A pictorial overview of pubovisceral muscle avulsions on pelvic floor magnetic resonance imaging
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Despite extensive research aimed at clarifying (failing) pelvic organ support, the complete aetiology of pelvic organ prolapse (POP) is still not fully understood. During vaginal delivery, the pelvic floor can be irreversibly traumatised, resulting in pubovisceral muscle avulsions. The aetiology of these avulsions is discussed in this pictorial overview. Normal female pelvic floor anatomy is described and variations are exemplified using magnetic resonance (MR) images. The clinical relevance of detecting pubovisceral muscle avulsions is specified.
T2-weighted MR imaging has multiplanar capabilities with high diagnostic accuracy allowing for detailed visualisation of the pelvic floor. Together with the use of a three-dimensional (3D) post-processing program, the presence and severity of pubovisceral muscle avulsions can be quantified.
Pelvic floor MR imaging is a non-invasive method that enables adequate identification of pubovisceral muscle avulsions which are known risk factors for the development of POP. They can be scored with good to excellent inter- and intra-observer reliability.
Radiologists and urogynaecology subspecialists should be familiar with MR imaging findings of pubovisceral muscle avulsions as this birth-related trauma is observed in over 36 % of vaginally parous women.
• Pelvic organ prolapse (POP) is a growing problem for both patients and for our healthcare system
• Pubovisceral muscle avulsions are known risk factors for pelvic organ prolapse (POP)
• T2-weighted MR imaging visualises pubovisceral muscle avulsions adequately
• Pubovisceral muscle avulsions are scored with good to excellent inter- and intra-observer reliability
KeywordsInjuries Magnetic resonance imaging Parturition Pelvic floor Pelvic organ prolapse
The opening within the bony pelvis comprises the biggest potential hiatus within the human body. It is closed off by the pelvic floor which acts as a supportive layer to all pelvic and abdominal organs. The pelvic floor must maintain urinary and faecal continence, but also permit urination and defecation. Furthermore, in women, intercourse and vaginal birth should be possible. It is understandable that with these contradicting properties the pelvic floor is prone to failure, which could ultimately lead to symptoms of pelvic floor dysfunction .
Pelvic organ prolapse (POP), urinary incontinence (UI) and faecal incontinence (FI) are the most common of all disorders assembled under the term “pelvic floor dysfunction”. Up to 20 % of the female population will have symptoms of POP and/or UI severe enough to require surgery . Another one out of six women will undergo additional surgery because of postoperative POP recurrence [3, 4]. Even though a vast amount of research has increased our knowledge on pelvic floor dysfunction since the first documentation in the Kahun Gynaecological Papyrus (ca. 1800 B.C. [fragments of the Kahun Papyri were discovered by Flinders Petri in 1889 and are kept at the University College London]), the complete disease mechanism of POP is still not fully understood: women with several risk factors may have normal pelvic organ support, whereas others develop POP even though they exhibit none of the known contributors.
Pelvic organ prolapse has both non-obstetric and obstetric causes. The former group includes conditions suggestive of weak connective tissue and a family history of POP (congenital factors), but also advancing age, heavy lifting and chronically increased intra-abdominal pressure [5, 6]. Furthermore, smoking, being from middle-European descent and a higher body mass index are reported contributors [5, 7, 8]. But of all these factors, none has as big a negative impact on pelvic organ support as vaginal delivery. Women who have given birth vaginally are 4–11 times more likely to develop POP .
But how do these risk factors influence pelvic organ support? What alters in the pelvic floor that leads to a lacking supportive function? And how can we visualise these alterations? One explanation lies in the damage to the pubovisceral component of the levator ani muscle. Avulsions are (partial) detachments of the pubovisceral muscle from their insertion on the pubis. Pubovisceral muscle avulsions can occur during vaginal delivery and are readily visualised using magnetic resonance (MR) imaging .
In this pictorial overview, we describe how the presence and severity of pubovisceral muscle avulsions can be interpreted on pelvic floor MR imaging. Both normal female pelvic floor anatomy and pubovisceral muscle avulsions are visualised. Furthermore, clinical implications are discussed.
