Clinical Review

A summary of the new ACOG report on neonatal brachial plexus palsy. Part 2: Pathophysiology and causation

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A look at the many variables that determine whether or not obstetric brachial plexus injury will occur in a given delivery


 

References

Obstetricians are often blamed for causing neonatal brachial plexus palsy (NBPP). For that reason, understanding the true pathophysiology and causation of this birth-related entity is of extreme importance.

In Part 1 of this two-part series, I summarized findings from the new report on NBPP from the American College of Obstetricians and Gynecologists (ACOG), focusing on whether the phenomenon of shoulder dystocia and NBPP can be predicted or prevented.1 Here, in Part 2, I focus on ACOG’s conclusions concerning pathophysiology and causation of NBPP, as well as the College’s recommendations for applying that knowledge to practice.

Some infants are more susceptible than others to the forces of labor and delivery
Babies emerge from the uterus and maternal pelvis by a combination of uterine ­contractions and maternal pushing (endogenous forces) aided by the traction forces applied by the birth attendant (exogenous forces). Research over the past 2 decades has shown that endogenous forces play a significant—if not dominant—role in the causation of NBPP.

Stretching and potential injury to the brachial plexus occur when the long axis of the fetus is pushed down the birth canal while either the maternal symphysis pubis or sacral promontory catches and holds either the anterior or posterior shoulder of the fetus, respectively. This conjunction of events generates a stretching force on the tissues that connect the fetal trunk and head—the neck—under which lies the brachial plexus. The same anatomic relationships and labor forces also vigorously compress the fetal neck against the maternal symphysis pubis or sacral promontory and may cause compression injury. Any traction applied by the clinician accentuates these stretching and pressure forces acting on the nerves of the brachial plexus.

How the neonate responds to these forces depends on the tensile strength of its tissues, the metabolic condition of the fetus after a potentially long labor (as measured by acid-base status), the degree of protective muscle tone around the fetal shoulder and neck, and other fluctuating conditions. In other words, because of the many variables involved, some fetuses are more or less susceptible to injury than others.

Maternal forces alone can cause NBPP
The ACOG report1 makes an important statement:

Maternal forces alone are an accepted cause of at least transient NBPP by most investigators.

Some plaintiff attorneys and their expert witnesses have tried to make the case that, although endogenous forces can cause temporary brachial plexus injuries, they cannot cause permanent brachial plexus injuries. However, as the ACOG report goes on to state:

No published clinical or experimental data exist to support the contention that the presence of persistent (as opposed to transient) NBPP implies the application of excessive force by the birth attendant. A single case report describes a case of persistent NBPP in a delivery in which no traction was applied by the delivering physician and no delay occurred in delivering the shoulders.2 Therefore, there is insufficient evidence to support a clear division between the causative factors of transient NBPP versus persistent NBPP.1

The report acknowledges that the clinician can increase brachial plexus stretch by applying downward lateral traction to the neonate’s head during delivery efforts. However, contrary to claims often made by the plaintiff bar, in the presence of shoulder dystocia, even properly applied axial traction will necessarily increase the stretching of the brachial plexus. The report also notes that traction applied in the plane of the fetal cervicothoracic spine typically is along a vector estimated to be 25° to 45° below the horizontal plane of a woman in lithotomy position, not in an exact straight line with the maternal trunk. This degree of delivery force below the horizon is defined as normal “axial traction.”

Exogenous forces have yet to be definitively measured
Multiple attempts have been made to quantify the amount of force applied by clinicians in various delivery scenarios. However, in the published studies in which this force has been “measured,” the accuracy of the findings has not been validated. The three studies in which delivery force was directly measured in a clinical setting “provide a limited assessment of exogenous forces” and “do not address the angle at which forces were applied.”3–5 All other studies used artificial models.

As a result, few conclusions from such studies are directly applicable to the clinical arena. Moreover, in other studies using simulated birth scenarios, there was no feedback to participating clinicians as to whether the force they applied would have been sufficient to deliver the “fetus.” It was therefore difficult for participants in such studies to “determine how the situation corresponds with the force they would apply clinically.”1

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Next Article:

Tackle the challenging shoulder dystocia emergency by practicing delivery of the posterior arm

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