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Available online at www.sciencedirect.com
www.elsevier.com/locate/semperi
Shoulder dystocia: Risk factors, predictability, and preventability Shobha H. Mehta, MDa,n, and Robert J. Sokol, MDb a
Department of Gynecology, Obstetrics, and Women's Health, Henry Ford Health System, MI Department of Obstetrics and Gynecology, Wayne State University School of Medicine, MI
b
article info
abstra ct
Keywords:
Shoulder dystocia remains an unpredictable obstetric emergency, striking fear in the hearts
Shoulder dystocia risk factors
of obstetricians both novice and experienced. While outcomes that lead to permanent
Macrosomia
injury are rare, almost all obstetricians with enough years of practice have participated in a
Diabetes
birth with a severe shoulder dystocia and are at least aware of cases that have resulted in
Operative vaginal delivery
significant neurologic injury or even neonatal death. This is despite many years of research trying to understand the risk factors associated with it, all in an attempt primarily to characterize when the risk is high enough to avoid vaginal delivery altogether and prevent a shoulder dystocia, whose attendant morbidities are estimated to be at a rate as high as 16–48%. The study of shoulder dystocia remains challenging due to its generally retrospective nature, as well as dependence on proper identification and documentation. As a result, the prediction of shoulder dystocia remains elusive, and the cost of trying to prevent one by performing a cesarean delivery remains high. While ultimately it is the injury that is the key concern, rather than the shoulder dystocia itself, it is in the presence of an identified shoulder dystocia that occurrence of injury is most common. The majority of shoulder dystocia cases occur without major risk factors. Moreover, even the best antenatal predictors have a low positive predictive value. Shoulder dystocia therefore cannot be reliably predicted, and the only preventative measure is cesarean delivery. & 2014 Elsevier Inc. All rights reserved.
Overview of risk factors There are copious studies elucidating risk factors associated with shoulder dystocia.1–3 They are limited, however, by their retrospective nature, as well as the dependence on the proper identification and documentation of shoulder dystocia, generally reported to be 0.2–3.0%4 but with some studies suggesting a higher rate.5–7 In addition, all risk factors have the drawback of poor positive predictive value; many patients with one or more risk factors go on to deliver without event. In the following section, we review each of the major risk
factors associated with shoulder dystocia, with the accumulated evidence to associate them with shoulder dystocia as well as predictability, building a case for the continued difficulty in prevention of shoulder dystocia.
Macrosomia Macrosomia is considered to be the “most significant determinant of risk” for shoulder dystocia.8 Approximately half of all shoulder dystocias occur in large-for-gestational-age infants.9–11 The incidence of shoulder dystocia increases with
n Corresponding author at: Department of Gynecology, Obstetrics, and Women's Health, Henry Ford Health System, 3031 West Grand Blvd, 8th floor, Detroit, MI 48202. E-mail address: [email protected] (S.H. Mehta).
http://dx.doi.org/10.1053/j.semperi.2014.04.003 0146-0005/& 2014 Elsevier Inc. All rights reserved.
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each 500 g increase in birth weight (BW), reaching a 10-fold increase by 4500 g.9,12 Stotland et al., reviewing over 146,000 deliveries in California between 1995 and 1999, found the shoulder dystocia incidence to be 1.5% with BW ¼ 2500– 3999 g, 8.8% with BW ¼ 4000–4499 g, 17.4% with BW ¼ 4500– 4999 g, and 23.3% with BW Z5000 g. An adjusted odds ratio (OR) for shoulder dystocia was 6.29 (CI: 5.83–6.77) for BW ¼ 4000–4999 g, 13.05 (CI: 11.70–14.56) for BW ¼ 4500–4999 g, and 17.52 (CI: 13.54–22.68) for BW Z5000 g.12 Macrosomia is limited as a risk factor by its poor predictive value. Overall, 70–90% of all macrosomic fetuses deliver without sequelae.11,13 The incidence of shoulder dystocia is 1% with BW o4000 g14 but because of the higher proportion of deliveries o4000 g, this weight range [average-for-gestationalage infants] accounts for 40–60% of all shoulder dystocia.15–18 Another major limiting factor of BW is the difficulty in predicting it accurately. This is the case whether utilizing maternal perception, Leopold maneuvers, or current use of ultrasound. Most literature on the relationship of offspring weight to shoulder dystocia utilizes BW after delivery, not projected estimated fetal weight (EFW), which of course is not available to the physician at the time of critical decisionmaking regarding counseling, preparation, labor, and delivery of the patient. While EFW underestimation does not appear to be related to an increased rate of shoulder dystocia,19 it is not completely clear what knowledge of the true BW would have on the rates of shoulder dystocia, neonatal injuries, or cesarean delivery. Melamed et al.20 did find that the cesarean delivery rate was 2–2.5 times higher when the EFW was 4000– 4499 g, regardless of actual BW.
