While hormonal and mechanical effects of pregnancy may be contributory, vaginal birth is considered the single most important modifiable risk factor for development of pelvic floor dysfunction, in particular pelvic organ prolapse (POP).¹ Nerve injuries during vaginal birth have been proposed as the potential pathway, however, a lack of evidence of permanent denervation and muscle atrophy in women who have sustained obstetric fistula argues against this as an important mechanism.² Traction and tearing of pelvic floor musculofascial structures are likely the missing link between childbirth and pelvic floor morbidities.

Significance of Pelvic Floor Injuries and Limitations of Clinical Management

Anal Sphincter Trauma

Third/fourth degree perineal tears, or obstetric anal sphincter injuries (OASI), are the most important modifiable risk factor for anal incontinence in women of reproductive age. OASI has been extensively researched and is well recognised by O&G professionals.³ However, management is sometimes suboptimal. Under- and over-diagnosis of OASI are not uncommon, and some tears are missed.^4,5 With a quoted incidence of 3-5%, the true incidence of OASI is likely substantially higher (Figures 1-2).⁵ Both missed tears and under-diagnosis can result in inadequate repair and bowel symptoms.

On the other hand, over-diagnosis can cause psychological stress and unnecessary caesarean births. Inaccurate clinical diagnosis makes research and quality control activities dubious. Hence, OASI should not be used as a key performance indicator (KPI) in maternity services unless confirmed by imaging. The quality of OASI repair also needs attention. Imaging studies after primary repair have shown residual anal sphincter defects in 16-40% of cases (Figure 3).^6,7

In terms of prevention, a Cochrane meta-analysis demonstrated that a policy of selective episiotomy reduced OASI incidence by 30%,⁸ but an inappropriately executed episiotomy can cause rather than prevent anal sphincter injuries (Figure 4). Episiotomy angle, length, depth, and incision starting point are all associated with risk of OASI.⁹

Fig. 1. A case of missed diagnosis of OASI. (A) Exoanal imaging showing a normal anal sphincter during pregnancy. (B) A missed 3B anal sphincter trauma marked by * revealed on postpartum exoanal imaging. A 2nd degree perineal tear was documented by attending staff in delivery suite. EAS: external anal sphincter; IAS: internal anal sphincter; AC: anal canal.. Source: Guzman Rojas R, Shek K, Langer S, Dietz H. Prevalence of anal sphincter injury in primiparous women. UOG. 2013; 42:461–466.

Fig. 2. (A) Overdiagnosis: A case with a documented clinical Grade 3c perineal tear. Postpartum exoanal imaging revealed no defect, distortion, thinning of the anal sphincter. (B) Underdiagnosis: A case with a documented Grade 3a perineal tear. Postpartum exoanal imaging revealed abnormalities (indicated by arrows) at the EAS and IAS implying a 3c/4th degree tear. Source: Gillor M, Shek KL, Dietz HP. How comparable is clinical grading of obstetric anal sphincter injury with that determined by four-dimensional translabial ultrasound? UOG. 2020; 56:618–623.

Fig. 3. (a) Well-repaired 3c tear without significant residual anal sphincter defects. Discontinuity of =>30° in only 2/6 slices of interest at the EAS (slices 2 and 3). (b) Badly-repaired 3c tear with discontinuity of =>30° in 6/6 slices of interest at the EAS (slices 2-7). Source: Dietz HP, Shek KL and Low GK. Validation of new ultrasound algorithm for estimating prevalence of anal sphincter trauma in a urogynecological population. UOG. 2011;60:800-04.

Fig. 4. A poorly incised episiotomy. The episiotomy (linear hypoechoic scar at the perineum indicated by arrows) was incised towards, not away, from the anal sphincter. There was an underlying unrecognised and unrepaired 3b sphincter trauma with measurements showing defect angles. Source: Dietz HP. Exoanal imaging of the anal sphincters. Journal of Ultrasound in Medicine. 2018; 37:263–280.

