Nivedita Gauthaman1 and Stergios K. Doumouchtsis2, 3
(1)
Department of Urogynaecology, St. Georges University Hospitals, NHS Foundation Trust, Blackshaw Road, Tooting, London, SW17 0QT, UK
(2)
Department of obstetrics and gynaecology, Epsom and St Helier University Hospitals NHS Trust, London, UK
(3)
University of Athens, Medical School, Athens, Greece
Nivedita Gauthaman (Corresponding author)
Email: nivegau@doctors.org.uk
Stergios K. Doumouchtsis
Email: sdoum@yahoo.com
Abstract
Perineal trauma during childbirth is common. Although in the majority of cases perineal trauma does not have a major impact in the woman’s future pelvic floor function, severe degrees of childbirth trauma such as levator ani trauma and obstetric anal sphincter injuries (OASIS) may result in significant pelvic floor dysfunction including urinary and faecal incontinence. Levator ani muscle trauma is diagnosed in one third of women who have vaginal birth. Levator trauma can lead to weakening of the pelvic floor muscles, widening of the urogenital hiatus and pelvic organ prolapse in the future. In this chapter, the biomechanics of the second stage of labour and its effects on the pelvic floor and levator ani complex are discussed along with the effects of prolonged second stage on the pelvic floor. Different modes of delivery including instrumental deliveries have a different impact on the risk of perineal trauma and levator ani complex. The effect of fetal malposition and malpresentation and multiple pregnancies on perineal trauma as well as the role of episiotomy are also discussed in this chapter.
Keywords
Obstetric anal sphincter injuries (OASIS)BiomechanicsSecond stage of labourLevator ani traumaPerineal injuryForceps deliveryVentouse deliveryEpisiotomyMalpresentationMalposition
Introduction
The second stage of labour and various types of vaginal birth have been associated with a variable impact on risks of pelvic floor trauma. Severe degrees of perineal trauma, particularly OASIS and levator ani trauma, can have long-term consequences such as faecal and urinary incontinence, pelvic floor dysfunction and pelvic organ prolapse in the future. Our understanding of the biomechanics of the second stage of labour has improved in recent years with the advent of dynamic imaging modalities and research based on modelling techniques in an effort to simulate vaginal childbirth and its effects on the pelvic floor muscles and the perineum. Fetal malpositions, malpresentations, multiple pregnancies, instrumental deliveries and the use of episiotomy have been identified in several studies as factors affecting the risks of pelvic floor trauma.
The Second Stage of Labour and Its Effects on the Pelvic Floor
Evidence on the effects of the second stage of labour per se on the pelvic floor prior to the birth of the fetus is limited. Studies to evaluate the impact of the second stage on the pelvic floor are challenging due to technical reasons and possible ethical concerns. With biomechanical modeling, changes on tissues of the pelvic floor have been studied to some extent. These are part of various changes to the pelvic floor during the different stages of labour. During the first stage there is increasing strength and frequency of uterine smooth muscle contractions, progressive dilatation of the cervix and descent of the fetal presenting part into the pelvis. The second stage of labour commences after the cervix is fully dilated and there is further descent of the fetal presenting part. This stage is complete when expulsion of the fetus from the introitus takes place with maternal voluntary efforts. There is progressive increase in the intrauterine pressures during uterine contractions and voluntary pushing efforts and an additional increase in intrauterine pressures has been estimated with ventouse traction of 113 N [1] and forceps traction of 200 N [2].
During the second stage of labour the descent of the fetal head causes progressive distension of the perineum and anal dilatation. The stage of fetal crowning occurs when the vulvovaginal opening is persistently dilated by the fetal head, the biparietal diameter has passed through the level of the ischial spines and there is no retraction of the fetal head between contractions. The perineum is thinned out and may undergo spontaneous tearing in primigravidae. This is less common in multiparous women. Episiotomy is usually performed at the stage of crowning to facilitate delivery with less risk of anterior perineal trauma.
