Methods
Women having their first vaginal delivery over a 16-month period between March 2016 and June 2017 at University Hospital Lewisham were invited to participate. Women were given a patient information leaflet about the study at 20-22 weeks gestation during anomaly scan appointment. Women were approached by a dedicated clinical research fellow in the maternity triage, antenatal ward and the labour ward in their early labour. During the initial contact, women were informed about the study and invited to participate. The inclusion criteria were a singleton pregnancy of 37 weeks of gestation or more, maternal age 18 years old or more, women having their first vaginal delivery, cephalic presentation, and being able to read and understand English.
All consenting women had their perineum examined after delivery by the accoucher. The clinical examination was conducted according to the individual’s usual practice. All women then had their perineum re-examined by a trained clinical research fellow – this was carried out in a systematic fashion with adequate light and consisted of a rectal and vaginal examination. We minimised the measurement and technique variability by having all clinical examinations performed by a single trained research fellow who is experienced in diagnosing OASIs. All perineal tears were classified according to RCOG classification (18). When an OASI was identified by the clinical research fellow (KW) that was missed by the doctor or midwife who conducted the initial examination it was verified by a senior registrar or consultant obstetrician, thereby minimising the risk of overdiagnosing OASIs by the clinical research fellow.
3D TPUS was performed at rest using a GE Voluson 730 system with a 4-8 MHz transabdominal curved array volume transducer after delivery and before suturing. Women were scanned by the trained clinical research fellow (KW) in the supine position with knees semi-flexed after emptying their bladder. The transverse transducer placement allows the probe to keep a minimal distance from the anal sphincter and focus on the first two centimetres of the anal sphincter. The probe is then tilted to visualise the puborectalis muscle which appears as a ‘U’ shape. It is then angulated to obtain a transverse view of the anal canal and to visualise the whole length of the anal sphincter complex. A minimum of three ultrasound volumes are acquired and used for assessment of the external and internal anal sphincter. The 3D volume images are stored for offline assessment. Tomographic imaging (TUI) allows for evaluation of the entire EAS from the subcutaneous part of the EAS to its cranial termination. It is adjusted to have eight slices, with the interslice interval varying according to the individual sphincter length. Slice one corresponds with the puborectalis level, slice two the most cranial aspect of the EAS (deep level), slice three to six with the superficial level, and slice seven and eight the subcutaneous level. The EAS is seen in slices 2-8 and the IAS in slices 2-6. The angle of discontinuity of the ring shape of the EAS or IAS is captured with TPUS after delivery and prior to suturing and was later measured blindly. An EAS defect is diagnosed when three or more of the seven slices (between slice two and eight on TUI) showed a discontinuity of 30 degrees or more in the ring of muscle of the external sphincter (mixed echoic ring). Similarly IAS defects are diagnosed when two or more of the five slices (between slice two and slice six on TUI) show a discontinuity in the sonographic image of the internal sphincter (hypoechoic ring) (38). One independent investigator (KW) who was blinded to both information regarding the delivery and clinical assessments analysed the ultrasound images. Ten percent of the images were randomly selected and analysed by two independent investigators who were also blinded. Discrepancies were reviewed by a third blinded investigator.