Transperineal ultrasound assessment of the anal sphincter after obstetric anal sphincter injury (OASI).

Abstract

Obstetric anal sphincter injury (OASI) is the most common cause of anal incontinence and anorectal symptoms in women1. Reported rates of anal incontinence following primary repair of OASI range between 15% and 61%, with a mean rate of 39%2, 3. Other possible complications of OASI include perineal pain, dyspareunia and, less commonly, abscess formation, wound breakdown and rectovaginal fistulae. Symptom onset may occur immediately after delivery, several years postpartum or only late in life, when aging of tissues adds to the delivery insult. Having sustained an OASI may impact significantly on a woman's physical and emotional health. Missed OASI, inadequate repair or lack of follow-up are potential sources of litigation4. The reported incidence of OASI may be as high as 4–6.6%4, averaging 2.9% in the UK3. The incidence is higher in primiparae (6.1%) than it is in multiparae (1.7%)3. In recent years, we have seen an increase in awareness and structured training programs, which have apparently resulted in an increase in the detection rate of OASI3. The following risk factors have been identified, with various risk rates reported3: Asian ethnicity (odds ratio (OR), 2.27; 95% CI, 2.14–2.41), primiparity (relative risk (RR), 6.97; 95% CI, 5.40–8.99), birth weight > 4 kg (OR, 2.27; 95% CI, 2.18–2.36), shoulder dystocia (OR, 1.90; 95% CI, 1.72–2.08), occipitoposterior position (RR, 2.44; 95% CI, 2.07–2.89), prolonged second stage of labor (RR, 2.02; 95% CI, 1.62–2.51, after 4-h duration). Instrumental delivery and the use of episiotomy have been studied extensively, resulting in the following evidence5: the OR for OASI during vacuum delivery without episiotomy is 1.89 (95% CI, 1.74–2.05); vacuum delivery with episiotomy is protective (OR, 0.57; 95% CI, 0.51–0.63), while forceps delivery without episiotomy carries the highest potential risk (OR, 6.53; 95% CI, 5.57–7.64); and the OR for forceps delivery with episiotomy is 1.34 (95% CI, 1.21–1.49). Other potential risk factors, such as advanced maternal age at first birth, vaginal birth after Cesarean delivery and type of obstetric care provider, have been suggested with various levels of evidence4. Possible protective factors include obesity, perineal massage (RR, 0.91; 95% CI, 0.86–0.96)6, perineal protection at crowning3, warm compression during the second stage of labor (RR, 0.48; 95% CI, 0.28–0.84)3, mediolateral episiotomy in instrumental delivery5, a wide angle of the mediolateral episiotomy (at least 60° away from the midline when the perineum is distended)7 and prelabor Cesarean section4. However, clinicians and patients alike should be aware that knowledge of a patient's risk factors does not necessarily allow accurate prediction of OASI3. OASI is usually diagnosed in the period immediately postpartum. The classification is based on the extent of lacerations to the external and internal anal sphincters (EAS and IAS, respectively) and epithelium, as defined by Sultan and coworkers (Figure 1) and adopted by the World Health Organization2, 3, 8. The degree of damage impacts on the development of symptoms, with 3C and 4th-degree tears carrying a graver prognosis than do 3A and 3B tears9. It is not unusual for a tear to be missed in the labor ward: the reported rates of missed OASI range from 26% to 87%10. Since all women having a vaginal delivery are at risk of sustaining OASI, prior to suturing they should be examined systematically, including a digital rectal examination, to assess the severity of damage3. Overall, outcomes following OASI are not encouraging; adequate intra- and postpartum management is important both in order to prevent symptoms and to counsel women regarding a subsequent delivery. There are published guidelines which discuss this extensively (including a Royal College of Obstetricians and Gynaecologists Green-top guideline3 and a Society of Obstetricians and Gynaecologists of Canada practice bulletin4). Pelvic floor physiotherapy can manage anal incontinence successfully3. Management and counseling, preferably in dedicated perineal clinics that provide a combination of perineal examination, anal manometry and ultrasound evaluation of the anal sphincter, can aid decision-making regarding future delivery3, 4. It is widely accepted that women who have sustained an OASI in a previous pregnancy and who are symptomatic or have abnormal endoanal ultrasonography (EAUS) and/or manometry should be counseled regarding the option of elective Cesarean delivery3. The available imaging modalities include EAUS2, transvaginal ultrasound11 and transperineal/translabial ultrasound (TPUS)12-14, with or without three-dimensional (3D) imaging. EAUS is still considered the modality of choice and is also used widely by gastroenterologists and colorectal surgeons. TPUS has emerged within the last decade or so, arising originally as a result of the wide availability of new high-resolution wide-sector probes and volume imaging commonly used in obstetric ultrasound practice. There have been only a few studies comparing these modalities and current understanding is that TPUS is at least as good as EAUS15. TPUS evaluation of OASI shows characteristic signs which correlate well with the clinical complaints of incontinence12, 13. Advantages of this modality include its wide applicability and acceptability by patients because of its painless and non-intrusive nature. Unlike EAUS, in TPUS, using either a transvaginal or a transabdominal probe, there is no distortion of the anal canal by the transducer during the examination, and evaluation of the anal sphincter and mucosa is possible in the resting position. The advanced ultrasound systems available in both tertiary and primary care facilities support the establishment of dedicated one-stop clinics administered by a combination of gynecologists, perinatologists, urogynecologists, sonographers and physiotherapists4. The aim of this Opinion is to describe the methodology for TPUS assessment of the anal sphincter in women who have sustained an OASI. A two-dimensional (2D) ultrasound examination may be performed with any machine of good resolution. 