Ultrasound evaluation of posterior compartment defects

Ultrasonographic imaging is gaining a key role in the understanding of pelvic floor disorders of the posterior compartment. Endoanal and endorectal ultrasonography (EAUS/ERUS), endovaginal ultrasonography (EVUS) and dynamic transperineal US (DTPUS) are nowadays increasingly used in clinical practice for patients suffering from fecal incontinence, pelvic organs prolapse, obstructed defecation and anorectal sepsis. These non-invasive techniques not only provide a superior depiction of the pelvic anatomy but also yield unique dynamic information.
Recently, several new ultrasound techniques have been developed that could significantly improve the diagnostic value of ultrasonography (US) in this field. Three-dimensional (3D) and real-time four-dimensional (4D) imaging have been introduced into routine medical practice. These techniques overcome some of the difficulties and limitations associated with conventional two-dimensional (2D) US. Although 2D cross-sectional images may provide valuable information, it is often difficult to interpret the relationship between different pelvic floor structures because the 3D anatomy must be reconstructed mentally. Three-dimensional reconstructions may closely resemble the real 3D anatomy and can therefore significantly improve the assessment of normal and pathologic anatomy. Complex information on the exact location, extent, and relation of relevant pelvic structures can be displayed in a single 3D image. Interactive manipulation of the 3D data on the computer also increases the ability to assess critical details.
It seems likely that these new diagnostic tools will be increasingly used in the future to provide more detailed information on the morphology and function of examined organs, to achieve better accuracy in the diagnosis of complex diseases, to facilitate planning and monitoring of operations, and for surgical training.
EAUS has become the gold standard for the morphological assessment of the anal canal. It can differentiate between incontinent patients with intact anal sphincters and those with sphincter lesions (defects, scarring, thinning, thickening, and atrophy). Tears are defined by an interruption of the circumferential fibrillar echo texture. Scarring is characterized by loss of normal architecture, with an area of amorphous texture that usually has low reflectivity. The operator should identify if there is a combined lesion of the internal (IAS) and external anal sphincters (EAS) or if the lesion involves just one muscle. The number, circumferential (radial angle in degrees or in hours of the clock site) and longitudinal (proximal, distal or full length) extension of the defect should be also reported. In addition, 3D-EAUS allows to measure length, thickness, area of sphincter defect in the sagittal and coronal planes and volume of sphincter damage. EVUS can assess the levator ani muscle. Avulsion of the levator ani from the inferior pubic rami can be accurately evaluated and the levator ani gap measured. Levator ani damage is a risk factor for the development of pelvic organ prolapse and is correlated to recurrence after reconstructive surgery.
DTPUS has been shown to demonstrate rectocele, enterocele, and rectal intussusception with images comparable to defecography. The extent of a rectocele is measured as the maximal depth of the protrusion beyond the expected margin of the normal anterior rectal wall. On sonographic imaging, a herniation of a depth of greater than 10 mm has been considered diagnostic.  The rectal intussusception may be observed as an invagination of the rectal wall into the rectal lumen or the anal canal during maximal Valsalva maneuver. Enterocele is ultrasonographically visualized as downward displacement of abdominal contents into the vagina, ventral to the rectal ampulla and anal canal. Small bowel may be identifiable due to its peristalsis. The extent of an enterocele is measured against the inferior margin of the symphysis pubis. Pelvic floor dyssynergy can be documented during Valsalva maneuver because the anorectal angle (ARA) becomes narrower, the levator hiatus (LH) is shortened in the anteroposterior dimension and the puborectalis (PR) thickens in evidence of a contraction. The most relevant utility of EAUS applies in the detection of localized EAS and/or IAS defects in patients with obstructive defecation disorders.
The configuration of perianal sepsis and the relationship of abscesses or fistulae with IAS and EAS are the most important factors influencing the results of surgical management. Preoperative identification of all loculate purulent areas and definition of the anatomy of the primary fistulous tract, secondary extensions, and internal opening plays an important role in adequately planning the operative approach in order to ensure complete drainage of abscesses, to prevent early recurrence after surgical treatment, and to minimize iatrogenic damage of sphincters and the risk of minor or major degrees of incontinence. EAUS has been demonstrated to be a very helpful diagnostic tool in accurately assessing all fistula or abscess characteristics. It can be easily repeated while following patients with perianal sepsis to choose the optimal timing and modality of surgical treatment, to evaluate the integrity of or damage to sphincters after operation, and to identify recurrence of fistula. It also gives information about the state of the anal sphincters, which is valuable in performing successful fistula surgery. A fistula tract affecting minimal muscle can be safely excised, but where the bulk of external sphincter muscle is affected, it is best treated by seton drainage or mucosal advancement flap.

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