Sydney Pelvic Floor Health

The Anal Sphincter

Anal sphincter trauma

The anal sphincter is generally imaged by endo-anal ultrasound, using high resolution probes with a field of vision of 360 degrees. This method is firmly established as one of the cornerstones of a colorectal diagnostic workup for anal incontinence and covered extensively in the colorectal and radiological literature. Due to the limited availability of such probes in Gynaecology, Obstetricians and Gynaecologists have taken to using high-frequency curved array probes placed exoanally, i.e., transperineally (Peschers et al. 1997), in the coronal rather than the midsagittal plane as described for all other applications in this text.

There are advantages to this approach- not just from the point of view of the patient. Exoanal imaging reduces distortion of the anal canal and allows dynamic evaluation of the anal sphincter and mucosa at rest and on sphincter contraction, which seems to enhance the definition of muscular defects. However, resolutions may be inferior, and good comparative studies are still lacking.

Figure 1 demonstrates transducer placement and schematic appearances on translabial imaging of the anal sphincter complex in the coronal plane. Figure 2 shows normal appearances in the three orthogonal planes as imaged with 3D volume ultrasound. The mucosa is visualized as a hyperechoic area, often star-shaped, representing the folds of the empty anal canal. The internal anal sphincter (IAS) is seen as a hypoechoic ring, the external anal sphincter (EAS) as an echogenic structure surrounding the internal sphincter. On contraction the anal canal narrows slightly, the mucosal star may be less pronounced, and defects of the sphincter will become more obvious.

The sphincter can be seen in standard pelvic floor volumes obtained on pelvic floor muscle contraction, but optimal resolutions require that the transducer be rotated by 90 degrees, since spatial resolution is best in the main transducer plane (see Figure 1). This is particularly useful for depiction of the internal anal sphincter. A 3D or 4D volume data set also allows tomographic imaging (Yagel et al., 2007), similar to the TUI we use to standardise the diagnosis of levator trauma (see Figure 3). We rate a significant (or, after repair, ‘residual’) defect as a discontinuity of the EAS that extends over >30 degrees and that is visible in at least 4 out of 6 tomographic slices (see Figure 4).

This method has recently been validated against symptoms of fecal incontinence (FI) in women presenting to our urogynaecological service. Table 1 shows the results: defects that are evident in 4 or more slices seem strongly associated with FI (Guzman Rojas 2014). Internal sphincter damage seems relatively less important. Mathematical modelling using this data showed that 86% of all FI in this population of over 500 women was explained by the appearance of external and internal anal sphincter.

In a large prospective series (n=320 births) we recently identified a large number of defects of the external anal sphincter, 22% qualifying as ‘residual defects’ (Guzman Rojas 2012). Most of this EAS damage had not been detected in Delivery Suite, and in three cases were diagnosed with an intact perineum after childbirth. This probably implies truly occult trauma, since under-diagnosis appears very unlikely. On the other hand, we documented 2 episiotomies with unrecognised full extension and 2 second-degree tears that were very likely under-diagnosed. Table 2 shows the prevlence of residual EAS defects in women after vaginal delivery.

There seems to be a need for better education of our Labour Ward staff in the recognition of 3rd degree tears, since a substantial majority of those tears seems to be missed (see Figure 5 for an example). On the other hand, there is anecdotal evidence of truly ‘occult’ EAS defects, ie., defects that are masked by intact skin or fascia. The significance of such partial defects remains doubtful.

The quality of our repairs is another issue. After repair of 3rd and 4th degree tears ultrasound commonly demonstrates residual defects (see Figure 4 for an example), and the extent of such incompletely or inadequately repaired defects seems associated with fecal incontinence (Shek et al., 2012), see Table 3. Pelvic floor ultrasound may well have a major role to play in the evaluation of patients after traumatic delivery, but further studies are needed to define the role of exoanal in comparison to endoanal ultrasound. Such studies should validate both methods against symptoms as it is not acceptable to simply declare endoanal ultrasound the ‘gold standard’, as has occurred in the past.

If one considers OASIS and levator trauma jointly, it soon becomes obvious that a large minority of women who succeed in giving birth vaginally will suffer substantial trauma that is clearly associated with future morbidity.

We are legally and morally required to share this information with our antenatal patients, and we need to develop means of prevention and effective treatment. We need to make pelvic floor trauma a key performance indicator of obstetric services with a view to providing remedial therapy to delay or prevent subsequent pathology, and to facilitate practice improvement projects. The increasing availability of modern imaging equipment and the skills of using it for pelvic floor assessment means that it is now becoming possible to provide such services as a post-natal routine.

Finally, it has to be mentioned that it is not uncommon in older women to find defects of the internal anal sphincter which are clearly not obstetric in nature. Usually there is a history of haemorrhoidectomy (Figure 6), and such defects are often symptomatic.

Fig 6: IAS defect between 4 and 6 o’clock several years after surgical treatment of hemorrhoids. Such defects are often symptomatic.

Figure 1: Schematic illustration of exo-anal sphincter imaging in the coronal plane.

Figure 2: Normal appearances on translabial (exoanal) 3D imaging of the anal sphincter complex in the transverse plane (top left), the midsagittal plane (top right) and the axial plane (bottom left).

The Anal Sphincter fig 3.jpg

Figure 3: Normal appearances on tomographic imaging of the anal sphincter. The midsagittal reference plane is in the top left hand corner.

Figure 4: Determination of defect angle in patient after poorly repaired 3B perineal tear.

Table 1: Result of logistic regression modelling of the risk of FI in two models for (a) external and (b) internal anal sphincter findings on tomographic ultrasound imaging (n=501).

Table 2: The prevalence of residual EAS defects in a cohort of primiparae after vaginal delivery of a singleton baby at term (n=247).

Table 3: The relationship between residual EAS defects and levator trauma on the one hand and fecal incontinence 2-3 months after repair of a 3rd/ 4th degree tear (n=140).

Figure 5: Normal findings (A) before and an unrecognised residual EAS defect after a normal vaginal delivery in a primiparous woman who was rated as a minor perineal tear in Delivery Suite.

Figure 6: Symptomatic rectovaginal fistula (arrow) in patient after repair of 4th degree perineal trauma (singleton delivery at term, primipara). There are both EAS and IAS defects.