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35. Shoulder Dystocia
35.1 Introduction
Shoulder dystocia is an unexpected, uncertain, and unpreventable obstetric emergency. It is the nightmare of obstetricians, nurses, midwives, and other healthcare providers.
Shoulder dystocia
Spong and colleagues defined shoulder dystocia as a “prolonged head-to-body delivery time (e.g., more than 60 s) and/or the necessitated use of ancillary obstetric maneuvers.” The 60-s interval was selected as, in their study, it was approximately two standard deviations above the mean value for head-to-body time for uncomplicated deliveries [5].
35.1.1 Incidence
Incidence of shoulder dystocia varies worldwide from 0.6% to 3% among vaginal deliveries of fetuses in vertex presentation [6]. Differences in reported incidence may be present because of clinical variations in defining shoulder dystocia and the population studied and because of overdiagnosing or underdiagnosing milder forms of shoulder dystocia [2]. Studies involving the largest number of vaginal deliveries (34,800–267,228) report incidences between 0.58% and 0.70% [7–12].
35.1.2 Pathophysiology
Of the three diameters of the pelvic brim, the anteroposterior is the narrowest, the oblique is larger, and the transverse diameter is the widest diameter. The fetal bisacromial diameter (12 cm) enters the pelvis at an oblique angle, with the posterior shoulder ahead of anterior one. It then rotates to the anteroposterior position at the pelvic outlet with the external rotation of the fetal head. The anterior shoulder then slides under the pubic symphysis for delivery.
During descent, if the fetal shoulders remain in an anteroposterior position or descend simultaneously rather than sequentially into the pelvic inlet, then the conditions are set for shoulder dystocia. In these cases, it is the anterior shoulder that becomes impacted behind the pubic symphysis and the posterior shoulder almost always descends below the sacral promontory. In extremely rare cases, both the shoulders may remain above the pelvic brim, resulting in bilateral shoulder dystocia.
If the anterior or posterior shoulders remain impacted, the descent of fetal head continues; this may result in the stretching of the nerves in the brachial plexus which may cause nerve injury of fetus, further leading to neonatal brachial plexus palsy.
Compression of the umbilical cord or compression of the vessels in the fetal neck by a tight nuchal cord or by combination of both may result in fetal acidemia. With the delivery of the head, the volume of uterine contents is reduced, and the uterus contracts down which diminishes or stops the blood flow to the intervillous space. Since the fetal chest is compressed, the respiratory effort of fetus and oxygenation are impeded.
35.2 Risk Factors
Risk factors for shoulder dystocia
Sr. no. | Prepregnancy | Antepartum | Intrapartum |
|---|---|---|---|
1 | Previous shoulder dystocia | Diabetes mellitus | Oxytocin augmentation |
2 | Prior macrosomia | Excessive maternal weight gain | Prolonged active phase of first stage of labor |
3 | Pre-existing diabetes | Suspected macrosomia | Prolonged second stage of labor |
4 | Maternal obesity (BMI >30 kg/m2) | Short stature | Secondary arrest in second stage |
5 | Prior gestational diabetes | Post-term induction | Protracted or failure of descent of head |
6 | Advanced maternal age | Operative or assisted vaginal delivery (forceps/vacuum) | |
7 | Inappropriate maneuvers (fundal pressure) | ||
8 | Epidural anesthesia |
35.2.1 Shoulder Dystocia and Macrosomia
A definite relationship exists between fetal size and shoulder dystocia [17], but it is not a good predictor: partly because it is difficult to predict fetal size accurately but also because the majority of infants with birth weight of equal to more than 4500 g do not develop shoulder dystocia [18]. Equally important, 48% of the births complicated by shoulder dystocia occurs with infants weighing less than 4000 g [8]. The overall incidence of shoulder dystocia varies based on fetal weight, occurring in 0.6–1.4% of all infants with a birth weight of 2500–4000 g, increasing to a rate of 5–9% among the fetuses weighing 4000–4500 g born in mothers without diabetes [14].