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Operative vaginal delivery Operative vaginal delivery is associated with an increased risk of shoulder dystocia with a relative risk of 4.6–28.0, depending on station of application and other risk factors.4 A study including over 175,000 deliveries with birth weight of infants over 3500 g (3% SD rate) by Nesbitt et al.21 found that shoulder dystocia increased by approximately 35–45% in vacuum- or forceps-assisted deliveries. For non-diabetic mothers with assisted deliveries, this translated to shoulder dystocia rates of 8.6% for infants weighing 4000–4250 g, 12.9% for infants 4250–4500 g, 23% for 4500–4750 g, 29% for infants 4750–5000 g. Adjusted OR with assisted delivery was 1.9 (p ¼ 0.0001). In addition to macrosomia, a randomized trial of forceps vs. vacuum delivery by Bofill et al.6 demonstrated a stronger association for shoulder dystocia with vacuum delivery.
Previous shoulder dystocia The occurrence of shoulder dystocia in a pregnancy increases the risk of recurrence in a subsequent pregnancy. For those patients who undergo a trial of labor with a successful vaginal delivery, the rate of shoulder dystocia appears to be 10-fold higher than the general population.29 The best predictors of injury in these patients appear to be BW (as it compares to index shoulder dystocia delivery BW) and severity of prior neonatal injury.30 The risk of recurrence appears to be significantly increased with increasing offspring BW, 29.2% with BW 45000 g and prior history vs. 17.4% with BW 45000 g and no prior history.31 For those patients with an EFW less than the BW of the index shoulder dystocia delivery, or who lack a history of permanent brachial plexus injury, trial of labor may be reasonable.
Diabetes mellitus Maternal obesity/excessive weight gain Mothers with diabetes are at increased risk for having shoulder dystocia than their non-diabetic counterparts. Diabetes increases the overall risk of shoulder dystocia by more than 70%.21 This is due in part to the higher BW of infants of diabetic mothers. Langer et al.,22 in a population of greater than 75,000 patients, found that the incidence of macrosomia was 21% among diabetic mothers vs. 7.6% among non-diabetics. Furthermore, gram for gram, the incidence of shoulder dystocia and injury is higher in diabetic mothers. This is thought to be related to the physiologic differences in growth of offspring of diabetic mothers, who have larger shoulder and extremity circumferences, higher percentage body fat, and thicker upper extremity skin folds,23–25 an asymmetry of somatic growth ahead of overall growth.26 Even “milder” diabetics have higher rates of shoulder dystocia; A1 (dietcontrolled gestational diabetics) had a higher rate of shoulder dystocia than the general population (3% vs. 0.9%).27 Keller et al.28 found that the incidence of shoulder dystocia was 11.4% among A1 diabetics and 14.6% among A2 (medicationrequiring) diabetics. As with other risk factors, diabetes remains limited by its poor positive predictive value. Diabetes, whether gestational or pre-existing, has a sensitivity of 7.2%, specificity of 94.4%, positive predictive value of 1.4%, and negative predictive value of 98.9% for the occurrence of shoulder dystocia.8
It has been speculated that obesity leads to an increased risk of shoulder dystocia due to an increase in soft tissue within the maternal pelvis, which impedes vaginal delivery—a dystocia due to soft tissues.32 Some studies have shown a higher prevalence of obesity in pregnancies with shoulder dystocia, appearing to double the risk,33–35 and to increase the severity of injury when it does occur following shoulder dystocia.36,37 An independent relationship between maternal obesity and shoulder dystocia has been questioned, however, including in a study by Robinson et al., which found a crude OR of 2.1 for maternal obesity (CI: 1.4–3.2). However, following multivariate logistic regression, maternal obesity was no longer a significant risk factor for shoulder dystocia.38 Because of the powerful relationship between maternal obesity and diabetes and fetal macrosomia,39 two significant risk factors of shoulder dystocia, it may be more difficult to discern the impact of maternal obesity independently on shoulder dystocia. Furthermore, only 5% of obese women (4250 lb) experience shoulder dystocia.40
Labor dysfunction Studies have found an association between labor abnormalities and the occurrence of shoulder dystocia; however, the
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type of disorders noted varies between studies. Acker et al. noted arrest disorders in the 3000–3499 g group and protraction while arrest disorders in the 3500–3999 g group with subsequent shoulder dystocia.14 In the macrosomic group, they found no labor abnormalities in the 4000–4499 g group, while arrest disorders were noted in the 4500 g group with subsequent shoulder dystocia.9 Other studies have found abnormalities in the first phase of labor.41,42 A few studies have noted a prolonged second stage of labor in those patients with shoulder dystocia,1,3,42 one study showed an increased incidence of SD from 11% to 39%.3 This finding has been noted particularly in nulliparous patients in some studies.43,44 Still others have found no labor abnormalities associated with shoulder dystocia,45 and one study found a precipitous second stage to be associated with shoulder dystocia.46 These varying findings, and again a large number of patients with abnormalities in labor pattern who ultimately have an uneventful vaginal delivery, make labor abnormalities another poor predictor of shoulder dystocia. There are suggestions, though, that such a finding, particularly in the setting of fetal macrosomia, is a reason to avoid an operative vaginal delivery.1,9,44
