Introduction

In his 2002 paper, Pecorari states the following:

Unfortunately for obstetricians and midwives, in court Erb palsy has been causally connected with shoulder dystocia and errors in management, although it is not always true. Perhaps the lack of an obvious explanation has contributed to the identification of the birth attendant as a handy scapegoat.

When there is a permanent brachial plexus injury following shoulder dystocia, responsibility for this injury is often presumed to be with the obstetrician who supposedly did not foresee that a shoulder dystocia was likely to occur or mishandled it when it did. Yet a review of the literature does not substantiate such assumptions. Gherman in his 1998 paper summarizes the refutation to these claims:

We feel that some cases of brachial plexus injury are unavoidable events. Recent reports have noted that brachial plexus palsies occur:

(1) In the absence of characteristic risk factors

(2) In the absence of shoulder dystocia

(3) In the posterior arm of infants whose anterior shoulder was impacted behind the symphysis pubis

(4) In vertex-presenting fetuses delivered by atraumatic Cesarean section

(5) Without apparent relationship to the type or number of maneuvers used to disimpact the fetal shoulder

(6) In association with other peripheral nerve injuries

(7) With electromyelographic evidence of muscular denervation during the immediate postpartum period [indicating pre-delivery injury].

Jennett commented in a similar vein in 1997:

Evidence continues to accumulate that renders a univariate theory of the causation of brachial plexus injury untenable . . . Intrauterine maladaption is responsible for some instances of brachial plexus injury.

He notes that in his series of deliveries from 1977 to 1990, 22 of 39 (56%) brachial plexus injuries were not associated with shoulder dystocia. In that same paper he quotes Pearl (1993) and Gimovsky (1995), both of whom reported brachial plexus injuries in babies delivered from the occiput posterior position without shoulder dystocias. He further cites Walle (1993) who observed in his patient population that 1/3rd of 175 shoulder dystocias involved the posterior shoulder.

There are many other similar reports:

Hardy (1981) reported 36 infants with brachial plexus injuries of whom only 10 had shoulder dystocia noted at birth.

Gilbert (1990) initially published a study of 1000 infants with brachial plexus injury in which 39% did not have shoulder dystocia at delivery. In a supplementary article in 1999, he reported that 47% of babies with brachial plexus injury in his now larger series experienced deliveries in which no shoulder dystocia was noted. Even among macrosomic fetuses in this series, 26% of brachial plexus injuries occurred in the absence of shoulder dystocia.

Gram (1997) noted that only 8 babies had shoulder dystocia deliveries in a group of 15 who experienced brachial plexus injury. In the other seven cases, there had been no birth trauma.

In Gonik's 1991 paper, 71% of all injured infants in his series were the product of deliveries without recognized shoulder dystocia.

Ouzounian (1997) reported 4 babies with brachial plexus injury in which there was not even downward traction during delivery.

In Hillard's (1997) series of babies with Erb palsy, 15 of 51 babies had not experienced shoulder dystocia during delivery.

Sandmire (1996) reported 17 babies in his series of 36 with brachial plexus injuries whose deliveries did not involve shoulder dystocia. This article included his personal review of the literature concerning brachial plexus injury (BPI) with and without shoulder dystocia:

As can be seen, 51% of brachial plexus injuries in over 1727 deliveries of macrosomic babies did not involve shoulder dystocias.

What does cause brachial plexus injuries?

The standard explanation for brachial plexus injury is that it results from excessive downward traction by the obstetrician on the fetal head during the delivery of the anterior shoulder. This supposedly overstretches the brachial plexus thus injuring it.

It is in fact true that the majority of brachial plexus injuries, whether permanent or not, do follow shoulder dystocias. There also appears to be a correlation between the severity of the shoulder dystocia and the degree of brachial plexus injury. But there is also much evidence in the literature that brachial plexus injuries are caused by factors other than shoulder dystocia-related trauma.

What other than shoulder dystocia might cause brachial plexus injury?

The tractor-trailer theory

Sandmire (2000) and others have recently proposed an explanation for brachial plexus injuries that explains much of the data that has been collected over the years. Obstetricians have long sought to understand the mechanism of those brachial plexus injuries that occur following extremely rapid second stages of labor, some of which are as short as one or two contractions. Sandmire studied what happens to the various parts of the fetus during uterine contractions and maternal pushing. He noted that the forces of contractions and maternal pushing act on the long axis of the fetus. If the fetus's anterior shoulder were to get stuck behind the maternal pubic bone and continued pressure were applied to the long axis of the fetus, the baby's brachial plexus would undergo considerable stretching.

This may be compared to what happens when a tractor-trailer truck approaches a low overpass at high speed. While the tractor may pass under the bridge, the trailer -- taller than the tractor -- will ram into the overpass with high impact. The momentum of the tractor will result in large forces acting to separate it from its attached trailer. Sandmire suggests that an equivalent force acts upon a baby's brachial plexus during some shoulder dystocia deliveries.