The diagnosis of fetal macrosomia is precise. Several ultrasound measurements to predict macrosomia and an alert for shoulder dystocia have been proposed like abdominal circumference (AC > 350 mm) [19], newborn shoulder width [20], and 3D ultrasound weight estimation [21]. However, ACOG supports the use of the 4500 g cutoff for diagnosis of macrosomia as it is; at this rate, sharp increase is seen in risk of morbidity for infants and mothers [22].
The prediction of macrosomia by ultrasound is limited by the fact that fetal weight prediction is less accurate at higher birth weights. Moreover, the third trimester ultrasound scans have a sensitivity of just 60% for macrosomia (over 4.5 kg) [23].
35.2.2 Shoulder Dystocia and Diabetes
Infants of diabetic mothers tend to have a two- to fourfold increased risk of shoulder dystocia as compared to infants of the same birth weight born to nondiabetic mothers [15, 17]. The shoulder girth of fetus is composed of tissues that are insulin sensitive and respond to hyperglycemia and hyperinsulinism, while the head circumference and brain growth are less affected. As a result, higher shoulder-to-head circumference ratio is observed in infants of diabetic mothers further causing increased rates of shoulder dystocia.
Cohen reported that shoulder dystocia was predicted in 100% of diabetic pregnancies when the differences between the ultrasonic measurements of biparietal diameter minus the abdominal circumference exceeded 2.5 cm [24].
35.2.3 Obesity and Shoulder Dystocia
WHO defined obesity as a body mass index (BMI) of 30 or greater. Worldwide, a staggering rise has been observed in the prevalence of obesity. Etiology of obesity is multifactorial, mainly attributed to urbanization, changing dietary habits and sedentary lifestyles. A number of complications are attributed to obesity including increased risk of pregnancy loss, congenital malformations, gestational hypertension, and gestational diabetes mellitus. Intrapartum complications include increased risk of cesarean section, labor dystocia, instrumental delivery, and postpartum hemorrhage. Postpartum complications include increased rates of infection, thromboembolism, and increased hospital stay. Obese women tend to have increased risk of macrosomic infants as compared to non-obese women. Weiss reported that the incidences of macrosomia, defined as the birth weight of over 4000 g, was 8.3% in nonobese group, 13.3% in an obese group, and 14.6% among morbidly obese women [25]. The relationship between obesity and shoulder dystocia may be largely related to the increase in incidence of macrosomia seen in this group, rather than to obesity alone [26]. To support this notion, Robinson reported no increase in shoulder dystocia among obese women with infants of normal weight [27]. Thus, to conclude, obese women with macrosomic baby pose as a risk factor for shoulder dystocia, but obesity alone cannot be taken as independent risk factor for shoulder dystocia.
35.2.4 Recurrent Shoulder Dystocia
A recent study showed that recurrent shoulder dystocia was higher than the general population (3.7% vs. 0.7%, OR 7.36, 95% CI 3.68–14.23, P < 0.01) [28]. However, the rate of recurrence appears to be lower than previously estimated by Bingham et al. [29] in 2010. According to their study, about 12% of parturients with a history of shoulder dystocia have a recurrent dystocia in subsequent pregnancy with a risk of about 1 in 8 (OR 8.25). Brachial plexus injury occurs in 19/1000 vaginal births during the first episode of shoulder dystocia and in 45/1000 vaginal births after recurrent shoulder dystocia [29]. However, the true incidence of recurrent shoulder dystocia remains unknown because obstetrician and patients often do not choose to attempt a trial of labor when there was a history of complicated delivery or an injured infant with brachial palsy [2]. However, in cases of previous shoulder dystocia and brachial plexus injury or other complication, cesarean section could be justified in next pregnancy. The woman and her attendants should always be explained about the risk of recurrence and should be given option of either the cesarean section or vaginal delivery. The factors such as the severity of any previous neonatal injury, any serious maternal intrapartum event, predicted fetal weight, and choice of mother and family should all be considered and discussed with the woman and her family while making plans for route of delivery.
35.3 Maternal and Neonatal Consequences
Complications of shoulder dystocia
Fetal complications | Maternal complications |
|---|---|
Brachial plexus injury (most common) | Postpartum hemorrhage (both atonic and traumatic) |
Fetal distress with or without permanent neurological damage | Complete perineal tears |
Fetal death | Extension of episiotomy, vulval hematoma |
Clavicular fractures | Bladder atony/rupture |
Fractures of humerus | Symphyseal separation or diathesis with or without transient femoral neuropathy |
Rectovaginal fistula | |
Uterine rupture |
35.3.1 Fetal Complications
- 1.
