The cause of Erb's Palsy? The verdict is in.
In 1998, the American College of Obstetrics and Gynecology Precis Volume V confirmed over 100 years of brachial plexus investigation, "brachial plexus injuries result from excessive lateral traction." In 1998, ACOG Precis Volume V stressed that "the key to preventing injury is avoidance of excessive traction to the fetal head."
It is well known that many infants are injured in the first 60 seconds of traction. However, it is well known that physicians rarely will record "strong traction" in their delivery notes. In fact, the vast majority of shoulder dystocias are not recorded by the physicians in their delivery notes.
The authors claiming that intrauterine forces cause Erb's Palsy ignore the protective effect of the amniotic fluid (bag of waters) and the uterine muscle and the protective effect of fetal muscle. Intrauterine causation is a bogus theory and is not accepted by obstetricians outside the United States and is not accepted by non-obstetricians within the United States as reliable or established theory. At best, it can be described as novel.
A review of literature reveals that no other country supports this unreal causation theory of intrauterine forces as the cause of Erb's Palsy. There is no mention in the literature of the forces of the uterus causing nerve injury without injury to overlying muscle, skin, fat, fascia, or bone of the baby. The in-utero causation of Erb's Palsy is only reported in the United States obstetric literature and is written by defense expert witnesses.
Excessive persistence with downward head traction is the most common cause of Erb's Palsy. Most brachial plexus injuries and clavicular fractures are associated with downward traction and lateral stretching of the fetal neck.
Creasy's textbook on Maternal Fetal Medicine 516 (2d ed) states that brachial plexus injuries result from excessive lateral traction on the head of the infant during the delivery process. On page 516, the authors note that only the "gentlest traction" should be used. Excessive traction at this point is the source of brachial plexus injuries.
The seventeenth and most recent edition of Williams Obstetrics, the "Bible of Obstetrics", notes that the brachial plexus injury results from stretching or tearing of the upper roots of the brachial plexus, which is readily subjected to extreme tension as a result of pulling laterally upon the head, sharply flexing toward one of the shoulders. In extracting the shoulders, care should be taken not to impose excessive lateral flexion of the neck.
In all cases, the essential element of causation is traction on the nerves. The traction theory is supported by pathologic and clinical evidence.
There are significant, peer-reviewed, published studies showing: (1) There is a relationship between the force exerted on the head during shoulder dystocia and the production of Erb's Palsy; (2) Some physicians do not know when they are pulling significantly harder than at other times, as shown by actual measurements of forces applied. Physician documentation of excessive traction is completely unreliable, both because physicians do not recognize it when it happens and because physicians are not likely to document potentially incriminating actions once they see a baby with a limp arm; (3) Many physicians apply significant upward as well as downward traction to babies with shoulder dystocia, often exceeding the "Erb's threshold"; (4) Skill and experience contribute to avoiding shoulder dystocia and avoiding Erb's Palsy if shoulder dystocia occurs. The difference among physicians is more likely due to differences in skill and experience in manipulating the child without the use of excess force than it is due to "luck", "chance" or "intrauterine events"; and (5) The reported frequency of Erb's Palsy in the absence of lateral traction, including upward traction for posterior arm injuries, is extremely small compared to the reported frequency of Erb's Palsy in the presence of lateral traction. The injury is caused by the traction in about 98 to 99 percent of all cases.
Maternal pushing with the head out does not cause traction on the head; it causes impaction. The shoulder does not get stuck until the head is out.
Maternal pushing further impacts the shoulder or shoulders. It does not stretch the neck. The shoulders are not at risk for a permanent injury until the head is out. Traction or stretch injuries require a force and a resistance. If the shoulders are stuck in labor the forces of labor do not cause direction traction on the head.
Erb's Palsy in the absence of shoulder dystocia is a very rare event.