Levator Trauma

Levator trauma is a form of birth trauma that is even more common than OASI, and poorly recognised.³ It is often undiagnosed because it is mostly occult – it often occurs behind intact vaginal skin. Levator trauma is probably more important to our patients and the health care system than OASI. An Australian woman’s lifetime risk of requiring surgery for POP by age 85 years is 19%.¹⁰ One third of surgeries for prolapse and urinary incontinence are for recurrence.¹¹ In the US, 200,000 POP surgeries are performed annually, 9.5 times more than for faecal incontinence.¹ The demand for prolapse surgery will increase further because of ageing populations in developed countries. Understanding the etiology of POP is important to develop prevention. Current literature data suggest vaginal birth is the most important modifiable risk factor for its development, and levator trauma is likely the most major etiology, especially for prolapse of bladder and uterus.^1,12,13

On simulated vaginal birth, it was estimated that the most ventromedial aspect of the levator ani has to stretch by a factor of more than three,¹⁴ though the degree of muscle distension varies greatly in the pregnant population.¹⁵ Detachment of the puborectalis muscle from its insertion at the inferior pubic ramus, termed “levator avulsion”, has been reported in 2-36% of vaginally primiparous women.¹⁶ Older maternal age at first vaginal birth and forceps delivery are the best-established predictors.^17,18 Occipito-posterior birth, macrosomia, a prolonged second stage, vaginal birth after caesarean section (VBAC), and a lower BMI have also been implicated as risk factors for levator trauma. Levator avulsion is associated with pelvic floor muscle weakness, abnormal distensibility of the levator hiatus, and increased pelvic floor muscle strain.¹⁹ Women seem to notice the effect on intercourse. Levator trauma can also have a psychological impact on women, with 68% reporting symptoms compatible with post-traumatic stress disorder following a traumatic birth with levator avulsion.²⁰ Apart from avulsion, the levator ani may sustain microtrauma leading to irreversible over-distension of the levator hiatus during vaginal birth.²¹

POP is a form of hernia, and the levator hiatus is the hernial portal. Normally, the levator ani maintains near closure of the levator hiatus, and pelvic organ support structures are subjected to minimal tension as a result. If the levator ani muscle is anatomically or functionally defective, the levator hiatus will open, subjecting pelvic organ support structures to excessive loading because of the pressure differential between intra-abdominal and atmospheric pressure. These support structures will gradually stretch and fail over time. Both levator avulsion and an abnormally distensible hiatus are independent risk factors for POP^{22, 23} and prolapse recurrence after surgical repair.^24,25 The risk of prolapse recurrence after native tissue repair can be as high as 90% in women with a highly abnormal pelvic floor in whom cure is almost impossible without mesh use.²⁵

Transperineal Ultrasound Imaging

Transperineal ultrasound is increasingly used for levator ani and anal sphincter imaging. The equipment required is a 3D/4D ultrasound system and a curved array volume transducer, the same as is used for obstetric imaging. Compared with other techniques such as MRI and endoanal ultrasound, the equipment for transperineal imaging is widely available to O&G clinicians. The technique is simple, cheap, has superior spatial and temporal resolution, and allows real-time assessment of pelvic floor functional anatomy. It has been shown to be repeatable and valid in pelvic floor and anal sphincter imaging and has been internationally standardised.²⁶

Unlike endoanal imaging, transperineal ultrasound is non-intrusive, causes less patient discomfort, does not distort anatomy, and has moderate to good agreement with endoanal ultrasound.²⁷ Furthermore, it allows visualisation of perineal scars for quality control after episiotomy. The starting point, direction, angle, length, and depth of incision can be assessed.²⁸ With the same ultrasound system and transducer, one can perform a comprehensive pelvic floor assessment, including the lower urinary tract and pelvic organ support, to facilitate patient counselling and care.

Diagnostic Technique of Anal Sphincter Imaging

For exoanal imaging, the curved array volume transducer is placed transversely on the perineum, angled 40-60 degrees to the anal canal to obtain a coronal or transverse view of the anal canal. The internal anal sphincter (IAS) is typically seen as a hypoechoic ring, and the external anal sphincter (EAS) appears as an echogenic ring surrounding it (Figure 5).

Figure 6 shows the main steps in producing a tomographic representation of the anal canal for assessment. A volume on pelvic floor muscle contraction (PFMC) is acquired as it seems to improve tissue discrimination. However, the anal sphincter can also be assessed at rest for women unable to perform a PFMC. On multiplanar imaging, the cranial limit of the EAS is identified in the mid-sagittal plane dorsally before switching to tomographic imaging.

On tomographic imaging, a set of eight transverse slices of the anal canal is obtained, with the most inferior or caudal slice placed just caudal to the IAS and the most superior or cranial slice just above the cranial limit of the EAS. The six central slices are assessed for sphincter integrity (Slices 2-7 in Figure 6C). A defect is diagnosed if there is a disruption of 30 degrees or more of the circumference of the EAS.

