequent abnormality is a defect
of the EAS that commences at the level of the perineal body. The external
defect can be seen as discontinuity of the EAS or as an area of reduced echogenicity
with posterior shadowing (Figure 11, 12). Endoanal ultrasound plays an important
role in confirming the presence and extent of a sphincter defect prior to
consideration of surgery. Associated fistula formation may also be demonstrated (Figure 13). Endoanal ultrasound can also assess
the post-operative appearances following sphincter repair (Figure 14).
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Figure 11. Obstetric injury in a 32-year-old woman presenting with faecal
urgency 6 months after a forceps delivery. There is a defect in the
external sphincter between 1 O'clock and 3 O'clock (arrow).
The normal external anal sphincter is shown (arrowhead).
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Figure 12. Severe obstetric trauma. There is a defect in the external anal
sphincter between 9–12 O'clock and 2–3 O'clock (arrowheads).
There is disruption of the internal anal sphincter (arrows).
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Figure 13. Obstetric injury and fistula formation. There is a defect in the
anterior portion of the external anal sphincter with an anterior anal fistula (arrow).
The tiny focus of increased reflectivity is from gas within pus in the fistula
track.
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Figure 14. Obstetric injury, post repair. A defect in the internal anal sphincter
remains between 10 O'clock and 3 O'clock. The external anal sphincter (EAS)
has been repaired but there are still persistent changes in the EAS between
12 O'clock and 3 O'clock.
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Anal sepsis
Endoanal ultrasound can be used to determine the presence of an abscess (Figures 15–17) or a primary track [6]. Unfortunately it is difficult to
demonstrate complex tracks reliably owing to its limited field of view. MRI
with a phased array coil is often more useful than ultrasound and is generally
better tolerated in patients who have anal pain.
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Figure 15. Crohn's disease with recurrent fistula formation. There is
disruption in the internal anal sphincter (arrow), which may be
the result of previous sepsis compounded by surgery. There is evidence of
an extra-sphincteric collection (arrowhead).
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Figure 17. Low intersphincteric fistula in the (a) mid anal canal
and (b) low anal canal. There is an intersphincteric collection
low in the anal canal. The site of the internal opening is inferred by the
point at which the collection enters the internal anal sphincter (a,
arrow). The external anal sphincter appears intact.
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Tumour
Anal malignancy characteristically appears as a hypoechoic mass on ultrasound.
Endoanal ultrasound enables the presence and depth of penetration of tumour
to be assessed providing pre-operative staging. Ultrasound can also be used
to detect recurrence of tumour following surgery or radiotherapy.
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Acknowledgments
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We would like to thank Mr W Kmiot, FRCS for his support and help in
setting up the endoanal ultrasound unit.
Received for publication December 13, 2000.
Accepted for publication July 20, 2001.
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References
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Bartram CI, Sultan AH. anal endosonography in faecal incontinence. Gut 1995;37:4–6.[Free Full Text]
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Halligan S, Sultan A, Rottenberg G, Bartram CI. Endosonography
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Speakman CT, Burnett SJ, Kamm MA, Bartram CI. Sphincter injury
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Sultan AH, Kamm MA, Hudson CN, Thomas JM, Bartram CI. Anal-sphincter
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Deen KI, Williams JG, Hutchinson R, Keighley MR, Kumar D. Fistulas
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Abstract
Indications