3D examination has the advantage of allowing extensive evaluation of the anal sphincters along their entire length and width, thus improving standardization of measurements and facilitating examination. 3D TPUS is performed with a 3D transabdominal or transvaginal probe developed for obstetric imaging (e.g. RAB 8–4-MHz or 5–9-MHz 3D transvaginal probe, GE Healthcare Ultrasound, Milwaukee, WI, USA; AVV 531, Hitachi Medical Systems, Tokyo, Japan; V 8-4, Philips Ultrasound, Bothell, WA, USA; 3D 4–7 EK, Medison, Seoul, South Korea). We have employed Voluson ultrasound systems in recent years (Voluson 730 Expert, E8, E6 and E10, GE Healthcare Ultrasound). The transabdominal transducers combine an electronic curved-array RAB 4–8 MHz with mechanical sector technology, which allows a fast sweep of at least 70° through the field of view. 3D and 4D ultrasound imaging allow real-time acquisition of volume ultrasound data comprising successive planes acquired during the sweep. The data can be stored for offline analysis and post-processing with the help of dedicated software (e.g. 4D View, GE Healthcare Ultrasound). It can be displayed in orthogonal planes or rendered volumes. Assessment of the longitudinal (sagittal) plane may aid in location and evaluation of the area of rupture, which may be more cephalad to the anus and not amenable to diagnosis by standard clinical examination (‘button-hole tear’2). A transvaginal probe, such as the 5–9-MHz 3D probe, placed in the area of the fourchette, may also be effective for examination of the anorectum by the transperineal approach. The area is scanned in the transverse and sagittal planes. When the probe is placed appropriately in the midsagittal plane, the anorectum can be visualized fully. Additionally, we find that, with just a little pressure exerted on the probe, the entire sphincter can be visualized, including the distal part. Transvaginal and transabdominal probes each have their own advantages and disadvantages. Their use is dependent on operator preference, and they are rarely used interchangeably. The transvaginal probe is characterized by better resolution and high-quality imaging, but the drawback is decreased visualization at the 12 o'clock position. The transabdominal probe allows better visualization at the 12 o'clock position and a thorough examination of the pelvic floor is possible simply by tilting the transducer. Using a transvaginal probe requires increased pressure on the fourchette in order to prevent the probe from sliding backwards and may cause discomfort or pain. The transducer can be covered with a glove, condom or thin plastic wrap to maintain asepsis. Powdered gloves create reverberations that impact on image quality and should be avoided. Mechanical cleaning and alcoholic wipes should be used for disinfection. The patient is placed in the dorsal lithotomy position with hips flexed and slightly abducted. Pelvic tilt can be improved by bringing the patient's heels closer to her buttocks. She is asked to void before the examination but bowel emptying is not mandatory. If a transabdominal probe is used, a midsagittal view is obtained by placing the transducer on the perineum in the area of the fourchette and perineal body. The transducer axis is oriented in the midsagittal plane; the symphysis pubis, bladder neck and urethra, compressed vagina, distal part of the rectum with anorectal junction, and proximal part of the anal canal are seen. A transverse view is obtained by turning the transducer 90° counter-clockwise and tilting it backwards towards the anal sphincter. Similarly, if using a transvaginal probe, this is placed in the area of the fourchette and perineal body. The angle of acquisition differs from that of the transabdominal probe, but the operator can visualize the same structures. Maximum angle of acquisition and quality settings will provide the best volume for later offline analysis. Volume acquisition is performed in the transverse plane and volumes are saved and stored. When the probe is placed appropriately in the midsagittal plane, the anorectum is visualized fully, with the anal verge on one side of the screen and the anorectal angle on the other. The orientation can be in either direction as long as consistency is maintained. Rendering is initiated with the 2D transverse image plane positioned 1.5 cm from the distal margin of the anus, where the characteristic star shape of the anal mucosa can be visualized. During post-processing, the display is switched to show the multiplanar projection, in which the transverse plane of the anal sphincter complex should appear in panel A. The transverse plane can be adjusted along x-, y- and z-axes to obtain an image optimal for measurement. The