A significant EAS defect is diagnosed if such a sphincter abnormality is found in at least four of six tomographic slices (Figure 3b).²⁹ It has been recommended elective cesarean birth be offered to those with significant anal sphincter defects after OASI.³⁰

Fig. 5. (A) Placement of the transducer transversely on the perineum, inclined 40°-60° to the anal canal to obtain a transverse or coronal view of the anal canal (see in B). P: Perineum; EAS: External anal sphincter; TP: Transversus perinei muscle; IAS: Internal anal sphincter; IRF: Ischiorectal fossa. Source: Dietz HP. Pelvic floor ultrasound with permission. In: Fleischer A, editor. Sonography in Obstetrics and Gynecology: Principles and Practice. 8th ed. Columbus: McGraw Hill; 2017.

Fig. 6. Main steps in producing a tomographic representation of the anal canal for assessment. (A) The anal sphincter is identified on B mode 2D imaging in the transverse or coronal plane. (B) One switches to 4D sectional planes and acquires a volume on PFMC. During contraction, the mid-sagittal or B plane (image 2B) is monitored to ensure the entire anal canal is within the field of vision and at optimal depth, and to allow identification of the fascial plane between EAS and levator ani. EAS is outlined by dots ventrally and dorsally. (C) One then switches to tomographic imaging in the transverse or coronal or A plane, with the mid-sagittal plane as reference slice (top left image in C). The sphincter is evaluated in slices 2-7, with the first slice located above the EAS and slice 8 located below the IAS. Source: Dietz HP. Ultrasound imaging of maternal birth trauma. Int Urogynecol J. 2021;32:1953-62.

Diagnostic Technique of Levator Ani Imaging

Levator trauma is usually occult, unless exposed by a large vaginal tear.³¹ Avulsion can be diagnosed by digital palpation, but this requires substantial training. It can also be assessed on 2D oblique parasagittal ultrasound imaging; however, this is less valid because of a lack of anatomical landmarks. The current gold standard for the diagnosis of levator avulsion on imaging is by tomographic ultrasound using the transperineal technique, utilising the same 3D/4D ultrasound system and curved array volume transducers as for exoanal imaging. The transducer is placed longitudinally on the perineum (Figure 7).

Figure 8 shows the main steps in producing a tomographic representation of the levator ani for assessment. An ultrasound volume obtained on PFMC, or at rest for women unable to perform PFMC, is acquired for assessment. Using tomographic imaging, a set of six axial plane slices is obtained from 2.5mm below to 10mm above the plane of minimal hiatal dimensions, with the interslice interval set at 2.5mm. The plane of minimal hiatal dimensions is identified in the mid-sagittal plane where the distance between the pubic bone and the levator is the shortest (Figure 8B).²⁶

Full/complete levator avulsion is diagnosed if the puborectalis muscle has detached from its insertion on the inferior pubic ramus in all three central slices, i.e. the plane of minimal hiatal dimensions and the slices 2.5mm above and below this plane on one side in unilateral and on both sides in bilateral levator avulsion (Figure 9).

Fig. 7. (A) Placement of transducer on the perineum to obtain standard mid-sagittal view of the pelvic floor. Schematic representation of resulting image in mid-sagittal view (B). SP: Symphysis pubis; U: Urethra; B: Bladder; V: Vagina; Ut: Uterus; P: Perineum; AC: Anal canal; R: Rectal ampulla; ARA: Anorectal angle. Source: Dietz HP. Pelvic Floor Ultrasound: a review with permission. AJOG. 2010;202:321-34.

Fig. 8. The four steps in producing a tomographic representation of the levator ani for assessment. (A) A volume on pelvic floor
contraction is acquired, observing the effect of the manoeuvre in the mid-sagittal plane (left) and a rendered axial plane (right) (8A). The minimal distance between symphysis pubis and anorectal angle (the “plane of minimal dimensions” shown by the white line in 8B) is identified in an orthogonal representation of a volume showing a pelvic floor contraction, with the mid-sagittal plane rotated anticlockwise until the plane of minimal dimensions is horizontal (8B). Then switch to multislice imaging in the axial plane which allows the construction of a set of 8 axial tomographic slices at 2.5mm interval (8C). The axial image is rotated 90 degrees to place the symphysis pubis at 12 o’clock. In a final step, slice location is adjusted so that the symphysis pubis appears open in the first of the three central slices (slice 3), closed in the central slice (slice 4), and invisible due to acoustic shadowing in slice 5 (8D). Source: Dietz HP. Ultrasound imaging of maternal birth trauma. Int Urogynecol J. 2021;32:1953-62.

