Intrapartum care
DOI: 10.1111/j.1471-0528.2006.00799.x
www.blackwellpublishing.com/bjog
Occult anal sphincter injuries—myth or reality?
Vasanth Andrews,a Abdul H Sultan,a Ranee Thakar,a Peter W Jonesb
a Mayday University Hospital, Croydon, Surrey, UK b School of Computing and Mathematics, Keele University, Stafforshire, UK
Correspondence: AH Sultan, Mayday University Hospital, London Road, Croydon, Surrey CR7 7YE, UK.
Accepted 28 September 2005.
Objectives To establish the true prevalence of clinically
recognisable and occult obstetric anal sphincter injuries (OASIS).
Design Prospective interventional study.
Setting Busy district general hospital.
Sample Two hundred and fifty-four women having their first
vaginal delivery over a 12-month period were invited. Two
hundred and forty-one (95%) participated and 208 (86%)
attended follow up.
Methods Women had a clinical examination at delivery by the
accoucheur and repeated by an experienced research fellow
immediately after delivery. All identified OASIS were verified and
repaired by the duty specialist registrar or consultant. Endoanal
ultrasound was performed immediately postpartum prior to
suturing and repeated seven weeks later.
Results Fifty-nine (24.5%) women sustained OASIS. The
prevalence of OASIS increased significantly from 11% to 24.5%
when women were re-examined. Of these, 30 occurred in deliveries
by midwives who missed 26 (87%) and 29 following deliveries by
doctors who missed 8 (28%) injuries. All clinically apparent OASIS
were also identified on endoanal ultrasound. In addition, three
(1.2%) women had an occult anal sphincter injury. Two of these
occult sphincter injuries were isolated to the internal anal sphincter
(IAS) and would not usually be clinically detectable.
Conclusions OASIS occur more frequently than previously
reported. Many remain undiagnosed and are subsequently
classified as occult when identified on anal endosonography.
Genuine occult injuries are rare. Training in perineal anatomy and
recognition of OASIS needs to be enhanced in order to increase
detection of OASIS and minimise the risk of consequent anal
incontinence.
Main outcome measures Prevalence of recognised and occult anal
sphincter injuries.
Please cite this paper as: Andrews V, Sultan A, Thakar R, Jones P. Occult anal sphincter injuries—myth or reality? BJOG 2006; 113:195–200.
Obstetric trauma is the major cause of anal incontinence in
women and is believed to affect 40,000 mothers (5%) annually in the UK.1 However, it is largely under reported due to
social stigma and embarrassment. Recognised obstetric anal
sphincter injuries (OASIS) occur in 0.4–19% of vaginal deliveries in centres practising mediolateral and midline episiotomies, respectively.2,3
Obstetric-related anal incontinence was previously believed
to be largely due to pudendal neuropathy.4,5 However, following the advent of anal endosonography most of these women
demonstrated ‘occult’ anal sphincter injuries (Fig. 1). A
strong association has been shown between anal incontinence
and occult anal sphincter injury diagnosed by ultrasound.6
There are now nine prospective studies6–14 showing that
between 20% and 41% of women sustain occult sphincter
injuries. However, it remains to be established whether these
injuries are truly ‘occult’ or represent ‘overt’ anal sphincter
injuries that have either been wrongly classified as a seconddegree tear1,15 or missed.16
Furthermore, as OASIS are scrutinised during risk management meetings, there is a disincentive to document these
injuries accurately. Previous teaching caused confusion in
classification17 and a recent study by Fernando et al.1 found
that 33% of practising obstetric consultants in the United
Kingdom were wrongly classifying external anal sphincter
(EAS) tears as second-degree tears. Sultan et al.15 interviewed
75 trainee doctors and 75 midwives and found that there was
considerable inconsistency in their classification, and that
most were unaware that anal incontinence was an important
consequence of vaginal delivery. Consequently, a new classification was suggested18 and this has now been accepted by
the RCOG19 and the International Consultation on Incontinence20 (Table 1). OASIS therefore represent third or fourthdegree tears. Concern has also been raised that some anal
ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
195
Introduction
Andrews et al.
Methods
Figure 1. Endoanal scan demonstrating anal sphincter defect. Anal
endosonographic image of the mid-anal canal demonstrating an anterior
EAS defect between the arrows. The adjacent hypoechoic ring (l) is the IAS
that also appears fragmented anteriorly.
sphincter injuries are being missed at the time of delivery.
