Shoulder dystocia is one of the most dreaded and dramatic complications encountered in obstetrics. It is a true emergency that can lead to high rates of maternal morbidity as well as neonatal morbidity and mortality. Various maneuvers can free the impacted shoulder to obviate fetal hypoxia. The steps are completed expeditiously, but mechanical force must be tempered to avoid maternal and fetal traumatic injury. The importance of this obstetric complication and its sequelae were emphasized by the appointment of a Task Force by American College of Obstetricians and Gynecologists president Dr. James T. Breeden. This working group published its monograph Neonatal Brachial Plexus Palsy in 2014.
Shoulder dystocia has been defined in several ways. Following complete emergence of the fetal head during vaginal delivery, the remainder of the body may not rapidly follow despite downward traction and maternal pushing. Resnik (1980) described shoulder dystocia as a condition requiring special maneuvers to deliver the shoulders following an unsuccessful attempt to apply downward traction. Benedetti (1989) more specifically defined it as an arrest of spontaneous delivery due to impaction of the anterior shoulder behind and against the symphysis pubis. The Task Force cited a commonly accepted definition that requires additional maneuvers following failure of gentle downward traction on the fetal head to deliver the shoulders. Other investigators use the head-to-body time interval as the defining factor (Beall, 1998; Hoffman, 2011). Spong and coworkers (1995) reported that the mean head-to-body delivery time in normal births was 24 seconds compared with 79 seconds in those with shoulder dystocia. These investigators proposed that head-to-body delivery time >60 seconds should define shoulder dystocia.
At this time, it is reasonable for the diagnosis to continue to rely on the clinical perception that the normal downward traction needed for fetal shoulder delivery is ineffective. But, whatever definition is used, any perceived shoulder dystocia is an obstetric emergency. The umbilical cord is compressed within the birth canal, and the placenta is variably separated.
Shoulder dystocia has been described in the medical literature for at least two centuries. Swartz (1960) quoted Smellie, writing in 1730: “A sudden call to a gentlewoman in labor. The child’s head delivered for a long time—but even with hard pulling from the midwife, the remarkably large shoulder prevented delivery. I have been called by midwives to many cases of this kind, in which the child was frequently lost.” In his premier edition of Obstetrics, J. Whitridge Williams (1903) warns against applying excessive traction to avert traumatic brachial plexus stretching. But even as late as 1966, shoulder dystocia received relatively little attention. For example, in the 13th edition of Williams Obstetrics by Eastman and Hellman (1966), only one page is devoted to the subject. By way of contrast, this entire contemporaneous chapter is devoted to shoulder dystocia and its management.
Understandably, shoulder dystocia incidences differ because of varied definitions and data-reporting methods. According to the American College of Obstetricians and Gynecologists (2015b), the rate lies between 0.6 and 1.4 percent. The incidence is likely rising because of increasing fetal birthweights and improved reporting rates (Fig. 24-1) (MacKenzie, 2007; Øverland, 2014). At the same time, however, the incidence of neonatal brachial plexus palsy appears to be either stable or declining (American College of Obstetricians and Gynecologists, 2014; Chauhan, 2014a). For example, Hopwood (1982) described an increasing rate of shoulder dystocia, which was reported to be 0.2 percent of deliveries between 1966 and 1976. This rose to 1.1 percent between 1976 and 1981. They attributed the rise to the general trend toward larger fetal birthweight, a finding confirmed by Modanlou and coworkers (1982). Table 24-1 summarizes shoulder dystocia rates with respect to birthweight in several studies. The incidence further varies depending on the specific patient population and the subgroups of patients reported, for example, diabetic versus nondiabetic women.
Steps have been made to make reporting of shoulder dystocia more reproducible. Beall and associates (1998) attempted to validate the definition by Spong noted earlier. Prolonged head-to-body delivery time and/or use of ancillary maneuvers were prospectively evaluated in 722 women. Of the 99 cases of shoulder dystocia using this definition, the practitioner subjectively labeled only 25 percent as “dystocia.” Those deliveries requiring special maneuvers were more likely to be described as dystocia by the practitioner. Deliveries meeting this definition of shoulder dystocia had increased neonatal birthweights, and all fetal injuries were in the identified group. They also reported that 50 percent of those deliveries that required special maneuvers did not have documentation in the medical record.