Forces in deliveries

Some investigators have actually used mechanical testing devices in an attempt to measure the pressure placed on the brachial plexus of an infant during shoulder dystocia deliveries -- with conflicting results.

Allen (1991) reported his use of tactile force sensing devices on the tips of gloves during a series of vaginal deliveries to measure the forces placed on a baby's head by an obstetrician's hands. The deliveries that were observed were categorized into three groups: Routine, difficult, and those involving shoulder dystocias. He found that twice as much force was applied to a baby's head during shoulder dystocia deliveries as compared with routine deliveries.

Gonik, however, one of Allen's co-investigators, subsequently published a mathematical model estimating the forces acting on the fetal neck overlying the roots of the brachial plexus. His findings mitigate the significance his and Allen's previous work. He showed that the forces applied by the clinician to the fetal neck were only 1/4 to 1/9 of those that resulted from uterine contractions and maternal pushing themselves in the second stage of labor.

Posterior shoulder

Most brachial plexus injuries occur to a baby's right arm (60%). This is because babies most commonly "present" into the mother's pelvis in the left occiput anterior position (LOA). The LOA position is when the back of the baby's head -- the occiput -- points towards the mother's left arm while the fetal face is oriented towards the mother's right buttock. In this fetal position the baby's right arm will be anterior -- and thus more likely to get caught under the mother's pubic bone. But brachial plexus injuries have also been reported in the posterior shoulder. It is thought that in these cases the posterior shoulder gets caught on and restrained by the sacral promontory while the remainder of the baby is being pushed forward by the mother's expulsive efforts or by her uterine contractions. The posterior brachial plexus would thus be stretched, potentially injuring it.

Pre-delivery (in-utero) injury

There are multiple reports of brachial plexus injuries which appear to have occurred sufficiently prior to delivery so as to not be causally related to it. The evidence for the timing of such in utero injuries comes from electromyelographic studies, the measurement of electrical transmission in muscle fibers.

It takes approximately ten days for a muscle to show an injury pattern on electromyography after the nerve innervation to that muscle is damaged. Therefore if muscle damage from a brachial plexus injury is measured by electromyography immediately after delivery, the injury had to have occurred at least a week or more before the delivery took place.

Koenigsberg's (1980) report of this phenomenon is the most detailed. He describes two cases of brachial plexus injury in which electromyelographic evidence suggested an intrauterine -- prior-to-birth -- origin. One 3,625 gram baby had a classic right Erb palsy at birth. But electromyelographic studies of this baby's deltoid, biceps and brachial radialis muscles shortly after birth revealed multiple fibrillations and positive injury waves in addition to a greatly reduced numbers of active muscle units. Electromyelographic studies on the opposite arm were normal.

A second baby, a 3,180 gram term infant delivered via Cesarean section, was noted at birth to have left upper arm muscle weakness, loss of movement in the left hand, and Horner's syndrome -- classic for severe brachial plexus injury. Electromyelographic studies showed clear-cut evidence of muscle fiber damage in the left arm. All studies on the right side were normal.

Based on the electromyelographic evidence, Koenigsberg concluded that the injuries to the two babies had to have occurred prenatally rather than being caused by any injury associated with delivery. Such muscle injuries from brachial plexus damage measured electromyelographically at birth have also been reported by Philpot (1995), Jennett (1992), and others.

Brachial plexus injuries following Cesarean section

Reports of brachial plexus injury in the absence of shoulder dystocia are subject to the criticism that perhaps shoulder dystocias were under-reported or that "excess' traction might have been placed on the baby's head during the course of a routine delivery. But reports of brachial plexus injury following Cesarean section are less subject to criticism. There are many such reports in the literature:

Ecker (1997): Two infants born by Cesarean section who sustained brachial plexus injuries, one of a nondiabetic mother, the other of a diabetic mother.

Hardy (1981): Two infants born in vertex position at Cesarean section who sustained brachial plexus injuries.

McFarland (1986): Four patients delivered by Cesarean section who experienced brachial plexus injuries.

Graham (1997): Reported an Erb palsy from cesarean section.

Gilbert (1999): Evaluated data on all brachial plexus injuries from California in the years 1994 to 1995. Of the 1,094,298 babies born in those two years there were 1,611 brachial plexus injuries reported (0.15%). Of these, 60 were from Cesarean sections.

The phenomenon of brachial plexus injury following cesarean delivery -- and thus not related to shoulder dystocia -- is real. As has been shown, there is much evidence to suggest that not all instances of brachial plexus injury are due to shoulder dystocia deliveries or to the actions of a physician during such deliveries. Thus the automatic assignment of responsibility to an obstetrician or midwife for a brachial plexus injury whenever a shoulder dystocia delivery occurs is inappropriate and not supported by the literature.

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Copyright © 2006 Henry Lerner