Levator Ani Trauma and the Second Stage of Labour
The levator ani muscle complex consists of five parts and is responsible for closing the urogenital hiatus against the opening forces exerted by intraabdominal pressure. The muscle is distributed in a U-shape around the urethra, vagina and rectum, and the force caused during the muscle contraction compresses the rectum, vagina and urethra, from back to front. Using 3D/4D ultrasound imaging and dynamic MRI, detachment of the puborectalis muscle from its insertion to the inferior ramus of the pubic bone can be visualised and is noted in one third of all women having a normal vaginal birth [3]. However, 85–90 % of primiparous women can have a vaginal birth without disruption of the pubovisceral muscles, a result of compensatory stretching of the perineal body. This has been called the “fusible link hypothesis” [4]. Geometric models have suggested that muscle damage in second stage may be due to overstretching. The pubococcygeal/pubovisceral muscle, which is the most medial in the levator ani complex sustains the largest tissue strain with a stretch ratio of 3.26 [3]. MRI studies of simulated vaginal childbirth have also demonstrated the maximum stretch ratio to be 3.5 corroborating similar findings with the geometric models [4]. The maximum stretch ratio is usually achieved at the time of crowning of the fetal head, therefore factors which increase the stretch ratio such as instrumental deliveries and increased head circumference of the fetus or malposition would further increase the risk of levator ani trauma.
The pudendal nerve is also at risk of injury during vaginal delivery. It is located relatively superficially in the pelvis and is therefore prone to stretch damage during childbirth. Stretch injury of the pudendal nerve has been reported in 38–42 % of vaginal deliveries [5]. Although the nerve injury is usually reversible due to re-innervation, severe injuries or total transection may result in protracted restoration of nerve function as shown in simulated childbirth using rat models [6]. Pudendal nerve trauma during childbirth has been implicated in postpartum faecal and urinary incontinence and is usually attributed to increased stretch and pressure injury during the second stage of labour.
Prolonged Second Stage of Labour and the Pelvic Floor
The American College of Obstetricians and Gynecologists defines prolonged second stage of labour as longer than 3 h in a nulliparous woman with regional anaesthesia and 2 h without regional anaesthesia [7]. Prolonged second stage of labour can be associated with malpositions, fetal macrosomia and relative cephalopelvic disproportion. A combination of raised intrauterine pressure in the second stage of labour due to intense contractions and maternal voluntary pushing efforts can lead to ischaemic injury of the pelvic nerves and muscles. This can lead to denervation injuries, which in some cases can be permanent, particularly if the active phase of the second stage of labour is prolonged. A neurophysiological study using concentric needle electromyography (EMG), pudendal nerve conduction tests and perineometer in 96 nulliparous women showed EMG evidence of re-innervation after vaginal delivery in 80 % of cases. Prolonged active second stage of labour (>1 h in primiparous women) and heavier babies showed the most significant EMG evidence of nerve damage, whereas passive second stage of labour did not increase the risk of denervation injury to the pelvic floor [8]. An MRI study in women 9–12 months after the first delivery showed that the use of forceps, anal sphincter rupture and episiotomy were associated with increased risk of levator defects. Women with levator injury were also found to have a 78 min longer second stage of labour in this study [9].
Regional Anaesthesia and Pelvic Floor Trauma
The relationship between epidural analgesia and perineal trauma during childbirth is not clear. It can be inferred that epidural analgesia may be associated with an increase in the rate of severe perineal trauma indirectly due to the increased likelihood of operative vaginal delivery and episiotomy associated with instrumental delivery. However, it can also be postulated that epidural analgesia in labour can exert a protective effect against severe perineal trauma by reducing the uncontrollable urge to push during delivery. To evaluate the independent risk of epidural analgesia in labour, logistic regression analysis was used in a study and epidural was not found to be an independent predictor of perineal injury [10]. A large population based study has shown that epidural analgesia in labour does not increase the risk of perineal trauma [11].
