A summary of the new ACOG report on neonatal brachial plexus palsy. Part 2: Pathophysiology and causation
A look at the many variables that determine whether or not obstetric brachial plexus injury will occur in a given delivery
Cadaver studies have been inadequate to assess the in situ response of the brachial plexus
Many plaintiff claims regarding the cause of brachial plexus injury use cadaver studies as evidence. However, most such studies were conducted between 98 and 140 years ago. In these older studies, quantitative evaluation was rare. And in the few more recent studies, there are several reasons why the data obtained are problematic:
- the nerves being studied were dissected free from supporting tissues
- nerve tissue deteriorates quickly postmortem
- some studies used adult tissues; there may be significant differences between adult and newborn nerve tissue that obscure comparison.
The ACOG report concludes the section on cadaver studies by stating:
Physical models also fall short
The problem with the use of physical models in evaluating NBPP centers on the need to find materials that have the same or similar properties as the tissues of interest. These sorts of bioengineering limitations generally do not allow for findings that have direct clinical applicability.
Of interest, however, is the finding of at least two groups of investigators that less traction is required when simulating delivery of a model infant when rotational maneuvers (Rubin’s) are employed rather than after McRoberts repositioning.
Computer models have yielded data on the relative effects of endogenous and exogenous forces
Sophisticated computer analysis has been used to investigate both endogenous and exogenous delivery forces. Results of such studies have shown that maternal endogenous forces exert twice as much pressure on the base of the fetal neck against the maternal symphysis pubis as do deliverer-induced exogenous forces.
Is there a threshold of force?
Data that include measurement of the force applied to the brachial plexus nerves of a live infant during a real delivery are almost nonexistent. One group—on the basis of a single case of transient NBPP and potentially flawed pressure measurements—has suggested that the threshold for NBPP in the human is 100 Newtons.3 However, other studies have shown that physician-applied forces in routine deliveries commonly exceed this hypothesized cutoff—yet the rate of NBPP remains low. In measuring delivery forces it must be remembered that significant variation exists between individual neonates, both in terms of mechanical properties and anatomy. Because of this variation—and the nonlinear behavior of nerve tissues—the specific force needed to cause a nerve injury or rupture in a given neonate has not been established.
Chapter 3 of the ACOG report closes with a statement:
NBPP and shoulder dystocia
Shoulder dystocia is defined as a delivery that requires additional obstetric maneuvers after gentle downward traction on the fetal head fails to deliver the fetal shoulders. The ACOG report makes the important point that shoulder dystocia is not formally diagnosed until a trial of downward axial traction has been unsuccessful in delivering the anterior shoulder. This point is a refutation of the frequent plaintiff claim that, once a shoulder dystocia is thought to be present, no traction whatsoever should be applied by the clinician at any time during the remainder of the delivery.
Shoulder dystocia incidence is rising
The reported incidence of shoulder dystocia has increased over the past several decades. It is unclear whether this increase is related to maternal obesity, fetal macrosomia, or more widespread reporting. However, paradoxes exist in the relationship among risk factors, shoulder dystocia, and brachial plexus injury:
- although there is an increased incidence of shoulder dystocia with increased birth weight, the mean birth weight of neonates with recognized shoulder dystocia is not significantly higher than the mean birth weight of all term infants
- strategies to reduce NBPP by preventing shoulder dystocia—including early induction of labor and prophylactic use of McRoberts maneuver and suprapubic pressure—have not been effective in reducing the incidence of NBPP.
The ACOG report makes the statement: “Maternal and fetal factors associated with shoulder dystocia do not allow for reliable prediction of persistent NBPP.”1
What is optimal management of shoulder dystocia?
The last obstetric part of the ACOG report takes as its focus the management of shoulder dystocia. It discusses the importance of communication among members of the delivery team and with the mother whose neonate is experiencing a shoulder dystocia. The report states:
