Perineal ultrasound

Perineal ultrasound (PNUS) is an effective, inexpensive, easily available but so far not well known diagnostic tool.

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

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

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

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

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

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

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

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

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.

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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