The effect of epidural analgesia on levator trauma is not clear. As epidural analgesia increases the duration of second stage with higher risk of instrumental delivery particularly forceps, it can potentially lead to levator ani injury and microtrauma. Prolonged pushing in second stage can lead to neuromuscular or vascular injury due to distension and stretching and longer second stage of labour has been associated with levator trauma [9]. However, intrapartum epidural may be beneficial by preventing premature pushing and may exert a protective effect on levator trauma by muscle relaxation of the pelvic floor with an effective epidural analgesia [12].
Spontaneous Vaginal Delivery and Perineal Trauma
It is estimated that 85 % of women who deliver vaginally will suffer some degree of perineal trauma in the UK. The incidence of OASIS varies between 1.7 and 18 % [13–15]. Variations are related to the standards of reporting and training to recognise such tears and the methods of identifying OASIS. Various other factors, such as instrumental deliveries, use of episiotomy and parity, also affect the incidence of OASIS, hence the variation in the rates. Gurol-Urganci et al. reported that the rate of anal sphincter injuries tripled in England between 2000 and 2012 (1.8–5.9 %). Improved recognition of these tears, standardisation of the classification of perineal trauma and the decline in the use of routine episiotomy probably contributed to the increasing incidence of OASIS [16].
Episiotomy is one of the commonest procedures performed in labour. Albeit a fairly common practice, there is no robust scientific evidence in support of performing episiotomies and it seems that it has just crept into clinical practice. Episiotomy is usually performed to increase the vaginal orifice shortly prior to the delivery of the presenting part. It is also performed by obstetricians prior to instrumental delivery particularly forceps, breech vaginal deliveries and in deliveries where shoulder dystocia is anticipated such as fetal macrosomia. Episiotomy is also given for indications such as rigid and inflexible perineum particularly in primigravidae, to expedite vaginal delivery in cases of fetal distress during the second stage of labour, and to avoid multiple vaginal tears by performing a controlled surgical incision.
A Cochrane review has recommended the practice of restrictive episiotomy as it has the benefit of reducing severe perineal trauma and posterior perineal trauma although there was an increased risk of anterior perineal trauma [17]. This review included 8 studies with a total of 5541 women. In the routine episiotomy group, 75.15 % of women actually had episiotomies. In the group with restrictive episiotomy, 28.40 % had episiotomy. The restrictive episiotomy group had shown less risk of severe perineal trauma (RR 0.67, 95 % CI 0.49–0.91), less need for suturing (RR 0.71, 95 % CI 0.61–0.81) and lesser complications with wound healing (RR 0.69, 95 % CI 0.56–0.85). The use of restrictive episiotomy was associated with a higher incidence of anterior perineal trauma (RR 1.84, 95 % CI 1.61–2.10). This review did not recommend a specific type of episiotomy (midline versus mediolateral) and left the choice of type of episiotomy with the accoucheur due to lack of evidence.
Intrauterine Fetal Demise and Perineal Trauma
Women delivering vaginally following intrauterine fetal demise appear to have a lower overall risk of perineal trauma compared to women with a live birth in a retrospective case matched study. This study included 323 women who delivered vaginally following intrauterine fetal death and was controlled for age, parity, gestational age and birth weight and excluded other significant factors, which contribute to perineal trauma such as instrumental delivery and episiotomy. The study concluded that women with intrauterine fetal death had a lower risk of perineal trauma (RR 0.16, 95 % CI 0.12–0.22) as well as lower risk of OASIS (RR 0.12, 95 % CI 0.03–0.50), which may be due to differences in biomechanics of childbirth in cases of intrauterine fetal demise [18].