Normal female pelvic floor anatomy
Failing pelvic organ support
The term POP is commonly used to describe any degree of downward pelvic movement of a vaginal compartment with the pelvic organs protruding into the vagina or even past the introitus. With anterior compartment prolapse, the bladder and/or urethra are involved, whereas in POP of the central compartment, the vaginal vault with the uterus or cuff scar (in case of prior hysterectomy) descents. The latter may contain the small intestine, bladder or sigmoid. In case of posterior compartment prolapse, the rectum, with or without the small intestine, descents into the vagina. Multiple vaginal compartments are often affected .
Previously, when a woman suffered from symptoms of pelvic floor dysfunction, imaging techniques such as cystourethrography and defecography were used in case physical examination was non-conclusive. However, pelvic floor MR imaging provides an overview of the entire pelvic floor, which is especially useful when patients have symptoms that indicate the involvement of multiple vaginal compartments. Julia Fielding explained when to use MR imaging and how to interpret these images in case of signs and symptoms of pelvic floor dysfunction . Nowadays, diffusions tensor MR imaging even allows three-dimensional (3D) visualisation and quantification of the female pelvic floor . The current article is an addition to the paper of Fielding and is aimed at the interpretation of pubovisceral muscle avulsions on T2-weighted pelvic floor MR imaging.
Pathogenesis and clinical implications of pubovisceral muscle avulsions
During vaginal delivery or even during an attempt at such, the levator hiatus of the labouring woman is stretched to enable the fetus to pass. The area of the average fetal head measures 70–100 cm2 , which is several times larger than the original area of the levator hiatus . Using an MR imaging-based computer model, Lien et al.  found that the pubovisceral muscle has to stretch up to 3.3 times its initial length during crowning of the fetal head. It is understandable that the pubovisceral muscle may hereby be damaged. The prevalence of levator ani muscle injury is reported to be up to 36 % in vaginally parous women and presents as a detachment, i.e. avulsion, of the pubovisceral component of the levator ani muscle [18, 19]. These pubovisceral muscle avulsions can be observed as a complete loss of connection to the pubis or as a partial detachment with apparent loss of muscle bulk, either unilaterally or bilaterally. Besides this direct type of injury, in which the pubovisceral muscle is torn off from its insertion on the pubis, the muscle can be indirectly injured due to denervation resulting in atrophy, or by ischaemia with reperfusion damage. The indirect types of injury are, however, not subject of this paper.
Several intrapartum factors have been found to be associated with pubovisceral muscle avulsions: the use of forceps, increased maternal age at first delivery and prolonged second stage of labour (dilation phase). Vacuum delivery, epidural analgesia or oxytocin administration did not increase the odds for pubovisceral muscle avulsions. In a recent study, we found that episiotomy, prior anterior vaginal wall reconstructive surgery, presence of POP symptoms and increasing POP of the central vaginal compartment on physical examination were correlated with more severe pubovisceral muscle avulsions. In case of symptoms of obstructive defecation, it was more likely that there was no defect of the levator ani muscle on pelvic floor MR imaging . The study was performed among a group of patients who had either recurrent POP or a discrepancy between clinical signs and symptoms of POP.
Pubovisceral muscle avulsions are more often observed in women with POP. DeLancey et al.  found an adjusted odds ratio of 7.3 for major pubovisceral muscle avulsions in women with POP compared with a matched control group. Furthermore, women with pubovisceral muscle avulsions were reportedly more susceptible to postoperative POP recurrence of the anterior vaginal compartment, as they were more often submitted to pelvic floor reconstructive surgery [21, 22].
Diagnostic methods for evaluating pubovisceral muscle avulsions
Pubovisceral muscle avulsions were first described in the 1940s by Gainey , but have only recently found their way into our textbooks . Thus, so far there is little awareness for the daily occurrence of this birth related injury. Fortunately, research has been growing steadily, hereby increasing our knowledge. Avulsions can be diagnosed by both palpation and imaging techniques.
MR imaging was the first imaging technique used to detect pubovisceral muscle avulsions. Significant advantages of MR imaging include its native multiplanar capabilities and superior soft-tissue differentiation, whereas considerable costs and limited availability during routine clinical care are cons. These disadvantages can be overcome by 3D perineal ultrasonography (US). Due to technical improvements, 3D US now enables us to visualise the pelvic floor in the axial plane as well . It is therefore more and more used in the research setting. Its capability to distinguish between different tissues remains inferior to MR imaging, but the clinical relevance of this might be limited. The Translabial 3D-Ultrasonography for Diagnosing Levator Defects (TRUDIL) study is currently being performed to estimate the diagnostic accuracy of 3D US compared with MR imaging . Specific MR imaging techniques can be used to evaluate pubovisceral muscle avulsions, such as endoanal MR imaging . In this paper, however, images obtained with T2-weighted MR imaging using an external surface coil were used.