Fig. 9. Tomographic ultrasound imaging showing a right full levator avulsion. The right puborectalis muscle is disconnected from the inferior pubic ramus (defect marked by asterisks).

Postpartum Imaging for Birth Injuries

In a recent qualitative study, women suffering from levator avulsion reported multiple barriers to help-seeking behavior and felt abandoned by a medical system that did not recognise or identify their trauma.²⁰ There is an urgent need to improve training and education of O&G professionals on maternal birth trauma.³  Women could be greatly assisted by perinatal clinicians who acknowledge their concerns and provide relevant diagnostic and therapeutic services.²⁰ A parliamentary inquiry into maternal birth trauma in NSW in 2023/24 has led to a recommendation to fund postpartum services, including physiotherapy and mental health support for women affected by birth trauma.³² Imaging should be incorporated in such postnatal services, and 3D/4D TPUS is the imaging technique of choice.

After primary repair of OASI, imaging helps to evaluate the accuracy of clinical diagnosis and the quality of repair. For women with an episiotomy, it can provide quality control. These imaging findings could facilitate the provision of feedback to colleagues in delivery suite, to help improve OASI diagnosis, management, and prevention. Postnatal imaging of women at high risk of pelvic floor injuries helps identify missed OASI and levator trauma. For symptomatic women, it helps validate and explain symptoms and guides referral and therapeutic intervention. Imaging supports more individualised advice if anal sphincter and/or levator trauma is identified, e.g. bowel management, weight management, and pelvic floor physiotherapy. Apart from obstetricians and pelvic floor physiotherapists, such postpartum services should involve other disciplines including psychologists, psychiatrists, urogynaecologists, and colorectal specialists.

When and Who Should Have Postpartum Ultrasound Imaging?

As tissue oedema, haematoma, pain, and suture material can make interpretation of imaging findings difficult, imaging is optimally performed 10-12 weeks post repair. As anal sphincter and levator trauma usually occur with first vaginal birth,^33,34 we propose postpartum imaging to be offered to vaginal primiparae at high risk of birth trauma. They include:

1. Primiparae =>35 years of age
2. Instrumental birth (forceps or vacuum)
3. Prolonged second stage
4. Shoulder dystocia
5. Fetal macrosomia of >4,000g
6. Occipito-posterior birth
7. Vaginal sidewall tears (a marker of levator avulsion)³¹
8. Symptoms of POP, e.g. dragging sensation or vaginal lump
9. After primary repair of OASI
10. Persistent bowel control issues including faecal/flatal incontinence/faecal urgency, regardless of the degree of tear documented at birth.

Conclusion

Morbidity and mortality have always been the KPIs in medical practices. Recently, natural childbirth ideology has shifted the focus to caesarean birth rates, which have become a core KPI of maternity services. To reduce caesarean birth rates, VBAC,³⁵ forceps delivery,³⁶ and tolerance of a longer second stage³⁷ have been promoted. These changes could have long-term adverse effects on women’s physical and mental wellbeing.^1,38 Changes in demographics in recent decades, with women having their first baby at an older age, and larger babies, make women more likely to sustain pelvic floor trauma during vaginal birth.^17,39 In a study in Norway, 45% of healthy women in the general population were found to have POPQ stage =>2, 13% had prolapse symptoms, and 19% levator avulsion 20 years after their first birth. Levator avulsion was significantly associated with symptoms and signs of prolapse.²² These findings showed that many women in the general population have symptoms and signs of POP associated with levator trauma years after their first birth. Early postpartum diagnosis of birth trauma and rehabilitation may help prevent disease progression and reduce future demand for treatment.

Over the last few years, ultrasound equipment manufacturers have been developing AI software to offer automated analysis of ultrasound data and reduce workflow time for anal sphincter and levator image analysis.^40,41 This should greatly facilitate the assessment and management of pelvic floor injuries. It is time that we offer postpartum ultrasound imaging for the identification and management of birth trauma, to establish maternal birth trauma as a KPI in obstetric services.