There are only a few conditions in which the fetal body does not deliver promptly following the head. In addition to shoulder dystocia, some other conditions are listed in Table 24-2.
Most cases of shoulder dystocia occur in women without risk factors. That said, several risk characteristics for shoulder dystocia may be identified before or during labor (Table 24-3). Unfortunately, such qualities are so common that they lack both sensitivity and specificity and thus have limited clinical utility (American College of Obstetricians and Gynecologists, 2014, 2015b).
Antepartum Fetal macrosomia Maternal obesity Diabetes mellitus Postterm pregnancy Male gender Advanced maternal age Excessive weight gain Prior shoulder dystocia Platypelloid pelvis Contracted pelvis Multiparity Intrapartum First-stage labor abnormalities Protraction disorders Arrest disorders Prolonged second stage of labor Oxytocin augmentation of labor Midforceps and midvacuum extraction Epidural analgesia |
A large fetus is a major risk factor for shoulder dystocia. The rate of shoulder dystocia relative to birthweight is listed in Table 24-1 and depicted in Figure 24-2. However, the degree of risk depends on the weight used to define fetal macrosomia. This condition implies growth beyond a specific weight, usually 4000 g or 4500 g, regardless of gestational age. The American College of Obstetricians and Gynecologists (2015a) proposes a defining weight that accounts for available data regarding Erb palsy, which is a sequela of brachial plexus stretch, and for the known inaccuracies of antepartum estimated fetal weights (EFWs). They suggest that 4500 g is an appropriate EFW beyond which the fetus should be considered macrosomic. Adoption of a lower EFW threshold would label many fetuses as “at risk” despite only a relatively small increase in the risk of morbidity as a consequence of their size. Although there is general agreement with this definition, many studies still use ≥4000 g to define macrosomia. As an example of associated risk, Hehir and colleagues (2015) reported that 0.4 percent of pregnancies in contemporary practice weighed >5000 g. Shoulder dystocia was identified in 14 percent of those delivered vaginally.
To minimize shoulder dystocia risks, a more liberal cesarean delivery policy for suspected macrosomia has been suggested. Gross and coworkers (1987a) used multiple discriminant analysis to define a model that could predict shoulder dystocia. They calculated that if cesarean deliveries were performed for all neonates weighing ≥4000 g, six operations would be required to prevent one case of shoulder dystocia. Rouse and coworkers (1996) constructed a decision analytic model to study this issue and concluded that elective cesarean delivery for sonographically diagnosed macrosomia was not medically and financially feasible for the nondiabetic population. Other investigators have reached similar conclusions (Gonen, 2000; Kolderup, 1997).
Similarly, to lower the risks for shoulder dystocia rates, labor induction because of “impending” macrosomia has generally not been considered feasible (Grobman, 2013; Hansen, 2014). At least one recent investigation, however, has shown such benefits (Boulvain, 2015). In this study, preemptive induction and delivery of fetuses judged to weigh >95th percentile decreased the shoulder dystocia rate significantly. Importantly, the cesarean delivery rate was not increased.
In view of inaccuracies of antepartum EFWs derived sonographically, along with the fact that no substantial evidence supports early vaginal delivery or cesarean delivery in preventing brachial plexus injuries, management solely based on sonographic EFW is not warranted (American College of Obstetricians and Gynecologists, 2015b; Lee, 2016).
Johnson and associates (1987) reported a 5-percent incidence of shoulder dystocia if maternal weight at delivery is >250 lb compared with 0.6 percent in women who weighed <200 lb. Other investigators have found similar correlation between maternal obesity and shoulder dystocia (Avci, 2015; Crane, 2013; Spellacy, 1985). These studies also note that this link is related to the common cofactor of fetal macrosomia and/or diabetes (Langer, 2016). Conversely, Lewis and colleagues (1998) showed no increased risk of shoulder dystocia or birth trauma with a maternal weight >90 kg.
The combination of fetal macrosomia in maternal diabetes mellitus escalates the frequency of shoulder dystocia (Langer, 1991; Nesbitt, 1998). Of possible explanations, fetuses of diabetic women have increased shoulder-to-head and chest-to-head size differences relative to comparable-weight fetuses of nondiabetic mothers (Modanlou, 1982).
