Perineal ultrasound (PNUS) is an effective, inexpensive, easily available but so far
not well known diagnostic tool.
Examination technique
Examination technique
PNUS requires profound knowledge of the anatomy and topography of the pelvic floor
and sphincters. The examination does not require specific preparations of the patient.
For better contrast and orientation, the urinary bladder should be well filled. A
probe coverage (e.g., custom made coating or a latex investigation glove filled with
ultrasound gel) should be used for hygienic reasons. Conventional transabdominal convex
3-7 MHz probes are used first for orientation including colour Doppler imaging. Higher-frequency
linear probes (5-17 MHz) can be used after orientation providing higher resolution
imaging of the perianal region.
The patient can be examined in a left sided position but other examination positions
are possible as well. Anatomical structures and landmarks should be used for better
orientation. Images should be documented using the corresponding system for endorectal
ultrasound (ERUS). PNUS is more flexible than endorectal ultrasound, particularly
when oblique orientated transsphincteric fistulas are to be examined. For best results,
examination should be started using low frequency ultrasound probes, subsequently
followed by the use of high frequency probes. Additional techniques can be used (e.g.,
panoramic view, 3-D ultrasound, sono-elastography).
The localisation of inflammatory and neoplastic lesions should be described in relation
to the sphincter apparatus. Detected fistulas can be further differentiated by ultrasound
examination in intersphincteric, transsphincteric and extrasphincteric forms.
Comparison to other imaging modalities
Comparison to other imaging modalities
Compared with other cross-sectional imaging procedures (e.g. computed tomography (CT),
magnet resonance tomography (MRI)) ultrasound technology has the disadvantage that
documentation of findings (e.g., by overview images) is limited. One way to make subjective
analysis of the findings accessible to other investigators is the use of 3D-ultrasound.
The so called "free-hand"-3D-method works with correlation algorithms and uses several
sensor-supported steps (data acquisition, correlation algorithms with segmentation,
visualization and quantification of the recorded data sets) until a three-dimensional
image is constructed. This method is particularly useful for endorectal ultrasound.
For evaluation of endorectal tumours, 3D-data sets were recorded prior to computation
of the axial layer. Preliminary experience with this technique has been reported,
however, until today no systematic studies were published. The same is true for evaluation
of the sphincter apparatus and perirectal fistulas by sono-elastography. The latter
displays sclerosing fistulas as homogenous blue (harder) tissue structures, whereas
acute inflammation with perifocal oedema is displayed as green/yellow (soft tissue).
Comparison of PNUS and ERUS
Comparison of PNUS and ERUS
ERUS often fails to demonstrate perianal fistula that run diagonally from the sphincter
apparatus. PNUS permits acquisition of those lesions by using variable section planes.
However, as PNUS cannot demonstrate the complete sphincter, it is recommended to use
PNUS in conjunction with conventional ERUS.
Indications
Indications
For staging of perirectal tumours, PNUS can be used complimentary to conventional
ERUS. A valuable extension of the diagnostic application of PNUS is its supportive
role during invasive interventions, such as drainage of fluids or targeted puncture
of tissue lesions.
PNUS is particularly useful if clinical examination, ERUS or MRI (with endorectal
coil) cannot be performed (e.g., due to severe pain when introducing the probe into
the anus, particularly in children). Indications of PNUS are summarized in Table [1].
Table 1 Indications of PNUS
The complete evaluation of the sphincter for diagnosis of incontinence remains the
domain of ERUS, however, PNUS can add additional information on extra-sphincteric
complications (e.g., fistula, abscesses). The classification of anorectal malformations
is also possible.
For complex overview of the pelvis and the perineum, MRI is still the imaging technique
of choice. However, when diagnosis of acute inflammatory processes of the anorectal
region has to be made, MRI is often not readily available, time-consuming and cost-intensive,
and might be supplemented by PNUS with better detail resolution compared to CT and
MRI.
Fistula and abscesses
Fistula and abscesses
The most frequent causes for perianal and pararectal fistula as well as abscesses
are crypto-glandular inflammations that expand into the sphincteric region, as well
as chronic inflammatory bowl diseases (e.g., Crohn's disease). In addition, venereal
and HIV-associated inflammatory and neoplastic diseases can also cause fistula and
abscesses. Rare diseases (e.g., the Langerhans cell histiozytosis) must also be considered,
particularly as they can be mistaken sonographically and radiologically as anal carcinomas.
Complex fistulas that involve the M. sphincter ani externus, M. levator ani and/or
M. obturatorius cannot be assessed by native PNUS. However, discrimination of the
fistula by PNUS can be enhanced by instillation of contrast agents (SonoVue) over
the external ostium of the fistula. Hydrogen peroxide or sparkling mineral water can
be also used as an economical ultrasound contrast agent.
Diagnostic of anal incontinence
Diagnostic of anal incontinence
The work-up of anal incontinence requires the exact knowledge of individual anatomical
and functional defects of the sphincter apparatus and the surrounding structures.
