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.