This has been demonstrated at Queen Charlotte’s Hospital,
London,16 where a clinical research fellow examined women
who sustained perineal trauma and found that 40% of OASIS
were being missed.
In view of the anatomical confusion, inconsistency in classification of perineal tears and evidence that OASIS are being
missed, we aimed to determine the true prevalence of clinically overt and occult anal sphincter trauma in women
having their first vaginal delivery.
Table 1. Classification of perineal tears
Intact perineum
No visible tear
First-degree tear
Second-degree tear
Injury to skin only
Injury to the perineum involving perineal
muscles but not the anal sphincter
Injury to the perineum involving the anal
sphincter complex:
3a: less than 50% external sphincter
thickness torn
3b: more than 50% external sphincter
thickness torn
3c: IAS torn
Injury to the perineum involving the anal
sphincter and anal epithelium
Third-degree tear
Fourth-degree tear
196
Women having their first vaginal delivery over a 12-month
period between February 2003 and January 2004 at Mayday
University Hospital were invited to participate. A trained
clinical research fellow (VA) recruited women prior to delivery during office hours, nights and weekends.
All consenting women had a repeat perineal and rectal
examination by the research fellow immediately after being
examined by the accoucheur. In addition, when the delivery
was conducted by a doctor, the midwife caring for the woman
examined the perineum and her findings were noted. When
OASIS were diagnosed they were confirmed by the on-call
specialist registrar or consultant. Labour and delivery details
were recorded prospectively.
Endoanal ultrasound (Fig. 1) was performed in the left
lateral position using a 10-MHz 360! rotating probe (B & K
Naerum, Denmark).6,21 All women had endoanal ultrasound
scans performed at delivery (prior to suturing) and repeated
at seven weeks postpartum. All scans were video recorded and
reviewed independently by two experts in endoanal ultrasonography (AHS & RT) who were blinded to the clinical
diagnosis.
Data were entered into a Microsoft Excel database and
analysed by SPSS version 11.0 (Chicago, Illinois, USA). To
investigate the change in dependent proportions a McNemar
test was used, and to compare proportions in independent
groups the x2 test was used. Women were given written information about the study in the antenatal period and all gave
written consent when admitted in labour.
Results
Over the study period there were a total of 1495 primiparous
women who had a vaginal delivery of whom 254 were invited
and 241 (16%) consented to participate in the study. Two
hundred and thirty-two (96%) were nulliparous and nine
(4%) had had a previous caesarean section (none in the
second stage of labour).
One hundred and seventy-three (72%) deliveries were conducted by midwives and 68 (28%) by doctors. Sixty-three
(91%) of the deliveries by doctors were instrumental deliveries (40 vacuum extraction, 23 forceps deliveries). Ninety-eight
(41%) had a mediolateral episiotomy performed. The comparable episiotomy and instrumental delivery rates in our
institution during the same period were 39% and 21%,
respectively. The prevalence of OASIS increased significantly
from 11% to 24.5% (McNemar’s test P < 0.000) when women
were re-examined by the trained research fellow.
One hundred and twenty-nine of the 173 (75%) deliveries
were performed by midwives with at least five years experience. The diagnosis of injuries by the midwives and research
ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
Occult anal sphincter injuries—myth or reality?
fellow are shown in Table 2. Of the 173 deliveries, midwives
classified eight as OASIS of which only four had OASIS confirmed by the research fellow. In a further two cases, the midwife suspected OASIS and requested a doctor’s opinion but
neither of these were found to have OASIS. In total, 30 sustained OASIS but only 4 were correctly identified by the
midwife. Of the remaining 26 OASIS, the midwife made
a diagnosis of a second-degree tear in 25 and in one case
classified it as a first-degree tear. All 30 women with OASIS
had the diagnosis confirmed either by the on-call specialist
registrar or consultant and their anal sphincter was repaired
in theatre according to a standardised protocol.22
The remaining 68 deliveries were performed by doctors, 14
by senior house officers under supervision and 54 by specialist
registrars. None of the women delivered by doctors had an
intact perineum and only one woman had a first-degree tear.