In a study comparing shoulder dystocia rates in diabetic versus nondiabetic women, the incidence rose as birthweight increased for both nondiabetic and diabetic gravidas (Fig. 24-3) (Acker, 1985). In nondiabetic women, the shoulder dystocia incidence was 10 percent in those delivering newborns weighing between 4000 and 4499 g, compared with a 22.6-percent rate for those with neonates weighing >4500 g. These frequencies more than doubled in diabetic women. Cordero and associates (2015) reported a 28-percent rate of shoulder dystocia in macrosomic fetuses of diabetic mothers compared with a 15-percent rate in those born to nondiabetic gravidas.
Of preventive steps for diabetic gravidas, Conway and Langer (1998) noted that elective cesarean delivery for an EFW ≥4250 g and elective induction for an EFW ≥90th percentile but <4250 g significantly decreased the rate of shoulder dystocia—2.4 versus 1.1 percent. Pedersen (1954) described a trend of lower birthweights in neonates of diabetic women given long-term insulin therapy compared with those who had short-term therapy. Coustan and Imarah (1984) also found that insulin therapy decreased the incidences of macrosomia, operative delivery, and birth trauma in gestational diabetic gravidas. Berne and associates (1985) confirmed these observations and proposed the use of prophylactic insulin for this purpose. Somewhat related, Casey and colleagues (2015), however, reported that glyburide added to dietetic therapy did not lower the rate of shoulder dystocia in cases of mild gestational diabetes.
Gestations lasting longer than 42 weeks have an associated increased risk for shoulder dystocia (Fig. 24-4) (Eden, 1987; Johnson, 1987). Specifically, nearly half of shoulder dystocia cases are associated with a pregnancy extending beyond 41 weeks’ gestation (Hopwood, 1982; Johnstone, 1979). Spellacy and associates (1985) suggested that postterm pregnancy is a risk factor for macrosomia, which of course is a comorbid risk for shoulder dystocia. Because postterm pregnancies are a minority, the absolute risk for shoulder dystocia is low. Indeed, Acker (1985) and Øverland (2014) and their colleagues found that the overwhelming majority of postterm pregnancies were not associated with shoulder dystocia.
The recurrence rate for a woman with previous shoulder dystocia is much higher than for the general population. Estimates as high as 10 percent have been described (Bingham, 2010; Moore, 2008). Ouzounian and coworkers (2012) reported a recurrence risk of 3.7 percent compared with a baseline risk of 0.7 percent.
Male fetuses are more likely to be associated with shoulder dystocia. In several studies, male fetuses composed approximately 70 percent of cases (Hassan, 1988; Parks, 1978; Spellacy, 1985). Advanced maternal age as a risk factor for shoulder dystocia depends on the coexistence of diabetes mellitus and obesity (Langer, 1991; Øverland, 2014). Excessive maternal weight gain during pregnancy has also been linked with macrosomia and shoulder dystocia (Boyd, 1983; Lewis, 1998).
Shoulder dystocia is more common with extremely curtailed or prolonged second-stage labor (American College of Obstetricians and Gynecologists, 2014). That said, at least in 2016, the definition of a prolonged second stage of labor is undergoing debate and redefinition. In many older studies of shoulder dystocia, prolonged second-stage labor was defined to be >2 hours in a nullipara and >1 hour in multiparous women. Using this definition, a prolonged second stage was associated with an increased risk of shoulder dystocia (Acker, 1985; Gross, 1987a,b). In one study, Benedetti and Gabbe (1978) found that the overall incidence of shoulder dystocia was 0.37 percent of 8890 vertex deliveries. With a prolonged second stage and midpelvic delivery, the incidence of shoulder dystocia was 4.6 percent. Moreover, if birthweight exceeded 4000 g, if the second stage was prolonged, and if midpelvic delivery was performed, the incidence of shoulder dystocia rose to 23 percent. Acker and coworkers (1985) found that 22 percent of women with a shoulder dystocia had protraction disorders, and 8 percent had arrest disorders. That said, 70 percent of women with shoulder dystocia had a normal labor pattern.
The association of oxytocin administration may be a secondary factor in the development of shoulder dystocia, but no evidence supports a direct causal link. The apparent connection between shoulder dystocia and oxytocin administration may be related to factors such as labor disorders or fetal macrosomia.