Combinations of risk factors As alluded to above, several studies have found that a combination of risk factors significantly increases the risk of shoulder dystocia and should be avoided. One of the first examples of this was by Benedetti et al.,1 who in 1978 published an article noting that the combination of macrosomia 44000 g, prolonged second stage, and midpelvic operative vaginal delivery led to a 21% incidence of shoulder dystocia and a high rate of neonatal injury. Mehta et al.44 also noted that in the setting of fetal macrosomia and second stage of labor more than 2 h, performance of assisted vaginal delivery led to an increased rate of shoulder dystocia. Other studies have also cautioned against the use of operative vaginal delivery in the setting of macrosomia, including the one by Ouzounian et al.10 that found that at BW 44000 g, vacuum delivery led to an OR of 13.7 for shoulder dystocia; for BW 44500 g with vacuum, the OR was 21.5. Still others have examined the use of operative vaginal delivery in the setting of maternal diabetes and macrosomia, finding shoulder dystocia incidence of 12.2% for BW ¼ 4000–4250 g, 16.7% for 4250–4500 g, 27.3% for 4500–4750 g, and 34.8% for 4750– 5000 g.21
Predictability Following a retrospective analysis of shoulder dystocia cases and risk factors, Nocon et al.17 stated that “shoulder dystocia is an unpredictable event; infants at risk for permanent injury are virtually impossible to predict.” Gross et al.2 in their study of risk factors concluded that for neonates Z4000 g, shoulder dystocia cannot be predicted by clinical characteristics or labor abnormalities, and that the occurrence of shoulder dystocia is not an evidence of medical malpractice. Gherman et al.,4 in a comprehensive review of shoulder dystocia in 2006, stated that “when evaluated in a prospective fashion” pre-pregnancy and
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antepartum risk factors have “exceedingly poor predictive value for prediction of shoulder dystocia.” Lewis et al.47 found that only 25% of all shoulder dystocia cases had one or more significant risk factors. Moreover, Geary et al.48 found that the positive predictive value of shoulder dystocia for most risk factors was 2% and 3% when combined. The American College of Obstetricians and Gynecologists (ACOG)30 echoes these thoughts in its practice bulletin on shoulder dystocia, stating that “shoulder dystocia cannot be predicted or prevented because accurate methods for identifying which fetuses will experience this complication do not exist.”
Prevention While it does not completely eliminate the possibility of neonatal injury, performing a cesarean delivery can prevent shoulder dystocia and in large part its attendant morbidities. Rouse et al. has investigated the cost and benefits of performing cesarean delivery for the primary risk factor of macrosomia noted on fetal ultrasound. For non-diabetic women, they found that at a 4500 g threshold, 3695 cesarean deliveries are needed to avert a single brachial plexus injury at a cost of $8.7 million per permanent injury averted. At a 4000 g threshold, 2345 cesarean deliveries and $4.9 million are needed to prevent one permanent brachial plexus injury. The authors note that with the excess rate of mortality with cesarean delivery over vaginal delivery, one maternal death would result from every 3.2 permanent brachial plexus injuries avoided. They summarize that while there is societal and individual benefit from avoidance of permanent brachial plexus injury, it does not appear that a policy of elective cesarean delivery based on these estimated fetal weight thresholds is either “medically or economically” sound.49 It should be noted that the cesarean delivery rate in and of itself (not to mention medical cost) has changed dramatically since the data used to calculate these rates and costs. Now at upwards of 30%, this high rate of cesarean delivery likely alters the balance between the risks of shoulder dystocia and the costs of performing cesarean delivery, though to what extent is difficult to quantify and has not been reported. Some have emphasized that changes in maternal prepregnancy weight and weight gain may have the best preventative effect on shoulder dystocia, by decreasing the rates of gestational diabetes and fetal macrosomia that are often associated with it.50 Though possibly true in theory, this is an elusive goal that millions struggle to achieve on a daily basis for its myriad of health benefits. Until that is achieved, it may be best to consider throughout gestation and particularly late in labor the number of risk factors present in an individual, with consideration of avoidance of operative delivery in particular when these are present, and consideration of cesarean delivery when appropriate.
Summary Despite many studies elucidating risk factors, shoulder dystocia remains generally unpredictable due to the poor positive predictive value of even the strongest risk factors.