Neonatal Brachial Plexus Palsy (NBPP)
Brachial plexus injury is the most common and serious complication, occurring in 5–15% of neonates born after shoulder dystocia. The most common injuries can result in Erb’s or Erb-Duchenne palsy (injury to upper brachial plexus nerve roots, C5–C6) or Klumpke’s palsy (injury to the lower nerve roots, C8–T1). Rarely, the whole brachial plexus will be injured leading to flail arm.
A stretching force is transmitted to the tissues that connect the fetal trunk and the fetal head-neck, under which lies the brachial plexus, leading to neonatal brachial plexus palsy (NBPP). Conjunction of events generating stretching and compression forces leading to NBPP are enumerated as follows:
- (a)
Impaction of either the anterior or posterior shoulder behind the pubic symphysis or sacral promontory, respectively, while the long axis of the fetus is pushed down the birth canal with each uterine contraction.
- (b)
When the fetal neck is compressed against the maternal pubic symphysis or sacral promontory.
- (c)
Any amount of traction applied by the birth attendant.
Neonatal Response
Neonatal response to injury in form of stretching and compression forces in cases of shoulder dystocia varies and depends on many variables like tensile strength of fetal tissues, the degree of protective tone of muscles around fetal shoulders/neck, and the metabolic condition of fetus (measured by acid-base status).
In 2014, the ACOG Task Force on neonatal brachial plexus palsy (NBPP) reviewed the current literature, including consensus opinion as well as multiple published cases of peer-reviewed literature related to brachial plexus injury. This report concludes that NBPP occur in 1.5 of 1000 births. Only 50% of NBPP cases are preceded by shoulder impaction. Greater than 80% cases of NBPP occur in women without any risk factors. In fact, NBPP has not only been associated with routine vaginal deliveries not complicated by shoulder impaction but even with routine cesarean deliveries in 4% of cases. NBPP is associated with 4–40% of clinically apparent cases of shoulder impaction. Most injuries resolve, however, the incidence of persistent NBPP at 1 year of life ranges from 0.5% to 1.6% [31, 32].
Although shoulder dystocia and disimpaction maneuvers historically have been attributed for the etiology of these palsies, brachial plexus injury may occur in utero [33]. Probable mechanisms of intrauterine insult include the endogenous propulsive forces of labor, in utero position of the fetus, failure of the shoulders to rotate, and abnormal intrauterine pressures arising from uterine anomalies (such as fibroids, intrauterine septum, bicornuate uterus); all these injuries may also contribute to brachial plexus injury [13, 14].
The timing of brachial plexus injury can be determined by electromyelography within 24–48 h of delivery. Electromyelographic evidence of muscular denervation normally requires 10–14 days to develop. Its presence in the early neonatal period strongly suggests an insult predating delivery [13].
- (a)
- 2.
Fetal Asphyxia
In shoulder dystocia, the combination of hypoxia, obstructed cerebral venous return, and trauma during delivery renders the fetal brain vulnerable to damage. The umbilical artery pH is estimated to theoretically fall 0.04 pH units per minute in the interval between delivery of the fetal head and trunk (mean umbilical artery pH at term is 7.27). In general, to deliver a previously well-oxygenated fetus with normal acid-base status, the operator has 4–5 min to deliver the fetus before the possibility of permanent hypoxic damage.
Thus, there is time for the logical and systemic application of maneuvers to safely deliver the fetus before the permanent hypoxic damage. On the other hand, if the fetus is already hypoxic when shoulder dystocia occurs, the time to permanent hypoxic damage may be much shorter.
Severe cases of shoulder dystocia may even end in hypoxic ischemic encephalopathy and fetal death.
- 3.
Fractures
Fractures of clavicle and less commonly humerus are also reported in infants after shoulder dystocia. These fractures typically heal well without any deformities. Fracture or dislocation of the cervical spine is extremely infrequent but can be associated with desperate and ill-advised twisting maneuvers of the fetal head.
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