Multiple Births and Perineal Trauma
Twin vaginal births generally tend to happen in earlier gestations than singleton vaginal births. It can be surmised that the lower birth weight and head circumference of twins may cause less perineal trauma than their singleton counterparts. Data looking at perineal trauma in twin pregnancies are limited. A recent retrospective cohort study [19] comparing twin vaginal deliveries (1538) and singleton vaginal deliveries (91,312) in a single tertiary unit identified nulliparity (twins adjusted OR 5.9, 95 % CI 1.7–20.9; singletons adjusted OR 3.9, 95 % CI 3.5–4.4), occipitoposterior position (twins adjusted OR 3.00, 95 % CI 1.1–8.0; singletons adjusted OR 1.6, 95 % CI 1.3–2.00), instrumental delivery (twins adjusted OR 4.3, 95 % CI 1.2–15.4, singletons adjusted OR 2.4, 95 % CI 2.2–2.6) and birth weight (twin adjusted OR 1.1, 95 % CI 1.0–1.2; singletons adjusted OR 1.07, 95 % CI 1.06–1.08) to be independent risk factors for OASIS both in vaginal twin and singleton deliveries. The authors also concluded that no single risk factor posed a higher risk in twins than in singleton pregnancy. The OASIS rate in twin vaginal deliveries (1.27 %) was approximately half than in singleton deliveries (2.55 %) in this study. This is not different to the risk factors which have already been identified to increase the risk of perineal trauma in singleton vaginal births however it is still clinically relevant in counseling women with twin pregnancies prior to making decisions about mode of delivery.
Instrumental Delivery and Perineal Trauma
Ventouse and Forceps Delivery
Instrumental delivery either forceps or ventouse aims to expedite delivery of the fetus in the second stage of labour for various indications and accounts for 11 % of vaginal births in the UK [20]. Maternal indications include maternal exhaustion, prolonged second stage of labour and medical conditions such as pre-eclampsia, placental abruption, acquired or congenital heart disease. The most common fetal indication is fetal distress in the second stage of labour.
The choice of the instrument depends on operator’s preference and expertise, local variations in practice, clinical indications and type of maternal analgesia. Other factors which influence choice of instrument include fetal position, fetal station and availability of instruments. There are advantages and disadvantages in the use of ventouse versus forceps.
There are various types of ventouse available, namely soft cup (silastic) – usually recommended for occipitoanterior positions and anticipated easy delivery, semi-rigid cups – made from flexible plastic and more effective with higher success rates than soft cups, handheld disposable ventouse cups (Kiwi Omnicup®) – used for rotational and non-rotational ventouse delivery, rigid metal cups – anterior and posterior cups – useful for both rotational and non-rotational ventouse delivery. Common cups used are the Bird’s and the ‘malmstrom’ cups. Ventouse delivery has a number of advantages over forceps delivery as it is known to have less potential for trauma to maternal tissues compared to forceps delivery, can be performed without need for regional anaesthesia and episiotomy is not always required. The disadvantages of ventouse are that it cannot be used in preterm fetuses, it is contraindicated in face presentation, has higher failure rates leading to the use of sequential instruments and higher risk of injuries to fetal scalp and cephalhaematoma [21].
A Cochrane review [21] supports the use of ventouse as first line when the procedure is expected to be easy (occipitoanterior position, no cephalopelvic disproportion). Soft ventouse cup is recommended rather than the metallic cup due to less scalp injury (9 studies with 1517 women; RR 0.67, 95 % CI 0.53–0.86) and cephalhaematoma (6 studies with 669 women; RR 0.61, 95 % CI 0.39–0.95). However ventouse is more likely to fail to achieve a vaginal birth compared to forceps which increases the chances of sequential use of instruments, usually forceps, to complete the delivery and attendant risks to the mother and the fetus. The review also supported that forceps were more likely to be successful in achieving vaginal birth in studies including 2419 women (RR 0.65, 95 % CI 0.45–0.94).
There are various types of forceps available. The commonest ones used in the UK are Anderson’s and Neville Barnes forceps. These are typically used for midcavity and low forceps deliveries. Wrigley’s forceps are less commonly used and predominantly for outlet deliveries. There are no randomised controlled trials available to assess the efficacy of one type of forceps over the other and the choice depends on the operators familiarity and preference. Forceps are preferred over ventouse in prematurity (<36 weeks) and malpositions such as face presentation where ventouse is contraindicated, as well as for delivery of the aftercoming head in breech presentations where ventouse delivery is not applicable. Forceps are generally more successful in achieving vaginal birth over ventouse due to higher force applied but require training and expertise in assessing the correct fetal position and station. Forceps are generally associated with increased trauma to the maternal tissues and need for episiotomy.