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Prevention through cesarean delivery therefore comes at a high cost, particularly given the poor identification of those truly at risk, and a low benefit with the few number of permanent brachial plexus injuries avoided. As a result, shoulder dystocia remains a feared obstetrical emergency, whose best antidote may be preparation through simulation and other techniques illuminated in upcoming chapters.
r e f e r e n c e s
1. Benedetti TJ, Gabbe SG. Shoulder dystocia: a complication of fetal macrosomia and prolonged second stage of labor with midpelvic delivery. Obstet Gynecol. 1978;52:526–529. 2. Gross TL, Sokol RJ, Williams T, Thompson K. Shoulder dystocia: a fetal-physician risk. Am J Obstet Gynecol. 1987;156: 1408–1418. 3. Baskett TF, Allen AC. Perinatal implications of shoulder dystocia. Obstet Gynecol. 1995;86:14–17. 4. Gherman RB, Chauhan S, Ouzounian JG, Lerner H, Gonik B, Goodwin TM. Shoulder dystocia: the unpreventable obstetric emergency with empiric management guidelines. Obstet Gynecol. 2006;195:657–672. 5. Allen R, Sorab J, Gonik B. Risk factors for shoulder dystocia: an engineering study of clinician-applied forces. Obstet Gynecol. 1991;77:352–355. 6. Bofill JA, Rust OA, Devidas M, Roberts WE, Morrison JC, Martin JN Jr. Shoulder dystocia and operative vaginal delivery. J Matern Fetal Med. 1997;6:220–224. 7. Beall MH, Spong C, McKay J, Ross MG. Objective definition of shoulder dystocia: a prospective evaluation. Obstet Gynecol. 1998;179:934–937. 8. Dodd JM, Catcheside B, Scheil W. Can shoulder dystocia be reliably predicted? Aust N Z J Obstet Gynaecol. 2012;52:248–252. 9. Acker DB, Sachs BP, Friedman EA. Risk factors for shoulder dystocia. Obstet Gynecol. 1985;66:762–768. 10. Ouzounian JG, Gherman RB. Shoulder dystocia: are historic risk factors reliable predictors? Am J Obstet Gynecol. 2005; 192:1933–1938. 11. Gregory KD, Henry OA, Ramicone E, Chan LS, Platt LD. Maternal and infant complications in high and normal weight infants by method of delivery. Obstet Gynecol. 1998;92:507–513. 12. Stotland NE, Caughey AB, Breed EM, Escobar GJ. Risk factors and obstetric complications associated with macrosomia. Int J Gynaecol Obstet. 2004;87:200–206. 13. Bérard J, Dufour P, Vinatier D, et al. Fetal macrosomia: risk factors and outcome. A study of the outcome concerning 100 cases 44500 g. Eur J Obstet Gynecol Reprod Biol. 1998;77:51–59. 14. Acker DB, Sachs BP, Friedman EA. Risk factors for shoulder dystocia in the average-weight infant. Obstet Gynecol. 1986; 67:614–618. 15. Sandmire HF, O'Halloin TJ. Shoulder dystocia: the incidence and associated risk factors. Int J Gynaecol Obstet. 1988;26:65–73. 16. Langer O, Berkus MD, Huff RW, Samueloff A. Shoulder dystocia: should the fetus weighing greater than or equal to 4000 grams be delivered by cesarean section? Am J Obstet Gynecol. 1991;165:831–837. 17. Nocon JJ, McKenzie DK, Thomas LJ, Hansell RS. Shoulder dystocia: An analysis of risks and obstetric maneuvers. 18. Gherman RB. Shoulder dystocia: an evidence-based evaluation of the obstetrical nightmare. Clin Obstet Gynecol. 2002;45: 345–361. 19. Mehta SH, Blackwell SC, Hendler I, et al. Accuracy of estimated fetal weight in shoulder dystocia and neonatal injury. Am J Obstet Gynecol. 2005;192:1877–1880.