The M. sphincter ani internus can be displayed sonographically as circular, low-echogenic
structure with a cross-sectional diameter between 2-4 mm. The M. sphincter ani externus
presents as stronger echogenic structure with a thickness between 4-6 mm. The sono-morphological
diagnosis of incontinence remains a domain of ERUS, which allows a right-angled and
orthogonal inspection of both sphincters. The assessment of the sphincters by non-invasive
PNUS can give some complimentary information, particularly in women, but is limited
by diagonal angle of inspection. Combined results from ERUS, PNUS and rectal perfusion
manometry allow us to distinguish four major forms of anal incontinence: A purely
sensory form, a predominantly muscular form, a combination of both as well as a malfunction
of the rectal reservoir function. This is important, since each form of incontinence
requires a distinct therapeutic approach.
In several patients that were formerly diagnosed with "idiopathic incontinence", sonographic
evaluation now reveals a lesion of the sphincter apparatus as morphological correlate.
The most frequent causes of sphincter lesions in women are delivery traumas (e.g.
forceps delivery). Resulting scars are typically located between the ventral edge
of the anus and the vagina. Within the echo-rich external sphincter muscle, the scar
usually presents as echopoor tissue. In contrast, within the echopoor M. internus,
the scar presents as a comparative echorich structure. Sonographic evidence of disruptions
of the external anal sphincter correlates with faecal incontinence, particularly in
women.
Diagnostic of descensus uteri and other pelvic diseases
Diagnostic of descensus uteri and other pelvic diseases
PNUS can also give complimentary information for the diagnosis of descensus uteri
and other pelvic disease. The ultrasound transducer is placed above the external aperture
of the urethra, which allows imaging of the urethra and the posterior wall of the
urinary bladder. The degree of descensus uteri can be estimated from the delta of
the angel between both structures at rest and during Valsalva maneuver. For details,
we refer the reader to the specialized literature (e.g., published by Hans Peter Dietz).
Diagnosis of anal and rectal tumours
Diagnosis of anal and rectal tumours
Early anal carcinomas typically presents as superficially located echo-poor infiltration.
According to the valid UICC classification, anal carcinomas are subdivided according
to their size. For further staging and design of an individual therapeutic strategy,
imaging procedures have to be used, in order to determine the extension and the exact
position of the tumour in relation to neighbouring structures. In respect to lymph
node metastases, two different localisations have to be differentiated: the anal rim
carcinoma, which metastasises primarily inguinal; and the carcinoma of the anal canal,
which spreads primarily perirectal (Table [2]).
Table 2 TNM-Classification of anal rim and anal canal carcinoma
*Vagina, urinary bladder, prostate, sphincteric infiltration is not considered as
T4, Differences were marked in italic
The conventional pre-therapeutic staging of anal carcinomas relied on clinical investigation,
such as palpation and proctoscopy, as well as imaging techniques like computed tomography
or magnet resonance tomography. PNUS (in conjunction with ERUS) examination allows
a detail resolution of < 1 mm, which greatly improve estimation of the depth of tumour
infiltration. Due to the improved detail resolution by ultrasound imaging, several
groups have now suggested a revision of the UICC classification for anal carcinoma.
Instead, a new classification that considers the infiltration of the different portions
of the sphincter muscle was proposed.
The accepted first-line treatment for anal carcinomas is currently radio-chemotherapy.
Surgical intervention is usually limited to small anal rim carcinomas or cases of
extended disease and associated complications (e.g., bleeding or ileus). In some cases
of distal located carcinomas of the rectum, PNUS examination can provide useful additional
information; however, conventional ERUS remains the diagnostic method of choice. For
rectal carcinomas that are situated more proximal (> 4-6 cm) PNUS is much less reliable.
However, after surgical amputation of the rectum, PNUS and ERUS can supplement radiological
imaging, thereby improving quality of follow-up examinations.
Diagnostic and therapeutic interventions
Diagnostic and therapeutic interventions
A valuable extension of the diagnostic application of PNUS is its supportive role
during invasive interventions, such as drainage of liquids (e.g., inflammatory, post-operative)
or the targeted puncture of suspected lesions (e.g., infectious, neoplastic). For
this purpose, specialized transducers with a build in needle holder/applicator are
available. However, conventional curved array probes can be used for free hand punctures,
particularly if the lesion of interest is close to the surface and large enough in
size. A condition sine qua non for interventions in the very sensitive perineal region
is the appropriate use of local anaesthesia or analgo-sedation.
Fig. 1a
Fig. 1b
Suggested reading
Suggested reading
Dietrich CF, Barreiros AP, Nuernberg D, Schreiber-Dietrich DG, Ignee A. Perianal ultrasound].
Z Gastroenterol 2008; 46(6): 625-630.
Christoph F Dietrich and Ana Paula Barreiros, Germany