Forceps are associated with significant maternal trauma such as episiotomy, third or fourth degree tears with or without episiotomy (RR 1.89, 95 % CI 1.51–2.37), vulval and vaginal trauma (RR 2.48, 95 % CI 1.59–3.87) and higher incidence of facial injury of the fetus (RR 5.10, 95 % CI 1.12–23.25) compared to ventouse delivery [21].
Rotational (Kielland’s) forceps are used to achieve rotation of the fetal head from the occipito transverse or occipitoposterior position at the midcavity level of the pelvis. Ventouse can be used in these situations as well, however is fraught with increased risks of failure. Over the past 20 years the rising trend in the use of ventouse, fear of complications and subsequent litigation [22, 23], lack of experience with modern obstetric training and increasing obstetrician’s preference to resort to caesarean sections in second stage [24–27] has led to a decline in the use of rotational forceps. As rotation of the fetal head occurs at the midcavity level, the risk of perineal trauma should not be significantly increased over other types of forceps deliveries. Recent publications [28–30] have shown comparable risks of OASIS with the use of Kielland’s forceps compared to other non –rotational forceps/rotational ventouse deliveries. Rotational instrumental deliveries have a higher risk of injuries to the levator muscle complex [31]. Levator ani avulsion has been linked to higher risk of female pelvic organ prolapse [32]. Currently there are no techniques adapted to repair levator avulsion at the time of delivery, nonetheless this type of injury is often not recognised at the time of delivery.
Sequential Instruments
The use of sequential instruments usually involves completion of a vaginal delivery by forceps when a primary application of ventouse has failed. The main concern with the use of sequential instruments is the risk of neonatal morbidity such as retinal haemorrhage, intracranial haemorrhage and feeding difficulty compared to primary forceps deliveries as shown in a large retrospective study [33]. In another retrospective study comparing successful ventouse deliveries with failed ventouse deliveries either completed by forceps or second stage caesarean sections, the use of sequential forceps was found to increase the risk of OASIS significantly, although neonatal outcomes were comparable [34]. However in choosing sequential instruments considerable thought should be given to achieving a safe vaginal birth with minimum risks to the mother and fetus. The Royal College of Obstetricians and Gynaecologists recommends that this should be carefully considered versus the potential risks of a second-stage caesarean section [35].
In summary, although forceps appear to be more effective in achieving a vaginal birth thereby avoiding a second stage caesarean section there is risk of significant perineal trauma to the mother. Ventouse, particularly use of metal cups increases the risk of cephalhaematoma albeit with significantly less maternal perineal trauma. Soft ventouse cups have lower risks of cephalhaematoma and less perineal trauma but increased risks of failure to achieve a vaginal birth and subsequent use of a forceps with added maternal perineal trauma.
Episiotomy and Its Role in Instrumental Deliveries
Although the intention of performing an episiotomy is to facilitate delivery of the presenting part and to avoid extensive tears in the perineum by performing a controlled surgical cut, there is still controversy as to the benefit of episiotomy in preventing severe perineal tears. Midline episiotomies are generally thought to increase the risk of OASIS when compared to mediolateral episiotomies [36, 37].