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20. Melamed N, Yogev Y, Meizner I, Mashiach R, Pardo J, BenHaroush A. Prediction of fetal macrosomia: the consequences of false diagnosis. J Ultrasound Med. 2010;29:225–230. 21. Nesbitt TS, Gilbert WM, Herrchen B. Shoulder dystocia and associated risk factors with macrosomic infants born in California. Am J Obstet Gynecol. 1998;179:476–480. 22. Langer O, Berkus MD, Huff RW, Samueloff A. Shoulder dystocia: should the fetus weighing greater than or equal to 4000 grams be delivered by cesarean section? Am J Obstet Gynecol. 1991;165:831–837. 23. Cohen B, Penning S, Major C, Ansley D, Porto M, Garite T. Sonographic prediction of shoulder dystocia in infants of diabetic mothers. Obstet Gynecol. 1996;88:10–13. 24. Cohen BF, Penning S, Ansley D, Porto M, Garite T. The incidence and severity of shoulder dystocia correlates with a songraphic measurement of asymmetry in patients with diabetes. Am J Perinatol. 1999;16:197–201. 25. Modanlou HD, Komatsu G, Dorchester W, Freeman RK, Bosu SK. Large-for-gestational age neonates; anthropometric reasons for shoulder dystocia. Obstet Gynecol. 1982;60:417–423. 26. McFarland MB, Trylovich CG, Langer O. Anthropometric differences in macrosomic infants of diabetic and nondiabetic mothers. J Maternal Fetal Med. 1998;7:292–295. 27. Casey BM, Lucas MJ, Mcintire DD, Leveno KJ. Pregnancy outcomes in women with gestational diabetes compared with the general obstetric population. Obstet Gynecol. 1997;90: 869–873. 28. Keller JD, Lopez-Zeno JA, Dooley SL, Socol ML. Shoulder dystocia and birth trauma in gestational diabetes: a fiveyear experience. Am J Obstet Gynecol. 1991;165:928–930. 29. Mehta SH, Blackwell SC, Chadha R, et al. Shoulder dystocia and the next delivery: outcomes and management. J Matern Fetal Neonatal Med. 2007;20:729–733. 30. American College of Obstetricians and Gynecologists. ACOG practice bulletin clinical management guidelines for obstetricians-gynecologists. No. 40. Obstet Gynecol. 2002;100: 1045–1050. 31. Overland EA, Spydslaug A, Nielsen CS, Esklid A. Risk of shoulder dystocia in second delivery: does a history of shoulder dystocia matter? Am J Obstet Gynecol. 2009;200:506.e1–506.e6. 32. Crane SS, Wojtowycz MA, Dye TD, Aubry RH, Artal R. Association between pre-pregnancy obesity and the risk of cesarean delivery. Obstet Gynecol. 1997;89:213–216. 33. Jensen DM, Damm P, Sorensen B, et al. Pregnancy outcome and prepregnancy body mass index in 2459 glucose-tolerant Danish women. Obstet Gynecol. 2003;189:239–244. 34. Cedergren MI. Maternal morbid obesity and the risk of adverse pregnancy outcome. Obstet Gynecol. 2004;103:219–224. 35. Yogev Y, Langer O. Pregnancy outcome in obese and morbidly obese gestational diabetic women. Eur J Obstet Gynecol Reprod Biol. 2008;137:21–26. 36. Allen R, Petersen S, Moore P, et al. Do antepartum and intrapartum risk factors differ between mild and severe shoulder dystocia? Obstet Gynecol. 2003;189:S208. 37. Poggi SH, Spong CY, Allen RH. Prioritizing posterior arm delivery during severe shoulder dystocia. Obstet Gynecol. 2003;101:1068–1072. 38. Robinson H, Tkatsch S, Mayes DC, Bott N, Okun N. Is maternal obesity a predictor of shoulder dystocia? Obstet Gynecol. 2003;101:24–27. 39. Yu CK, Teoh TG, Robinson S. Obesity in pregnancy. BJOG. 2006;113:1117–1125. 40. Hassan AA. Shoulder dystocia: risk factors and prevention. Aust N Z J Obstet Gynaecol. 1988;28:107–109. 41. Gemer O, Bergman M, Segal S. Labor abnormalities as a risk factor for shoulder dystocia. Acta Obstet Gynecol Scand. 1999;78:735–736.
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42. Gross SJ, Shime J, Farine D. Shoulder dystocia: predictors and outcome: a five-year review. Am J Obstet Gynecol. 1987;156:334–336. 43. McFarland M, Hod M, Piper JM, Xenakis EM, Langer O. Are labor abnormalities more common in shoulder dystocia? Am J Obstet Gynecol. 1995;173:1211–1214. 44. Mehta SH, Bujold E, Blackwell SC, Sorokin Y, Sokol RJ. Is abnormal labor associated with shoulder dystocia in nulliparous women? Am J Obstet Gynecol. 2004;190:1604–1609. 45. Lurie S, Levy R, Ben-Arie A, Hagay Z. Shoulder dystocia: could it be deduced from the labor partogram? Am J Perinatol. 1995;12: 61–62. 46. Poggi SH, Stallings SP, Ghidini A, Spong CY, Deering SH, Allen RH. Intrapartum risk factors for permanent brachial plexus injury. Obstet Gynecol. 2003;189:725–729.
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47. Lewis DF, Edwards MS, Asrat T, Adair CD, Brooks G, London S. Can shoulder dystocia be predicted? Preconceptual and prenatal factors. J Reprod Med. 1998;43:654–658. 48. Geary M, McParland P, Johnson H, Stronge J. Shoulder dystocia: is it predictable? Eur J Obstet Gynecol Reprod Biol. 1995;62: 15–18. 49. Rouse DJ, Owen J, Goldenberg RL, Cliver SP. The effectiveness and costs of elective cesarean delivery for fetal macrosomia diagnosed by ultrasound. J Am Med Assoc. 1996;276: 1480–1486. 50. Gurewitsch ED, Allen RH. Reducing the risk of shoulder dystocia and associated brachial plexus injury. Obstet Gynecol Clin North Am. 2011;38:247–269.
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Available online at www.sciencedirect.com
www.elsevier.com/locate/semperi
Shoulder dystocia: Risk factors, predictability, and preventability Shobha H. Mehta, MDa,n, and Robert J. Sokol, MDb a
Department of Gynecology, Obstetrics, and Women's Health, Henry Ford Health System, MI Department of Obstetrics and Gynecology, Wayne State University School of Medicine, MI
b
article info
abstra ct
Keywords:
Shoulder dystocia remains an unpredictable obstetric emergency, striking fear in the hearts
Shoulder dystocia risk factors
of obstetricians both novice and experienced. While outcomes that lead to permanent
Macrosomia
injury are rare, almost all obstetricians with enough years of practice have participated in a
Diabetes
birth with a severe shoulder dystocia and are at least aware of cases that have resulted in
Operative vaginal delivery
significant neurologic injury or even neonatal death. This is despite many years of research trying to understand the risk factors associated with it, all in an attempt primarily to characterize when the risk is high enough to avoid vaginal delivery altogether and prevent a shoulder dystocia, whose attendant morbidities are estimated to be at a rate as high as 16–48%. The study of shoulder dystocia remains challenging due to its generally retrospective nature, as well as dependence on proper identification and documentation. As a result, the prediction of shoulder dystocia remains elusive, and the cost of trying to prevent one by performing a cesarean delivery remains high. While ultimately it is the injury that is the key concern, rather than the shoulder dystocia itself, it is in the presence of an identified shoulder dystocia that occurrence of injury is most common. The majority of shoulder dystocia cases occur without major risk factors. Moreover, even the best antenatal predictors have a low positive predictive value. Shoulder dystocia therefore cannot be reliably predicted, and the only preventative measure is cesarean delivery. & 2014 Elsevier Inc. All rights reserved.
Overview of risk factors There are copious studies elucidating risk factors associated with shoulder dystocia.1–3 They are limited, however, by their retrospective nature, as well as the dependence on the proper identification and documentation of shoulder dystocia, generally reported to be 0.2–3.0%4 but with some studies suggesting a higher rate.5–7 In addition, all risk factors have the drawback of poor positive predictive value; many patients with one or more risk factors go on to deliver without event. In the following section, we review each of the major risk
factors associated with shoulder dystocia, with the accumulated evidence to associate them with shoulder dystocia as well as predictability, building a case for the continued difficulty in prevention of shoulder dystocia.
Macrosomia Macrosomia is considered to be the “most significant determinant of risk” for shoulder dystocia.8 Approximately half of all shoulder dystocias occur in large-for-gestational-age infants.9–11 The incidence of shoulder dystocia increases with
n Corresponding author at: Department of Gynecology, Obstetrics, and Women's Health, Henry Ford Health System, 3031 West Grand Blvd, 8th floor, Detroit, MI 48202. E-mail address: [email protected] (S.H. Mehta).
http://dx.doi.org/10.1053/j.semperi.2014.04.003 0146-0005/& 2014 Elsevier Inc. All rights reserved.
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each 500 g increase in birth weight (BW), reaching a 10-fold increase by 4500 g.9,12 Stotland et al., reviewing over 146,000 deliveries in California between 1995 and 1999, found the shoulder dystocia incidence to be 1.5% with BW ¼ 2500– 3999 g, 8.8% with BW ¼ 4000–4499 g, 17.4% with BW ¼ 4500– 4999 g, and 23.3% with BW Z5000 g. An adjusted odds ratio (OR) for shoulder dystocia was 6.29 (CI: 5.83–6.77) for BW ¼ 4000–4999 g, 13.05 (CI: 11.70–14.56) for BW ¼ 4500–4999 g, and 17.52 (CI: 13.54–22.68) for BW Z5000 g.12 Macrosomia is limited as a risk factor by its poor predictive value. Overall, 70–90% of all macrosomic fetuses deliver without sequelae.11,13 The incidence of shoulder dystocia is 1% with BW o4000 g14 but because of the higher proportion of deliveries o4000 g, this weight range [average-for-gestationalage infants] accounts for 40–60% of all shoulder dystocia.15–18 Another major limiting factor of BW is the difficulty in predicting it accurately. This is the case whether utilizing maternal perception, Leopold maneuvers, or current use of ultrasound. Most literature on the relationship of offspring weight to shoulder dystocia utilizes BW after delivery, not projected estimated fetal weight (EFW), which of course is not available to the physician at the time of critical decisionmaking regarding counseling, preparation, labor, and delivery of the patient. While EFW underestimation does not appear to be related to an increased rate of shoulder dystocia,19 it is not completely clear what knowledge of the true BW would have on the rates of shoulder dystocia, neonatal injuries, or cesarean delivery. Melamed et al.20 did find that the cesarean delivery rate was 2–2.5 times higher when the EFW was 4000– 4499 g, regardless of actual BW.
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Operative vaginal delivery Operative vaginal delivery is associated with an increased risk of shoulder dystocia with a relative risk of 4.6–28.0, depending on station of application and other risk factors.4 A study including over 175,000 deliveries with birth weight of infants over 3500 g (3% SD rate) by Nesbitt et al.21 found that shoulder dystocia increased by approximately 35–45% in vacuum- or forceps-assisted deliveries. For non-diabetic mothers with assisted deliveries, this translated to shoulder dystocia rates of 8.6% for infants weighing 4000–4250 g, 12.9% for infants 4250–4500 g, 23% for 4500–4750 g, 29% for infants 4750–5000 g. Adjusted OR with assisted delivery was 1.9 (p ¼ 0.0001). In addition to macrosomia, a randomized trial of forceps vs. vacuum delivery by Bofill et al.6 demonstrated a stronger association for shoulder dystocia with vacuum delivery.
Previous shoulder dystocia The occurrence of shoulder dystocia in a pregnancy increases the risk of recurrence in a subsequent pregnancy. For those patients who undergo a trial of labor with a successful vaginal delivery, the rate of shoulder dystocia appears to be 10-fold higher than the general population.29 The best predictors of injury in these patients appear to be BW (as it compares to index shoulder dystocia delivery BW) and severity of prior neonatal injury.30 The risk of recurrence appears to be significantly increased with increasing offspring BW, 29.2% with BW 45000 g and prior history vs. 17.4% with BW 45000 g and no prior history.31 For those patients with an EFW less than the BW of the index shoulder dystocia delivery, or who lack a history of permanent brachial plexus injury, trial of labor may be reasonable.
Diabetes mellitus Maternal obesity/excessive weight gain Mothers with diabetes are at increased risk for having shoulder dystocia than their non-diabetic counterparts. Diabetes increases the overall risk of shoulder dystocia by more than 70%.21 This is due in part to the higher BW of infants of diabetic mothers. Langer et al.,22 in a population of greater than 75,000 patients, found that the incidence of macrosomia was 21% among diabetic mothers vs. 7.6% among non-diabetics. Furthermore, gram for gram, the incidence of shoulder dystocia and injury is higher in diabetic mothers. This is thought to be related to the physiologic differences in growth of offspring of diabetic mothers, who have larger shoulder and extremity circumferences, higher percentage body fat, and thicker upper extremity skin folds,23–25 an asymmetry of somatic growth ahead of overall growth.26 Even “milder” diabetics have higher rates of shoulder dystocia; A1 (dietcontrolled gestational diabetics) had a higher rate of shoulder dystocia than the general population (3% vs. 0.9%).27 Keller et al.28 found that the incidence of shoulder dystocia was 11.4% among A1 diabetics and 14.6% among A2 (medicationrequiring) diabetics. As with other risk factors, diabetes remains limited by its poor positive predictive value. Diabetes, whether gestational or pre-existing, has a sensitivity of 7.2%, specificity of 94.4%, positive predictive value of 1.4%, and negative predictive value of 98.9% for the occurrence of shoulder dystocia.8
It has been speculated that obesity leads to an increased risk of shoulder dystocia due to an increase in soft tissue within the maternal pelvis, which impedes vaginal delivery—a dystocia due to soft tissues.32 Some studies have shown a higher prevalence of obesity in pregnancies with shoulder dystocia, appearing to double the risk,33–35 and to increase the severity of injury when it does occur following shoulder dystocia.36,37 An independent relationship between maternal obesity and shoulder dystocia has been questioned, however, including in a study by Robinson et al., which found a crude OR of 2.1 for maternal obesity (CI: 1.4–3.2). However, following multivariate logistic regression, maternal obesity was no longer a significant risk factor for shoulder dystocia.38 Because of the powerful relationship between maternal obesity and diabetes and fetal macrosomia,39 two significant risk factors of shoulder dystocia, it may be more difficult to discern the impact of maternal obesity independently on shoulder dystocia. Furthermore, only 5% of obese women (4250 lb) experience shoulder dystocia.40
Labor dysfunction Studies have found an association between labor abnormalities and the occurrence of shoulder dystocia; however, the
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type of disorders noted varies between studies. Acker et al. noted arrest disorders in the 3000–3499 g group and protraction while arrest disorders in the 3500–3999 g group with subsequent shoulder dystocia.14 In the macrosomic group, they found no labor abnormalities in the 4000–4499 g group, while arrest disorders were noted in the 4500 g group with subsequent shoulder dystocia.9 Other studies have found abnormalities in the first phase of labor.41,42 A few studies have noted a prolonged second stage of labor in those patients with shoulder dystocia,1,3,42 one study showed an increased incidence of SD from 11% to 39%.3 This finding has been noted particularly in nulliparous patients in some studies.43,44 Still others have found no labor abnormalities associated with shoulder dystocia,45 and one study found a precipitous second stage to be associated with shoulder dystocia.46 These varying findings, and again a large number of patients with abnormalities in labor pattern who ultimately have an uneventful vaginal delivery, make labor abnormalities another poor predictor of shoulder dystocia. There are suggestions, though, that such a finding, particularly in the setting of fetal macrosomia, is a reason to avoid an operative vaginal delivery.1,9,44
Combinations of risk factors As alluded to above, several studies have found that a combination of risk factors significantly increases the risk of shoulder dystocia and should be avoided. One of the first examples of this was by Benedetti et al.,1 who in 1978 published an article noting that the combination of macrosomia 44000 g, prolonged second stage, and midpelvic operative vaginal delivery led to a 21% incidence of shoulder dystocia and a high rate of neonatal injury. Mehta et al.44 also noted that in the setting of fetal macrosomia and second stage of labor more than 2 h, performance of assisted vaginal delivery led to an increased rate of shoulder dystocia. Other studies have also cautioned against the use of operative vaginal delivery in the setting of macrosomia, including the one by Ouzounian et al.10 that found that at BW 44000 g, vacuum delivery led to an OR of 13.7 for shoulder dystocia; for BW 44500 g with vacuum, the OR was 21.5. Still others have examined the use of operative vaginal delivery in the setting of maternal diabetes and macrosomia, finding shoulder dystocia incidence of 12.2% for BW ¼ 4000–4250 g, 16.7% for 4250–4500 g, 27.3% for 4500–4750 g, and 34.8% for 4750– 5000 g.21
Predictability Following a retrospective analysis of shoulder dystocia cases and risk factors, Nocon et al.17 stated that “shoulder dystocia is an unpredictable event; infants at risk for permanent injury are virtually impossible to predict.” Gross et al.2 in their study of risk factors concluded that for neonates Z4000 g, shoulder dystocia cannot be predicted by clinical characteristics or labor abnormalities, and that the occurrence of shoulder dystocia is not an evidence of medical malpractice. Gherman et al.,4 in a comprehensive review of shoulder dystocia in 2006, stated that “when evaluated in a prospective fashion” pre-pregnancy and
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antepartum risk factors have “exceedingly poor predictive value for prediction of shoulder dystocia.” Lewis et al.47 found that only 25% of all shoulder dystocia cases had one or more significant risk factors. Moreover, Geary et al.48 found that the positive predictive value of shoulder dystocia for most risk factors was 2% and 3% when combined. The American College of Obstetricians and Gynecologists (ACOG)30 echoes these thoughts in its practice bulletin on shoulder dystocia, stating that “shoulder dystocia cannot be predicted or prevented because accurate methods for identifying which fetuses will experience this complication do not exist.”
Prevention While it does not completely eliminate the possibility of neonatal injury, performing a cesarean delivery can prevent shoulder dystocia and in large part its attendant morbidities. Rouse et al. has investigated the cost and benefits of performing cesarean delivery for the primary risk factor of macrosomia noted on fetal ultrasound. For non-diabetic women, they found that at a 4500 g threshold, 3695 cesarean deliveries are needed to avert a single brachial plexus injury at a cost of $8.7 million per permanent injury averted. At a 4000 g threshold, 2345 cesarean deliveries and $4.9 million are needed to prevent one permanent brachial plexus injury. The authors note that with the excess rate of mortality with cesarean delivery over vaginal delivery, one maternal death would result from every 3.2 permanent brachial plexus injuries avoided. They summarize that while there is societal and individual benefit from avoidance of permanent brachial plexus injury, it does not appear that a policy of elective cesarean delivery based on these estimated fetal weight thresholds is either “medically or economically” sound.49 It should be noted that the cesarean delivery rate in and of itself (not to mention medical cost) has changed dramatically since the data used to calculate these rates and costs. Now at upwards of 30%, this high rate of cesarean delivery likely alters the balance between the risks of shoulder dystocia and the costs of performing cesarean delivery, though to what extent is difficult to quantify and has not been reported. Some have emphasized that changes in maternal prepregnancy weight and weight gain may have the best preventative effect on shoulder dystocia, by decreasing the rates of gestational diabetes and fetal macrosomia that are often associated with it.50 Though possibly true in theory, this is an elusive goal that millions struggle to achieve on a daily basis for its myriad of health benefits. Until that is achieved, it may be best to consider throughout gestation and particularly late in labor the number of risk factors present in an individual, with consideration of avoidance of operative delivery in particular when these are present, and consideration of cesarean delivery when appropriate.
Summary Despite many studies elucidating risk factors, shoulder dystocia remains generally unpredictable due to the poor positive predictive value of even the strongest risk factors.
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Prevention through cesarean delivery therefore comes at a high cost, particularly given the poor identification of those truly at risk, and a low benefit with the few number of permanent brachial plexus injuries avoided. As a result, shoulder dystocia remains a feared obstetrical emergency, whose best antidote may be preparation through simulation and other techniques illuminated in upcoming chapters.
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