Of the 68 deliveries, 22 (32%) had OASIS identified by the
doctor performing the delivery and all were confirmed by the
research fellow. In a further seven cases OASIS were missed by
the doctor performing the delivery. In four cases, the career
senior house officer conducting the delivery failed to identify
the OASIS that were subsequently confirmed by the duty
specialist registrar and research fellow. In three further cases
the research fellow identified OASIS that were missed by the
duty specialist registrar and these injuries were all confirmed
by the on-call consultant. One woman with a second-degree
tear was wrongly classified as third degree by a senior house
officer. Specialist registrars missed 14% of OASIS, this being
significantly less than the 67% missed by senior house officers
(x2 test, P = 0.016). The midwife caring for each woman
delivered by a doctor was asked to examine the perineum in
the same manner as they would normally do after delivery.
Only 1 of the 29 (3.4%) OASIS was identified and no midwife
performed a rectal examination. Twenty- seven were classified
as second-degree tears and one as first degree. The classification of perineal trauma following deliveries performed by
doctors is shown in Table 3.
The results of the endoanal ultrasound scan are shown in
Table 4. Two hundred and nine (87%) attended follow up at
a mean of seven weeks and two days (SD = 16 days). There
were 11 discrepancies on scan reports initially that were
resolved on review. All women who had OASIS had a sonographic defect at delivery. In addition, there were three defects
on ultrasound that were not seen clinically. Two of these
involved the internal anal sphincter (IAS) alone and one
was a combined defect of the IAS and EAS. No de novo defects
were seen at follow up.
Discussion
Anal incontinence is a social stigma that can have a devastating effect on a woman’s quality of life. Although anal sphincter injury remains the major aetiological factor, it was
previously largely attributed to ‘occult’ anal sphincter injuries.6
Table 2. Rates of perineal trauma in deliveries conducted by midwives. The question mark (?) represents suspected anal sphincter injuries and
details of these are given in the Results section
Midwives diagnosis (%), n = 173
Intact perineum
32 (18.5)
First-degree tear
20 (11.6)
Second-degree tear
111 (64.2)
Third/fourth-degree tear
8 (4.6)
Third-degree tear*
2 (1.2)
Research Fellow diagnosis (%), n = 173
Intact perineum
First-degree tear
Second-degree tear
Third/fourth-degree tear
Intact perineum
First-degree tear
Second-degree tear
Third/fourth-degree tear
Intact perineum
First-degree tear
Second-degree tear
Third/fourth-degree tear
Intact perineum
First-degree tear
Second-degree tear
Third/fourth-degree tear
Intact perineum
First-degree tear
Second-degree tear
Third/fourth-degree tear
24 (13.9)
7 (4)
1 (0.6)
0
5 (2.9)
5 (2.9)
9 (5.2)
1 (0.6)
0
4 (2.3)
82 (47.4)
25 (14.5)
0
0
4 (2.3)
4 (2.3)
0
0
2 (1.2)
0
*Suspected anal sphincter injuries. See Results section for further details.
ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
197
Andrews et al.
Table 3. Rates of perineal trauma in deliveries conducted by doctors
Delivering doctors Research fellow
diagnosis (%)
diagnosis (%)
Intact perineum
0
First-degree tear
1 (1.5)
Second-degree tear
45 (66.2)
Third/Fourth-degree tear 22 (32.4)
0
1 (1.5)
38 (55.9)
29 (42.6)
This is the first prospective study to accurately establish the
prevalence of anal sphincter injury using the new internationally accepted classification of perineal trauma.18,20 Secondly,
this is the first study in which endoanal ultrasound was performed immediately postpartum and then repeated seven
weeks later to establish the true prevalence of occult sphincter
injuries. To avoid bias, all ultrasound images were reported
independently by two clinicians who were blinded to the clinical diagnosis.
We found that when a trained research fellow re-examined
postpartum women before suturing of the perineum, the
prevalence of OASIS increased significantly from 11% to
24.5%. This finding is consistent with another observational
study in which the rate of OASIS doubled when women were
re-examined.16 Our findings indicate that the majority of midwives and junior doctors (senior house officers) were failing
to identify OASIS. This is in keeping with questionnairebased studies in which the majority of practising doctors
and midwives indicated that they were inadequately trained
in diagnosing and repairing perineal trauma.1,15 We
therefore believe that our findings are not unique to local
practice but probably reflect a more widespread phenomenon. Doctors and midwives were not blinded to the intent
of the study because women subsequently found to have
OASIS had to have their anal sphincter injury repaired in
theatre with written consent. However, their prior knowledge
of the study did not appear to affect their clinical practice as
the rates of episiotomy and instrumental deliveries did not
differ from that of other primiparous women delivering in
this unit.