Pelvic floor MR imaging protocol
There is no international standardised pelvic floor MR imaging protocol that can be used for women with symptoms and/or signs of pelvic floor dysfunction. The following represents the protocol of the Radboud University Medical Centre, Nijmegen, The Netherlands.
MR imaging is performed after voiding with the patient in the supine position. No oral or intravenous contrast agents are administered. For bowel preparation, 10 mg bisacodyl is taken the day before imaging and 1 ml scopolaminebutyl is injected intramuscularly directly prior to imaging to reduce intestinal movement. Patients are asked to retain from ingesting fluids and eating solid food up to 1 and 4 h before the examination, respectively. Image quality is hereby enhanced. As the entire protocol also includes a dynamic phase, the anal canal is opacified with 120 ml ultrasound gel. The dynamic images are used to identify a possible enterocele and/or rectal intussusception. Only the static images are used for the grading of pubovisceral muscle avulsions. In our experience, the presence of ultrasound gel never interfered with the grading of pubovisceral muscle avulsions.
Static images of the pelvic floor are acquired in the axial plane using T2-weighted turbo spin-echo (TSE) sequences on a 3-T (TRIO; Siemens Medical Solutions, Erlangen, Germany) MR imaging unit (repetition time [ms]/echo time [ms] (TR/TE), 2,000/90; field of view (FOV), 200 × 200 mm; slice thickness 3 mm, no image spacing; 320 × 320 matrix; flip angle 150°). A 3D SPACE sequence is acquired of the entire pelvis with 1 mm isotropic voxels. Section orientation is parallel and perpendicular to the plane of the anal canal. A phased-array surface coil for signal reception is centred low on the pelvis. The entire MR examination time is approximately 25 min.
Pubovisceral muscle avulsions can be scored with adequate inter-observer reliability among women with either POP or urinary incontinence, and healthy controls . We recently extended the use of the levator defect scoring system to a group of women with recurrent POP or with a discrepancy between clinical signs and symptoms of POP. One of the observers in our study was a novice examiner who had no radiology experience, but who did participate in a case-based course. This was followed by the jointly scoring of 15 MR datasets with the second observer, an experienced abdominal radiologist. Again, good to excellent inter-observer reliability was obtained .
Scoring pubovisceral muscle avulsions
The research group of Professor John DeLancey (Ann Arbor, MI, USA) developed the levator defect scoring system to assess pubovisceral muscle avulsions. This scoring system does not just observe the amount of muscle present, as this is known to vary among nulliparous women and most likely to vary even more among vaginally parous women . Instead, it focuses on the type of injury seen after vaginal delivery and uses a scoring system that estimates what percentage of the expected muscle bulk is missing. Both the axial and coronal planes are used for scoring. The sagittal plane was not found to be useful for avulsion assessment.
Unilateral scoring of pubovisceral muscle avulsions
Complete connection of the pubovisceral muscle to the pubis with normal muscle bulk
Connection of the pubovisceral muscle to the pubis, but <50 % of the muscle bulk is missing
Connection of the pubovisceral muscle to the pubis, but ≥50 % of the muscle bulk is missing
Complete detachment of the pubovisceral muscle from its insertion on the pubis
Classification system of pubovisceral muscle avulsions
4–6 or unilateral score of 3
Challenges in scoring pubovisceral muscle avulsions
We have provided a structured overview on the anatomy, pathogenesis and diagnosis of pubovisceral muscle avulsions with a special emphasis on MR imaging. Assessing presence and severity of pubovisceral muscle avulsions can be easily learned when the provided scoring system is used. Good to excellent inter- and intra-observer reliability can hereby be obtained. Women with major pubovisceral muscle avulsions are more likely to develop POP and have previously undergone pelvic floor reconstructive surgery more often than those without levator trauma. The clinical implications of being diagnosed with a pubovisceral muscle avulsion remains topic of future research.
Conflict of interest
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