To the best of our knowledge, this is the first description of accurately measured dimensions of the perineal body on 2D ultrasound. This is a simple, non-invasive method of imaging the perineal body that can be performed in the clinic. The values of the perineal body dimensions were obtained during live scanning. This trans-perineal ultrasound scan can be performed in a clinical outpatient setting with a 7-MHz curved linear probe, such as those found in most modern Obstetrics and Gynaecology departments. This method of scanning the perineal body gives the clearest images observed so far in the literature. Previous studies had shown poor repeatability [10, 11]. This may be because, in those studies, the perineal body was measured on images that acquired the entire pelvic floor. When measuring small structures by ultrasound, such as the nuchal translucency or a gestational sac, it is important for the image to be optimised for these measurements. We acquired maximally zoomed-in images that were optimised specifically for the perineal body. Optimised images enable the accurate and repeatable measurement of the perineal body.
In prolapse, the perineal body area is less than 2.4 cm2, as defined by the lower value of the 95% confidence interval. A small perineal body (less than 2.4 cm2) is associated with prolapse.
The POP-Q PB does not correlate with the perineal body length on ultrasound, as demonstrated by these data and those of previous studies . The POP-Q PB only considers the length between the fourchette and the anus, but does not consider the thickness of the substance of this structure, which in cases of prolapse may be just skin. The ultrasound was aimed at assessing this area in more detail. The perineal body is known to be a confluence of the pelvic floor muscles. This study has shown that in prolapse, this confluence is damaged. It is found to be smaller in prolapse patients.
Other studies correlating POP-Q PB and POP-Q GH with the presence of prolapse have also shown similarly that increasing stages of prolapse are associated with POP-Q GH, but not with POP-Q PB . POPQ PB + GH have been shown to improve the prediction of levator injuries in women with incontinence and prolapse .
The POP-Q PB may not correlate with perineal body ultrasound measurements owing to the presence of perineocoeles. Perineocoeles have been demonstrated in cadaveric anatomy studies . In these cases, the perineal muscles weaken and sublux, causing the bowel to prolapse into the perineum . A perineocoele may co-exist with a rectocoele. Clinically, this is not immediately evident because the POP-Q PB distance does not discriminate between these entities.
These data suggest that the perineal body might be an important support structure for the anorectal complex. A small perineal body (less than 2.4 cm2) is strongly associated with posterior compartment prolapse. These findings are in support of Emmet’s and Kelly’s thinking, in which they emphasised the importance of perineorrhaphy, i.e., perineal body reconstruction in vaginal prolapse repair [1, 2].
These findings also supplement the theories for prolapse discussed above. Failure of the collagenous infrastructure puts the musculature of the pelvic floor under strain. The perineal body is the confluence of many pelvic floor muscles. It is possible that when put under strain from other structural changes, such as levator ani injury or collagen fascial compromise, the confluence of muscles is slowly pulled apart, leaving the anorectum unsupported. This possibility is supported by cadaver dissection work showing subluxation of the muscles that would normally attach on the perineal body .
Alternatively, it may be that primary damage of the perineal body, which may be a destabilising factor in itself, putting the fascial collagenous structures under undue strain, causing them to fail over time. For example, as with levator ani injury, the perineal body may also be damaged during childbirth . It may also be possible for both of these hypotheses to be true in different women. In advanced prolapse, procidentia, all the structural supports are likely to have failed, resulting in complete eversion of the vagina.
Beyond childbirth-related mechanical disruption, the pathophysiology of prolapse is also known to include multiple collagen gene defects , hormone-induced collagen alterations , neurological factors , changes in the extracellular matrix , age [24, 25], obesity , constipation , chronic pelvic floor stress , previous operations (such as hysterectomy) , and a wider transverse inlet of the bony pelvis .
The strength of this study is the novel ultrasound method for the assessment of the perineal body in the posterior compartment. This technique contributes towards a comprehensive clinical assessment of the pelvic floor.
Future research could further evaluate the role of the perineal body, using the technique described here, in understanding the pathophysiology of prolapse.