Assisted delivery with either forceps or vacuum extractor is associated with an increased risk for shoulder dystocia. Broekhuizen and associates (1987) noted a 3-percent incidence of shoulder dystocia in women delivered by vacuum extraction compared with 0.3 percent in a group delivered by forceps. This was partially explained by the vacuum group having a higher rate of midpelvic delivery, a higher mean birthweight, and an increased frequency of birthweights >4000 g. In a prospective randomized trial comparing forceps and vacuum extraction with 637 women, Bofill and colleagues (1997) showed higher shoulder dystocia rates with the use of vacuum extraction versus forceps delivery—4.7 versus 1.9 percent. Pelvic station and rotational maneuvers were not associated with an increased risk. But, Tempest and workers (2013) found shoulder dystocia incidences of 3.7 and 6.3 percent with rotational delivery by Kielland forceps and vacuum extractor, respectively. In a large California study, Nesbitt and colleagues (1998) examined all births >3500 g and calculated that operative vaginal delivery in diabetic and nondiabetic women was associated with a 35- to 45-percent rise in shoulder dystocia risk.
Unfortunately, shoulder dystocia is most often unpredictable and unpreventable. Various sonographic parameters that include biparietal diameter, abdominal area, femur length-to-abdominal circumference ratio, and EFW have been studied as predictors of birthweight and thus neonatal complications. Seigworth (1966) reported that the chest circumference was the same or greater than head circumference in 33 of 41 cases (80 percent) of shoulder dystocia. Kitzmiller and associates (1987) measured fetal shoulder width by computed tomography in diabetic women. A shoulder measurement exceeding 14 cm predicted a birthweight >4200 g. The sensitivity was 100 percent, specificity was 87 percent, positive-predictive value was 78 percent, and negative-predictive value was 100 percent.
Bochner and colleagues (1987) evaluated the utility of sonographic measurement of abdominal circumference at 30 to 33 weeks in gestational diabetics to predict macrosomia. An abdominal circumference ≤90th percentile for gestational age accurately predicted the absence of macrosomia, dystocia, and birth trauma. Fetal abdominal circumference >90th percentile between 30 and 33 weeks was not an accurate predictor of macrosomia at term but was associated with a rise in labor dystocia, shoulder dystocia, and birth trauma rates.
Because of the inaccuracies of these studies and the low positive-predictive values for shoulder dystocia, the evolution in obstetric thinking regarding the preventability of shoulder dystocia has been considerable. Although several risk factors are clearly associated with this complication, identification of individual instances before the fact has proved to be impossible. The American College of Obstetricians and Gynecologists (2015b) reviewed available studies and concluded that:
Most cases of shoulder dystocia cannot be accurately predicted or prevented.
Elective induction of labor or elective cesarean delivery for all women suspected of having a macrosomic fetus is not appropriate.
Planned cesarean delivery may be considered for the nondiabetic woman with a fetus whose estimated fetal weight is >5000 g or for the diabetic woman whose fetus is estimated to weigh >4500 g.
Proper management of shoulder dystocia requires prior consideration of risk factors, a well-conceived plan of action, and rapid execution. Table 24-4 summarizes one protocol. The basic clinical tenets are prompt recognition, expeditiously performed maneuvers to deliver the impacted shoulders, and avoidance of excessive forces to the fetus and mother. Because shoulder dystocia is an uncommon and unpredictable event, prospective clinical trials to determine optimal methods of management have not been and are not likely to be conducted. Nocon and coworkers (1993), in an analysis of risk of obstetric maneuvers for shoulder dystocia, concluded that no single delivery method for shoulder dystocia was superior to another with respect to neonatal injury. They further concluded that no protocol could serve to substitute for clinical judgment and that any reasonable methods were appropriate. This is emphasized by the American College of Obstetricians and Gynecologists (2015b) in that no “one maneuver” in shoulder dystocia management has been proved superior to another in preventing fetal injury.
Call for help Initial gentle traction Suprapubic pressure Evaluate for episiotomya McRoberts maneuver Rotation of anterior shoulder Woods screw maneuver Posterior arm extraction Rubin maneuver Clavicular fracture “All fours” maneuver Other Methods to Consider: Posterior axilla sling traction Zavanelli maneuver Abdominal rescue Symphysiotomya Fundal pressure to augment certain methodsa |
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