Introduction

Neonatal brachial plexus palsy (NBPP) is typically defined as a finding identified in the immediate neonatal period due to trauma to the fetal brachial plexus at some point prior to completion of delivery. Although many times it is empirically assumed that the inciting pathophysiologic event occurs during the second stage of labor (specifically with extraction of the fetal shoulders), in fact, the exact timing of this event is poorly understood. There have been arguments presented in the literature that support an in utero occurrence prior to the labor process, an intrapartum event prior to the identification and delivery of an impacted fetal shoulder, or during the disimpaction process related to endogenously and exogenously applied forces to the fetus.

The incidence of NBPP is reported to be approximately 1.5/1000 live births, and has not decreased in recent years.^1,2 According to the majority of reports, most injuries are transient with full return of function in 70–92% of cases, whereas in selected cases, permanent injuries after the identification of neonatal brachial plexopathy have been reported to occur with an incidence of 25-78%.² Chauhan and colleagues³ described some common obstetric conditions associated with the occurrence of neonatal brachial plexus palsy in their tertiary care center over a 23-year time period. These have been summarized in Table 5.1.

Table 5.1 Common obstetrical conditions associated with neonatal brachial plexus palsy

NBPP can be divided into six clinical presentations: 1) complete (total) brachial plexus palsy; 2) Duchenne-Erb palsy; 3) upper-middle trunk brachial plexus palsy; 4) Klumpke palsy; 5) fascicular brachial plexus palsy; 6) bilateral brachial plexus palsy.⁴ The two most common presentations of NBPP encountered in obstetrical practice are Duchenne-Erb and Klumpke palsies. The former occurs due to involvement of fifth and sixth cervical nerve roots, resulting in weakness of shoulder abduction, external rotation, elbow flexion, and forearm supination. There is minimal, if any, weakness in wrist and finger extension, and winging of the scapula may be present. Klumpke palsy results from involvement of the eighth cervical and first thoracic nerve roots, causing occasional weakness of elbow flexion, forearm supination, or wrist extension, but always with weakness of the hand, which may result in a claw-like deformity of the hand.⁴ The prognosis for injuries isolated to the upper nerve trunk is significantly more favorable than those that include the lower components of the brachial plexus. Sjoberg and colleagues⁵ reported that 66% of palsies involving both the upper and lower trunks were permanent as compared to only 21% of Duchenne-Erb’s palsy cases.

Risk factors for NBPP

It is difficult to tease out individual risk factors for brachial plexus palsy in the neonate, in that many overlap. Some, such as vaginal breech delivery with hyperabduction of the fetal arms, are primarily of historical interest because of the sharp decline in this route of delivery. Most authorities agree that the occurrence of injury in the clinical arena cannot be accurately predicted based on epidemiologically derived risk factors reported in the literature. In fact, there are a number of reports that describe brachial plexus palsies in newborn infants without any identifiable risk factors being present.^6,7 Obstetrical factors and the risk for brachial plexus palsy in the delivering neonate are listed in Table 5.2.⁸

Table 5.2 Obstetrical risk factors for neonatal brachial plexus palsy

Shoulder dystocia appears prominently in this table, and in most investigations related to NBPP. Shoulder dystocia is the initial failure to deliver the fetal shoulder after the head has been delivered. The anterior, posterior, or both shoulders can be involved and the delivery route can be vaginal or by cesarean delivery. The majority of times, shoulder dystocia is a subjectively determined event, with the attending clinician assessing the degree of difficulty associated with extraction of the fetal shoulders. Sometimes utilization of specific maternal or fetal maneuvers is used to establish the occurrence of a shoulder dystocia event, as is recommended by both the Royal and American Colleges of Obstetricians and Gynecologists.^9,10 An objective definition based on prolongation of the head-to-body delivery interval greater than 60 seconds, with or without ancillary obstetric maneuvers, has been suggested but is not commonly employed in clinical practice.¹¹ The incidence of shoulder dystocia deliveries has been reported to be 0.2–2.1 percent of live births.^11,12

Of interest, although a shoulder dystocia event has the highest odds ratio of NBPP as shown in Table 5.2,⁸ nearly half of all brachial plexus injuries occur without the identification of this obstetrical complication. In the largest series to date, 57% of NBPP were not associated with the occurrence of shoulder dystocia.¹² Although one may argue that this is because of under-reporting, in fact, multiple studies consistently support this finding. A confounding variable – neonatal weight – influences this relationship. In cases of NBPP, the finding of a shoulder dystocia event increases from 22% to 74% when birth weight goes from <3.5 kg to >4.5 kg, respectively. Some investigators have postulated that non-shoulder dystocia cases associated with NBP likely occur due to different mechanistic processes in labor^8,13 and there is controversy as to whether these non-shoulder dystocia cases have a better¹³ or worse⁸ prognosis for full recovery.

Pathophysiologic mechanisms of NBPP

Compression, traction (with or without shoulder-neck angle widening), vascular disruption, and inflammation are identified pathologic mechanisms by which the newborn’s brachial plexus can be transiently or permanently affected. Intrauterine compressive brachial palsy is usually associated with an observed deformity of the upper limb, and may be caused by compression from a uterine anomaly and uterine or pelvic mass during gestation.^14,15 Gonik and colleagues¹⁶ have also recently described a mathematical model that hypothesizes the occurrence of brachial plexus compression underneath the symphysis pubis at the time of vaginal delivery. This compression of the brachial plexus is thought to be the result of either exogenous force generated by the delivering clinician with extraction of the anterior fetal shoulder or related to maternal endogenous forces with attempts to expulse the obstructed fetus.

Inflammation-induced disruption of the brachial plexus due to in utero viral¹⁷ or bacterial¹⁸ infection has been reported. In these cases, other associated findings are typically seen, such as muscular atrophy of the affected arm or humeral osteomyelitis. Other less common causes of in utero NBPP include hemangiomas, exostosis of the first rib, and neoplasms in the region of the brachial plexus.

Traction/stretching as an etiology for brachial plexus palsy in the newborn has received the most attention, particularly related to the occurrence of shoulder dystocia at the time of delivery. For many years the naïve explanation for NBPP associated with shoulder dystocia was that too much traction was applied by the clinician in an attempt to clear (usually) the anterior fetal shoulder from underneath the maternal symphysis pubis. As an aside, this “excessive” force has been the battle cry of the medicolegal community, as plaintiff and defense attempt to justify or explain why the injury took place. Of critical importance, one needs to first define excessive compared to what. Clearly from the obstetrician’s perspective, whatever force is applied to complete the delivery process is necessary, because without this assistance the fetus might suffer hypoxic injury or death by remaining in situ. The term “excessive” must therefore mean the force that is in excess to the inherent strength of the structure in question, here being the fetal brachial plexus. Germane to this discussion, these threshold forces have yet to be defined in the clinical setting and more importantly, may vary considerably depending on other factors such as the fetal acid/base status, muscle tone of the surrounding neck muscles, the anatomic relationship of the fetal head to the aftercoming shoulders, and the direction of any force that is applied. An additional factor needs to be mentioned, and that is the delivering clinician has a difficult time accurately perceiving the force that is being applied, regardless of the clinician’s years of experience, gender, or body mass index.^19,20 With the use of a recently developed high-fidelity shoulder dystocia simulation mannequin, Crofts et al.²¹ demonstrated a very wide range of “diagnostic traction” peak forces (6-250 Newtons) being applied before the implementation of shoulder dystocia maneuvers.