correct plane has been obtained when the sphincter is visualized as in Figure 2a. The reference dot is placed in the middle of the anal mucosa. The EAS appears as a ring of hyperechogenicity surrounding the thin hypoechogenic ring of the IAS. The mucosal folds of the anal canal are visible as symmetric structures of mixed echogenicity creating a characteristic star shape radiating from the center. The transverse perinei muscles (superficial and deep) can be visualized superior/cephalad to the EAS in panel A. The zoom is set to 1.3–1.6. Panel B of the multiplanar image shows the longitudinal plane, which allows measurement of the length of the hyperechogenic EAS and the hypoechogenic IAS. The puborectalis muscle can be visualized as a U-shaped echogenic structure surrounding the EAS posteriorly, in the proximal part of the sphincter complex. The EAS and IAS widths can be measured in the transverse plane, 1.5 cm from the distal margin of the anus (Figure S1). Activating the tomographic ultrasound imaging (TUI) option will allow tomographic multislice imaging of the anal sphincter in the transverse plane (Figure 3). Nine slices should be displayed in a three-by-three matrix, with an interslice interval ideally of about 2 mm. This will suffice in most cases since the length of the EAS anteriorly rarely exceeds 3 cm. The TUI display is scrutinized for the presence of any sphincter defect. Although the minimum accepted requirements for diagnosis are the three central slices of the TUI image, in order to minimize false-positive or false-negative evaluations, it is imperative to examine all TUI slices. A defect in the EAS is suspected when discontinuity in the hyperechogenic ring is observed, whether it affects the whole EAS or only part of its thickness (Figures 4-8 and S2–S8). The size of the defect is measured by selecting a generic two-line angle from the menu and placing its vertex in the center of the anal mucosa (Figures 6, 7, S2 and S7). This can be done for both EAS and IAS, and reported as an angle measurement. A significant residual defect is defined as an EAS defect on ultrasound with ≥ 30° angle9. Four sonographic signs indicate damage to the anal sphincter following repair of OASI12, 13: discontinuity of the EAS and/or IAS; asymmetry in EAS and/or IAS widths, i.e. thickening of the IAS opposite the rupture site, with thinning or interruption in the area of repair, known as the ‘half moon’ sign (Figure 7); thickening of the EAS in the area of repair; and atypical rectal mucosal folds, these being considered abnormal if some deviation from the symmetrical star shape is observed. Sliding the region of interest to the left of the volume will depict the subcutaneous part of the sphincter. This part is distal to the IAS and therefore there should be no IAS in the image. In order to measure the defect correctly, the TUI image should include the subcutaneous part of the sphincter, which is generally considered to be the strongest part. In this TUI image, the first panel represents the midsagittal plane, followed by panels 2 and 3 which start at the subcutaneous part of the sphincter and continue towards the more proximal sphincter as the slices progress. Panel 9 will usually represent the upper part of the EAS where it terminates and merges with the levator ani. Corresponding images using a transvaginal probe can be seen in Figures 8 and S3–S6. Another severely damaged sphincter can be visualized in Figure S5 and a rectovaginal fistula is depicted in Figure S8. The following parameters may serve as a general guide but can of course be modified in each individual examination to optimize imaging: gray map 4, tint off/sepia according to preference, line filter low, persistence 4, enhancement 1, speckle reduction imaging at least 3, line density normal. The rendered view direction is set to ‘up’ without 3D tinting, but sepia is a good option as well. The image is optimized by combining the surface smooth and gradient light modes at an 80/20 mix with a low-threshold setting, transparency near 20 and dynamic contrast 8. The acquisition angle is set at 85°, with mid to high render quality. Further optimization can be achieved by using volume contrast imaging with a slice thickness of 2 mm and clear tinting. In selected cases, inversion mode can be used for evaluation of the IAS anatomy. Preferably, the analysis should be performed at least 10–12 weeks postpartum. This allows time for the initial edema to subside and enables a complete evaluation of the pelvic floor. Such an examination is not the subject of this article and will not be discussed further here. The report should include details of the examination technique and findings, such as extent of visualization of the anal canal, type of probe used, measurements of sphincter complex and extent of defect. It might, for example, read as follows: ‘The anal canal was visualized in its entirety with transperineal 3D sonography, using a transabdominal probe. In the longitudinal section, the length of the EAS is 10 mm. In the transverse section there is a residual full-thickness defect of the EAS between 10 o’clock and 2 o'clock, corresponding to an angle of 120°, which can be visualized in all sections of the TUI (2-mm interslice interval). The IAS and mucosal folds appear normal. The measurements in the 12 o'clock position are as follows. EAS: not present; IAS: 2 mm; transverse perinei muscles: 5 mm'. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.