All clinically diagnosed OASIS were identified by endoanal ultrasound. In three women, defects were identified by
endoanal ultrasound but not seen clinically. Two of these
were isolated IAS injuries. It is unclear whether these minor
IAS defects were due to clinical disruption or whether they
represent distortion of the IAS as a result of the shearing
forces during delivery.6 In practice, isolated IAS injuries
would not normally be clinically apparent as the EAS would
need to be torn as well. There was only one anal sphincter
injury diagnosed by scan that was not recognised at delivery
(involving both the IAS and EAS). This may therefore represent a genuine occult sphincter injury. No new defects were
identified by ultrasound at follow up that were not seen on the
first scan performed immediately after delivery, indicating
that all anal sphincter injuries occur at the time of birth
and not in the puerperium. This study also demonstrates that
following appropriate training, clinical examination can be as
accurate as anal endosonography in the immediate postpartum period. However, as postpartum anal endosonography is
technically difficult, time consuming, expensive and requires
additional expertise, it should not substitute accurate clinical
examination.
By performing anal endosonography in the immediate
postpartum period and repeating it seven weeks later, we have
demonstrated that genuine occult sphincter injuries are a rare
occurrence. Therefore, almost all ‘occult’ sphincter injuries
described previously6–14 probably represent missed tears.
Although two other studies have shown that endoanal ultrasound is feasible in the postpartum period neither study had
independent clinical verification nor were follow up scans
performed.23,24
Table 4. Endoanal ultrasound scan findings
Type of injury
No. of cases, n = 241 (%)
Anal endosonographic
defects prior to perineal
repair, n = 241 (%)
Anal endosonographic
defects at follow up,
n = 209 (%)
Intact perineum
First-degree tear
Second-degree tear
3a tear
3b tear
3c tear
Fourth-degree tear
All third/fourth-degree tears
(OASI)
29 (12)
17 (7.1)
136 (56)
28 (11.6)
30 (12.4)
0
1 (0.5)
59 (24.5)
1/29 (3.4)
0/17 (0)
2/136 (1.5)
28/28 (100)
30/30 (100)
0/0 (0)
1 (100)
59/59 (100)
1/24 (4.2)
0/16 (0)
2/111 (1.8)
0/27 (0)
6/30 (20)
0/0 (0)
0/1 (0)
6/58 (10)
198
ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
Occult anal sphincter injuries—myth or reality?
We have also demonstrated that clinical examination at the
time of delivery remains the cornerstone of diagnosis of anal
sphincter damage. In each case, the research fellow performed
a careful examination of the perineum and vagina. In keeping
with previous recommendations,25 he performed a rectal
examination to exclude rectal or anal sphincter injury. Visual
inspection combined with palpation by performing a pillrolling motion between the index finger in the rectum and
the thumb over the anal sphincter25 improved the detection
rate of OASIS. Unfortunately, although most midwifery and
obstetric textbooks recommend that a rectal examination
should be performed after perineal repair, very few indicate
the need for rectal examination before repair.
Conclusions
Our study has demonstrated that a large number of OASIS are
missed at delivery. These injuries that have previously been
classified as ‘occult’ do have clinical relevance as it has been
shown that 42% of women who have asymptomatic ‘occult’
anal sphincter defects after their first vaginal delivery develop
faecal incontinence after a subsequent vaginal delivery.26 It is
therefore important that doctors and midwives undergo more
focussed and intensive training to recognise these tears at
delivery. Recent innovations such as using a specially designed
latex model and fresh animal anal sphincters in dedicated
hands-on workshops22 (www. perineum.net) are proving very
popular. These hands-on workshops have been shown to
change clinical practice of doctors27 and midwives.28 Consequently, a fourfold change of practice was noted among midwives who began performing rectal examinations prior to and
after perineal repair.
Recognition of the deficiencies of current training and
more widespread implementation of re-education programmes need reappraisal so as to improve identification of
OASIS. Only then can we minimise the risk of developing
anal incontinence—a physically and socially devastating
affliction.
Acknowledgements
We would like to thank B & K Medical for loaning the scanner
to perform this project.
Competing interests
Mayday University Hospital provides training courses for
doctors and midwives in perineal repair.
Funding
Mayday Childbirth Charity Fund.
ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
Ethical approval
The study was approved by the Croydon Ethics and Research
Committee. j
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ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology