Keywords
shoulder dystocia - brachial plexus injury - prediction - medicolegal
Shoulder dystocia occurrences during vaginal delivery result in ∼5,000 new permanent
brachial plexus palsy cases annually in the United States. Up to 63,000 adults are
afflicted with permanent loss of upper extremity function.[1] Malpractice claims alleging permanent brachial plexus palsy was frequent and second
only to birth asphyxia in total dollars paid for obstetrics claims.[2]
Shoulder dystocia occurs in 0.2 to 3% of deliveries.[3] Multiple studies cited in the American College of Obstetricians and Gynecologists
(ACOG) Neonatal Brachial Plexus Palsy Task Force publication of the ACOG substantiate
that ∼10% of shoulder dystocia deliveries result in brachial plexus palsy, accounting
for 1.5 per 1,000 deliveries.[4] Permanent brachial plexus palsy affects ∼15% of these neonates.[4]
The medicolegal costs of claims involving permanent brachial plexus and other permanent
injury can exceed $1 million per claim. Over a 5-year period, one of the largest professional
liability insurers in the United States paid more than $10 million for 14 brachial
plexus palsy claims, an average of nearly $750,000 in malpractice payments and attorneys'
fees per claim. These costs represent only the claims for which the insurance company
paid for an injured child. During that same 5-year period, 45 unproven claims for
shoulder dystocia complications also cost millions of additional dollars in just attorney
and staff expenses (MagMutual Insurance Company, Atlanta, GA).
The ACOG guidelines for prevention and management of shoulder dystocia state that
shoulder dystocia cannot be predicted or prevented accurately, although there are
several known risk factors.[3] Nevertheless, those guidelines provide that cesarean delivery should be considered
in three scenarios to prevent shoulder dystocia: when estimated fetal weight >5,000 g
or when estimated fetal weight >4,500 g in a diabetic gravida or with a history of
shoulder dystocia in a previous delivery. In essence then, these recommendations do
constitute a prediction that the expected probability of shoulder dystocia is sufficiently
high under certain circumstances to justify elective cesarean delivery. Other than
the ACOG guidance, the prevailing position in obstetrics practice is that despite
knowing the risk factors for shoulder dystocia, one cannot determine accurately enough
when it will occur to justify avoiding vaginal delivery.[5]
[6]
[7]
[8] Importantly, none of these references examined a risk assessment based on combinations
of maternal and fetal size and shape, or sought to determine if those with persistent
injury were also unpredictable.
In an effort to address this problem, Dyachenko et al[9] and Hamilton et al[10] reported that the probability of experiencing shoulder dystocia with neonatal injury
(brachial plexus palsy, fracture, or encephalopathy) could be estimated for patients
after 36 weeks' gestation, with multivariable logistic regression modeling, utilizing
the advances in statistical modeling that were used in screening for trisomy 21. Their
formula computed the risk of shoulder dystocia with persistent brachial plexus palsy
specifically, using the variables, maternal height and body mass index (BMI), birth
weight, and birth weight percentile. The final iteration of this model computes the
risks of shoulder dystocia with and without temporary and permanent brachial plexus
injury and is currently available for purchase in a web-based software application
called the Shoulder Screen (PeriGen, Inc., Cary, NC).
We undertook this cross-sectional diagnostic test accuracy study to assess the diagnostic
accuracy of the Shoulder Screen in our patient population. The primary objective was
to evaluate how well this multivariable prediction model distinguished cases of known
shoulder dystocia from vaginal deliveries without shoulder dystocia. Other objectives
were to compare the performance of the model with the intervention criteria based
on birth weight promulgated by ACOG and to assess whether the model could potentially
decrease malpractice costs.
Methods
New Hanover Regional Medical Center is a tertiary hospital in North Carolina with
a residency program in obstetrics and gynecology where ∼4,000 deliveries occur annually.
The definition of shoulder dystocia used in clinical practice at this institution
is a delivery requiring use of specialized maneuvers, beyond gentle downward traction
of the head, to deliver the shoulders. All patients who experienced shoulder dystocia
during vaginal delivery for the period January 1, 2010, to December 31, 2013, during
which more than 17,000 deliveries occurred, were identified by querying the International
Classification of Diseases-9 codes for the diagnosis of shoulder dystocia. These patients
were designated as Group SD.
The patient group that did not experience shoulder dystocia was selected by choosing
every ninth patient from randomly selected medical record numbers of all patients
from the same population who had normal vaginal deliveries without shoulder dystocia
during the same time frame. Every ninth patient was chosen to comply with the Institutional
Review Board limitations placed on the number of patients whose medical records could
be abstracted. These patients were designated as Group NVD. Gravity, parity, maternal
height, maternal weight, BMI, gestational age at delivery, birth weight, birth weight
percentile, and Apgar scores were recorded for each birth. The medical records of
all infants delivered with a shoulder dystocia were analyzed to ascertain the presence
of brachial plexus injury, fracture of the clavicle or humerus (from X-rays), neonatal
encephalopathy, cardiopulmonary resuscitation (CPR) or use of positive pressure ventilation
in the delivery suite, or 5-minute Apgar score <6.
The variables required for the Shoulder Screen to calculate risk of shoulder dystocia
are parity, maternal height, maternal BMI, birth weight, and birth weight percentile.
The equation produces a number between 0 and 1, called the risk score. Each number
correlates with a specific risk of shoulder dystocia, as determined by Dyachenko et
al[9] and Hamilton et al.[10] The likelihood of shoulder dystocia with and without persistent injury is directly
related to the size of the risk score. Hamilton et al[10] showed that risk scores of >0.4 were present in 54.8% of patients with shoulder
dystocia and persistent brachial plexus palsy, 23.1% of patients with temporary brachial
plexus palsy, 16.5% of patients with uncomplicated shoulder dystocia, and in 2.5%
of women who delivered vaginally without shoulder dystocia.
For our study, a Shoulder Screen risk score for shoulder dystocia elevated enough
to justify offering a cesarean section in clinical practice was defined as a score
≥0.4. This score was based on the above-described reports of the prediction model's
detection rates and false-positive rates at higher and lower scores.[9]
[10]
We employed statistical measures under the STARD 2015 guidelines for reporting diagnostic
accuracy studies to accomplish our primary objective to evaluate how well this multivariable
prediction model distinguished cases of known shoulder dystocia from vaginal deliveries
without shoulder dystocia. These measures included sensitivity and specificity, false-positive
rate, positive and negative likelihood ratios, and the receiver operating characteristic
curve. We also examined test performance in a subgroup of patients whose shoulder
dystocia was complicated by infants experiencing fracture, brachial plexus injury,
encephalopathy, need for CPR or positive pressure ventilation in the delivery suite,
or 5-minute Apgar score <6. Categorical variables were compared using chi-square test
or Fisher's exact test, as appropriate. Continuous variables were assessed for normality
using the D'Agostino and Pearson's test and Kolmogorov–Smirnov's test. Variables that
were not normally distributed were compared with the Mann–Whitney's test and normally
distributed variables were compared using the Student's t-test. All tests were two tailed and a probability value of <0.05 was significant.
Results
There were 201 patients (199 with complete data) in Group SD and 199 patients in Group
NVD. Demographically, median values for Group SD revealed statistically significant
more advanced gestational age at delivery, higher maternal weight, greater BMI, and
higher Shoulder Screen test results than Group NVD patients. Also, the rates of nulliparity,
birth weight more than 4,500 g, and birth weight over the 90th percentile were higher
in Group SD. These results are presented in [Table 1]. No infant in either group weighed more than 5,000 g.
Table 1
Maternal and fetal characteristics
|
Group SD
n = 199
Median (IQR)
|
Group NVD
n = 199
Median (IQR)
|
p-Value
|
|
Gestational age
|
39.7 (39.0–40.6)
|
39.1 (38.4–39.7)
|
<0.0001
|
|
Maternal height
|
1.63 (1.57–1.68)
|
1.63 (1.57–1.68)
|
0.15
|
|
Maternal weight
|
84.8 (74.8–98.4)
|
75.8 (67.6–84.8)
|
<0.0001
|
|
Maternal BMI
|
31.9 (28.3–37.1)
|
29.2 (25.7–32.7)
|
<0.0001
|
|
Birth weight
|
3940 (3600–4218)
|
3260 (3005–3600)
|
<0.0001
|
|
Birth weight percentile
|
0.82 (0.58–0.93)
|
0.37 (0.16–0.61)
|
<0.0001
|
|
Shoulder Screen score
|
0.14 (0.03–0.039)
|
0.01(0.002–0.03)
|
<0.001
|
|
n (%)
|
n (%)
|
|
|
Nulliparity
|
91 (45.7)
|
33 (16.6)
|
<0.0001
|
|
Birth weight > 4,500 g
|
20 (10.1)
|
0 (0.0)
|
<0.0001
|
|
Birth weight > 0.90
|
65 (32.7)
|
23 (11.5)
|
<0.0001
|
Abbreviations: BMI, body mass index; IQR, interquartile range.
[Table 2] shows the sensitivity (23.1%), specificity (99.5%), false-positive rate (0.5%),
positive and negative predictive values (97.9 and 56.4%), positive and negative likelihood
ratios (46 and 0.77), and accuracy (61.3%) of the Shoulder Screen test in detecting
all the Group SD patients and distinguishing them from the Group NVD patients. The
false-positive rate represents potentially unnecessary cesarean deliveries that could
result from the Shoulder Screen.
Table 2
Standard performance measures of the Shoulder Screen test (a positive test = test
score >0.4)
|
Shoulder Screen test
|
Group SD
|
Group NVD
|
|
Positive
|
46
|
1
|
|
True positive (a)
|
False positive (b)
|
|
Negative
|
153
|
198
|
|
False negative (c)
|
True negative (d)
|
|
Sensitivity
|
a/(a + c)
|
23.1%
|
|
Specificity
|
d/(b + d)
|
99.5%
|
|
False-positive rate
|
1-specificity
|
0.5%
|
|
Positive likelihood ratio
|
Sensitivity/(1 − specificity)
|
46
|
|
Negative likelihood ratio
|
(1/sensitivity)/specificity
|
0.77
|
|
Accuracy
|
(a + d)/(a + b + c + d)
|
61.3%
|
Since our study population had no neonates with birth weight more than 5,000 g, we
compared the Shoulder Screen with the more stringent ACOG criteria described earlier
for diabetic patients for detecting patients at risk of shoulder dystocia, estimated
fetal weight of ≥4,500 g. These results are presented in [Table 3]. Sensitivity was significantly higher with the Shoulder Screen test compared with
the 4,500 g criterion (23.1 vs. 10.1%, p = 0.0007) in detecting shoulder dystocia. Since the majority of patients with shoulder
dystocia had infants weighed less than 4,500 g, the Shoulder Screen test detected
23.1% and the ACOG criterion detected only 10.1% of all patients with shoulder dystocia.
Table 3
Standard performance measures using BW only (positive = BW >4,500 g)
|
Group SD
|
Group NVD
|
|
Positive (BW >4,500 g)
|
20
|
0
|
|
True positive (a)
|
False positive (b)
|
|
Negative (BW <4,500 g)
|
179
|
199
|
|
False negative (c)
|
True negative (d)
|
|
Sensitivity
|
a/(a + c)
|
10.1%
|
|
Specificity
|
d/(b + d)
|
100%
|
|
False-positive rate
|
1 − specificity
|
0.0%
|
|
Positive likelihood ratio
|
Sensitivity/(1 − specificity)
|
NA
|
|
Negative likelihood ratio
|
(1 − sensitivity)/specificity
|
0.90
|
|
Accuracy
|
(a + b)/(a + b + c + d)
|
55.0%
|
Abbreviations: BW, birth weight; NA, not available.
Some of the infants born to Group SD patients had neonatal complications. A total
of 24/199 (12%) infants in this group had one or more of the following complications:
humerus fracture, brachial plexus injury, encephalopathy, requirement for CPR or positive
pressure ventilation, or a 5-minute Apgar ≤6. Brachial plexus injury was present in
four infants and was persistent at the time of discharge in one infant. The most common
injury was radiographically demonstrated humerus fracture in 8 (4%). The Shoulder
Screen at our selected risk score of >0.4 identified 33.3% of the infants with complications.
The ACOG birth weight criterion detected none of them. [Table 4] shows the detection rate for these patients using the Shoulder Screen compared with
detection using the ACOG.
Table 4
Detection rates using Shoulder Screen Test versus birth weight criterion in complicated
and uncomplicated shoulder dystocia
|
Shoulder Screen score >0.4
|
Birth weight >4,500 g
|
|
n
|
n (%)
|
n (%)
|
p-Value
|
|
SD with complication
|
24
|
8 (33.3)
|
0 (0)
|
0.0039
|
|
SD without complication
|
175
|
38 (21.7)
|
20 (11.4)
|
0.0104
|
The receiver operating characteristic curve in [Fig. 1] shows the sensitivity in Group SD and false-positive rates in Group NVD for each
possible Shoulder Screen test result that could be chosen as the threshold for clinical
intervention. The location of our selected risk score of >0.4 is shown in red. The
area under the curve is 0.86.
Fig. 1 Receiver operating characteristic curve for the Shoulder Screen multivariable test.
The red circle indicates the performance of the test at a risk score result of 0.4.
Discussion
The findings of this study suggest that some shoulder dystocias may be predicted,
and their complications, thereby, possibly prevented. Even though the Shoulder Screen
test, utilizing a score of >0.4 is not perfect, it did detect one-third of the potentially
seriously injured infants in our study. It performed better than the ACOG intervention
based on birth weight exceeding 4,500 g, especially since all infants with complicated
shoulder dystocia in our study weighed less than 4,500 g. Furthermore, with its application
to only diabetic patients, the ACOG clinical guideline would have even lower detection
rates than shown in [Table 4].
The strength of this study is that the patients were drawn from a population consisting
equally of private, commercially insured patients and poor, uninsured and Medicaid
insured patients.
A weakness of this study is that we were unable to show detection rates for persistent
brachial plexus palsy. Even though our study population consisted of more than 17,000
deliveries, only one case of persistent nerve injury occurred. Analysis of ∼100,000
deliveries would be required to show the detection rate for permanent brachial plexus
palsy. To overcome this problem, Dyachenko et al[9] and Hamilton et al[10] were able to use 221 malpractice claim cases involving shoulder dystocia with persistent
brachial plexus injury in their development of the Shoulder Screen.
Other researchers have reported attempts to predict shoulder dystocia with multivariate
statistical techniques, ultrasound to identify the infant with macrosomia or with
disproportionately large bisacromial or trunk diameter, and empiric risk scores based
on the number and type of risk factors. These methods have yielded mixed results.[7]
[11]
[12]
[13] Deaver and Cohen devised a test to predict risk of shoulder dystocia utilizing statistical
modeling and reported that a risk score of 0.72 would prevent 36% of brachial plexus
palsy and result in 14 unnecessary cesarean deliveries for each brachial plexus palsy
that was prevented.[14] However, none of these models approached the problem as an anatomical, biomechanical
misfit, and interaction between maternal and fetal size.[10] None had sufficient numbers to examine shoulder dystocia with permanent brachial
plexus palsy separately.
Criticisms of the Shoulder Screen test could include: (1) it is too cumbersome for
the busy practitioner to use; (2) it results in an unacceptable increase in the elective
cesarean delivery rate; and (3) ultrasound estimated fetal weight has to be used instead
of the actual birth weight in the clinical application of the model.
To gather the four (birth weight percentile is calculated in the model) clinical variables
used to compute the Shoulder Screen test score from a patient's medical record, access
the Web site, enter the data, and retrieve the score should be completed with minimal
effort and time and can be done by an assistant.
Concern about an increase in the cesarean rate is unfounded with the results of our
study. Selection of a Shoulder Screen test score of >0.4 as the point at which cesarean
delivery is offered would potentially increase the cesarean delivery rate by less
than 1%.
The evidence concerning the accuracy of estimated fetal weight by ultrasound in estimating
actual birth weight is conflicting. An accurate assessment of fetal birth weight is
paramount in deciding which patients are at high risk of brachial plexus palsy.[7]
[15]
[16]
[17]
[18]
[19]
[20]
[21] Like the Shoulder Screen, the ACOG guidelines on management of fetal macrosomia
are derived from studies utilizing birth weight, but the clinical application of the
guidelines utilizes estimated fetal weight. The ACOG practice bulletin advises that
the accuracy of estimated fetal weight using ultrasound biometry is no better than
clinical palpation maneuvers since ultrasound software formulas have a 13% error for
infant >4,500 g.[19] However, Dudley reported the mean deviation exceeded 14% of birth weight in only
5% of ultrasound weight estimates. At 4,500 g, the overestimation never reached 15%
and the underestimation of weight never reached 20%.[21] Furthermore, Alsulyman et al found an absolute error of 13% for birth weight ≥ 4,500 g
and 8% for birth weight <4,000 g.[22] The Shoulder Screen maintains its detection rate accuracy at plus or minus 16.3%
of actual birth weight.[9]
To evaluate the prospective clinical use of the Shoulder Screen, Daly et al published
a report of their experience utilizing the Shoulder Screen test in 8,767 deliveries
at two sites.[23] They also chose 0.4 as the high-risk score at which cesarean delivery was offered.
Estimated fetal weight by ultrasound was used as birth weight in the calculations.
Patients with risk scores of <0.4 planned vaginal delivery. Over the course of the
study, the incidence of shoulder dystocia fell, the rate of inductions of labor for
macrosomia decreased, while the primary cesarean delivery rate did not increase. The
population studied was too small to detect whether the occurrence of brachial plexus
palsy decreased.
The Shoulder Screen test is able to predict more cases of shoulder dystocia by lowering
the threshold risk score at which to offer cesarean section, but the false-positive
rate increases accordingly. Each practice and institution using the test chooses whatever
threshold score is prudent for its institutional goals, considering the advantages
of higher detection rates of the adverse outcome and the disadvantages with the associated
false positives that are potentially unnecessary cesarean deliveries.
The search for a method to make patients and clinicians safer from the medical and
economic harm from the serious injury of shoulder dystocia with brachial plexus palsy
is paramount. As a result malpractice costs associated with brachial plexus palsy
would decrease.
This study demonstrates that the Shoulder Screen test accurately detects a portion
of patients at increased risk for shoulder dystocia deliveries and deliveries with
permanent injury, utilizing ultrasound estimated fetal weight rather than birth weight
with a minimal increase in the primary cesarean delivery rate.
