Interpretation
The rate of OASIS in primiparous women in the United Kingdom is 6.1%
(19), which was similar to that detected by the accoucher. However, when
women were re-examined by the trained research fellow, the incidence of
OASIs increased from 6% to 9%. Previous studies have also shown that
OASIs are missed by doctors and midwives (6, 13, 14). Andrews et al
performed a prospective study of 254 primiparous women immediately after
delivery, and found that the prevalence of OASIs increased from 11% to
24.5% when women were re-examined by a trained clinical research fellow
(13). Of these, 87% of OASIs were missed by midwives compared to 24%
by doctors. Similarly, Groom et al independently re-assessed 121 women’s
perineum following a vaginal delivery and found that 40% of OASIs were
being missed by accouchers (6). Both of these studies however are more
than 15 year old and subsequently there has been an increase both in
awareness and training in the diagnosis and management of OASIs (16).
Hands on training courses in the management and diagnosis of OASIs are
now mandatory for trainees in Obstetrics and Gynaecology the UK and have
been shown to improve both the knowledge of perineal anatomy and
techniques of identifying (15-17). Most missed OASIs however appear to
be by midwives. In the UK there is no standardised assessment for the
midwives in the diagnosis and repair of perineal tears. For
obstetricians the Royal College of Obstetricians and Gynaecologists have
developed objective structured assessment of surgical and technical
skills for perineal tears. These are mandatory for obstetric trainees,
and consideration should be given to implementing these for midwives as
well.
This is the first study where TPUS was performed immediately after
delivery and prior to suturing. 90.5% of anal sphincter defects seen
clinically were confirmed on ultrasound and 99% of the clinically
intact anal sphincters appeared to have an intact anal sphincter on 3D
TPUS. We found that TPUS had a low positive predictive value to diagnose
a sphincter defect and a high negative predictive value to detect an
intact anal sphincter. This is in keeping with the findings of a
cross-sectional study of 250 women who sustained OASIs that were scanned
at 6-12 weeks postpartum by TPUS, where they found that 3D TPUS had a
low positive predictive value (0.37-0.63) and a high negative predictive
value (0.85-0.95) when compared with EAUS. They found that 3D TPUS could
identify an intact anal sphincter but only had a fair ability to
diagnose an EAS and IAS defect (9). Although Taithongchai et al’s (9)
study is the most adequately powered study to date to compare the TPUS,
TVUS with EAUS, it must be acknowledged that their training is of high
standard and may not be generalisable.
TPUS had also been used to evaluate the anal sphincter complex during
the early postpartum period. García-Mejido et al found that of the 146
women where TPUS was performed within 48 hours postpartum, 12 OASIs were
detected by clinical examination and all such injuries were confirmed by
TPUS (20). These results, however, should be interpreted with caution
due to the small number of women who sustained OASIs and the study was
not adequately powered.
In our study there were two anal sphincter defects (1%) seen on
ultrasound and classified as a second degree tear clinically. These may
have been missed by the clinical research fellow. It may also represent
a genuine “occult” sphincter injury. Genuine ‘occult’ anal sphincter
injuries are defects that are only seen on ultrasound and not detected
clinically (21). These injuries were previously believed to be
“occult”, but subsequently have been shown to represent clinically
missed OASIs (13). Our findings were consistent with another
observational study where 1% of OASI were seen on ultrasound but were
not clinically detectable (13).
We found two OASIs that were diagnosed clinically but not seen on TPUS.
The plausible explanation of the discrepancy between the clinical and
ultrasound findings might be because of the technical difficulties
performing an ultrasound examination immediately postpartum. It can be
challenging due to oedema, haematoma and suture material (22) which may
impair the image quality of the EAS. The IAS, however, seems to be
easier to interpret due to its hypoechoic nature. In our study eight
(3.5%) ultrasound images were excluded due to quality of the images.
This finding is consistent with another observational study by
García-Mejido et al (23) who performed TPUS on women immediately
postpartum and found that three anal sphincter defects (2%) were not
diagnosed on clinical examination but were detected in the subsequent
ultrasound scan. They excluded five (3%) images due to poor quality for
analysis caused by the intense perineal oedema.
All OASIs in our study were classified according to the RCOG
classification. Using this classification if less than 50% of the EAS
is torn it is classified as a 3a tear, and as a 3b tear when there is
more than 50% involvement. We found that there is poor correlation
between the size of an EAS defect seen clinically, when compared with
ultrasound findings using TPUS similar to that seen on EAUS (13).