The role of episiotomy in the prevention of OASIS in instrumental deliveries is debatable. A large retrospective population based study [38] including 284,783 vaginal deliveries obtained from the Dutch National Obstetric Database, showed an overall risk of third degree tears of 1.94 %. The study concluded that mediolateral episiotomy strongly protected against damage to the anal sphincter complex during delivery (OR 0.21, 95 % CI 0.20–0.23). Another population based retrospective observational study [39] of 21,254 women delivered by ventouse and 7478 women delivered by forceps showed that mediolateral episiotomy significantly protected against OASIS in both the ventouse group (OR 0.11, 95 % CI 0.09–0.13) and forceps delivery (OR 0.08, 95 % CI 0.07–0.11). However other studies did not support these findings. A retrospective cohort study including 33,842 vaginal births [40] showed that operative vaginal delivery particularly in combination with midline episiotomy was associated with an increased risk of OASIS in primi and multigravidae (nullipara OR 4.5, 95 % CI 3.7–5.4; multipara OR 14.6, 95 % CI 10.4–20.5). A prospective non randomised study in the UK [41] comparing the use and non-use of episiotomy for all operative vaginal deliveries showed that episiotomy did not reduce or greatly increase OASIS (9.9 % versus 7.1 %, adjusted OR 1.11, 95 % CI 0.66–1.87). A pilot randomised controlled trial conducted in two maternity units in the UK [42] involving nulliparous women did not show any conclusive evidence of the protective effect of routine episiotomy over restrictive episiotomy against anal sphincter injuries (OR 0.72, 95 % CI 0.28–1.87) in women who underwent operative vaginal deliveries. To date there is no conclusive evidence basis for routine use of episiotomy to avoid third/fourth degree tears with instrumental deliveries and operator judgement in the use of episiotomy is recommended by the RCOG [35].
Malpositions, Malpresentations and Perineal Trauma
Occipitoposterior position is the commonest malposition in labour and is diagnosed in 5 % of deliveries. It is due to deflexion of the fetal head either due to mechanical factors such as flat pelvis or due to weak uterine contractions leading to inadequate flexion of the fetal head. Occipitoposterior position is associated with primigravida, prolonged labour and epidural analgesia and often increases the risks of instrumental delivery which is also known risk factor for OASIS. Persistent occipitoposterior position has been associated with a sevenfold increase in the risk of OASIS [43]. Increased levator ani stretch can be attributed to occipitoposterior positions due to the higher diameter of the presenting part, however this association needs to be evaluated further.
Breech presentation is the commonest malpresentation and occurs in 3–4 % of term pregnancies. In modern obstetric practice caesarean section is increasingly performed for breech presentations, although there is a role for breech vaginal delivery particularly in multiparous women and favourable breech presentations such as flexed breech and frank breech with an average sized fetus. There are no studies which have specifically looked at perineal trauma in breech vaginal deliveries. Episiotomy would be preferred in breech vaginal deliveries in order to increase the outlet diameter and also to enable use of various manoeuvres for assisted vaginal breech delivery. Forceps has been used to facilitate the delivery of the aftercoming head in vaginal breech deliveries. The effect of episiotomy and the use of forceps for the aftercoming head may increase the risk of perineal trauma and OASIS but there are no published studies to confirm this. A case of the fetal foot causing isolated rectal tears in frank breech delivery with intact anal sphincters has been reported [44].
Shoulder Dystocia and Perineal Trauma
Shoulder dystocia (SD) is an obstetric emergency which is defined as failure to deliver the anterior, posterior or both shoulders of the fetus requiring additional manoeuvres to achieve delivery. It occurs when the shoulder of the fetus cannot pass below the pubic symphysis causing delay in internal rotation, fetal descent and delivery. It can be associated with serious neonatal morbidity and mortality such as brachial plexus injury, birth asphyxia and upper limb, clavicular and rib fractures. Maternal complications usually relate to perineal trauma particularly obstetric anal sphincter injuries (OASIS). There is limited evidence of associations between specific manoeuvres for shoulder dystocia and risk of OASIS.
In a recent retrospective study [45] which included cases of shoulder dystocia over a period of 5 years in a tertiary teaching unit, SD was associated with a three-fold increase in the risk of OASIS. The use of internal manoeuvres (OR 2.182: 95% CI 1.173-4.059), increased number of manoeuvres ≥4 (OR 4.667: 95% CI 1.846-11.795), Wood’s screw manoeuvre (OR 3.096: 95% CI 1.554-6.169), reverse Woods’ screw manoeuvre (OR 4.848: 95% CI 1.647-14.277) and removal of the posterior arm (OR 2.222: 95% CI 1.117-4.421) were all associated with a significant increase in the likelihood of OASIS. The authors concluded that to effectively manage shoulder dystocia with consideration of perineal trauma, these factors need to be considered in designing further prospective studies and developing management protocols for shoulder dystocia in the future.
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