Correspondence Pathogenesis of Obstetric Brachial Plexus Palsy To the Editor: I read with interest the article by Zafeiriou and Psychogiou [1], in which the authors explained the pathogenesis of obstetric brachial plexus palsy by focusing on fetal size, shoulder dystocia, and the re- lationship between the magnitude of propulsive force generated by uterine contraction and the magnitude of traction force generated by the obstetrician during delivery. The authors did not mention the re- sistance of the brachial plexus to stretch injury as a factor in the oc- currence of obstetric brachial plexus palsy. I think that sufficient published experience gives credence to this factor, involving: (1) posterior shoulder involvement, (2) occurrence of brachial plexus palsy after cesarean section, (3) the presence of shoulder girdle mus- cle atrophy at birth in a patient with electromyographically timed obstetrical brachial plexus palsy, (4) the presence of congenital de- formation of the arm in patients with obstetric brachial plexus palsy, and (5) the association of motor-cortex fetal-brain malformation with contralateral brachial plexus palsy [2-4]. This experience has led me to consider the probability of obstetric brachial plexus palsy (P) as directly proportional to the magnitude of stretching force (pro- pulsive and traction forces) acting upon the most vulnerable segment of the brachial plexus (f), and inversely proportional to the resistance of that segment (r). This assertion can be expressed as a simple for- mula (P = f/r). This formula can explain the occurrence of obstetric brachial plexus palsy in deliveries in which the magnitude of the stretching force seems insufficient to have injured the brachial plexus. I would like to read the authors’ opinion regarding the rela- tive importance of the intrinsic resistance of the brachial plexus and the extrinsic resistance of the shoulder girdle muscles, joints, and bones in the development of obstetric brachial plexus palsy. Israel Alfonso, MD Brachial Plexus Program Department of Neurology Miami Children’s Hospital Florida International University Miami, FL 33155 References [1] Zafeiriou DI, Psychogiou K. Obstetrical brachial plexus palsy. Pe- diatr Neurol 2008;38:235-42. [2] Sandmire HF, DeMott RK. Erb’s palsy causation: A historical perspective. Birth 2002;29:52-4. [3] Alfonso I, Diaz-Arca G, Alfonso DT, et al. Fetal deformations: A risk factor for obstetrical brachial plexus palsy? Pediatr Neurol 2006;35:246-9. [4] Alfonso I, Alfonso DT, Price AE, Grossman JAI. Cortical dyspla- sia and obstetrical brachial plexus palsy. J Child Neurol 2008; in press. Response: We thank Dr. Alfonso for his letter in response to our recent review concerning obstetric brachial plexus palsy [1]. Although different theories regarding an etiology have been presented in the literature, the exact pathogenesis of the disease has not been fully elucidated. Shoulder dystocia, endogenous forces of labor, intrauterine maladap- tation, clinician-applied forces, and posterior shoulder involvement are recognized etiologic factors. However, the precise mechanisms of action of several potential risk factors are not fully understood. Therefore, the resistance of the brachial plexus to stretch injury could explain, to some extent, how damage to the brachial plexus is caused. Although there is inadequate evidence to support this view, we agree to a certain extent with the theoretical basis postulated by Dr. Alfonso. The occurrence of brachial plexus palsy after cesarean section, the presence of shoulder girdle muscle atrophy at birth in patients with obstetric brachial plexus palsy, and posterior shoulder involvement could also be well-explained by decreased resistance of the brachial plexus, resulting from shoulder muscle weakness subsequent to uterine constraint because of malformations of the uterus, and mal- adaptation, malpositioning, and impaction of the posterior aspect of the shoulder on the sacral promontory. However, we would like to focus on two points: (1) birth palsy in newborn babies delivered by cesarean section [2], as well as in cases where intrauterine posture almost certainly plays a role in the etiology of brachial palsy [3], is extremely rare; and (2) as far as obstetric brachial plexus palsy in- volving the posterior arm is concerned, there is no scientific expla- nation to support this theory or to clarify the involved mechanisms. Alfonso et al. previously associated fetal deformations and ob- stetric brachial plexus palsy [4]. One of the mechanisms suggested by these authors involves decreased resistance of the shoulder gir- dle structures, which can be caused by a congenital deformation of the arm. Although we agree on this point with Alfonso et al. [4], we need to highlight that this association lacks evidence-based proof, because it is based only on retrospective studies. Searching the literature for motor cortex fetal malformations as- sociated with contralateral brachial plexus palsy, we did not find much evidence in support of this view. Until more extensive data are available, the above correlation remains speculative. Regarding the formula P = f/r, as suggested by Dr. Alfonso to express the probability of obstetric brachial palsy, we have to raise certain objections. First, there is considerable doubt regarding its practical efficacy, because it is a rather simplified expression with insufficient studies to prove the exact mathematical relation- ship between the probability of obstetric palsy and resistance of the brachial plexus. Moreover, it is difficult to identify deliveries with any certainty, in which the magnitude of stretching forces seems insufficient to have injured the brachial plexus, because the stretch-injury mechanism applies in most cases. Until prospective, multicenter studies address the etiology of the problem and reli- able biomechanical models are developed, different theories will arise in this specific setting, in hopes of better defining the factors that lead to injury. In conclusion, the present theory, although not evidence-based, offers a plausible explanation in some cases of obstetric brachial injury, but remains to be proven. Dimitrios I. Zafeiriou, MD, PhD Katerina Psychogiou, MD First Department of Pediatrics Aristotle University of Thessaloniki 54622 Thessaloniki, Greece References [1] Zafeiriou DI, Psychogiou K. Obstetric brachial plexus palsy. Pediatr Neurol 2008;38:235-42. [2] al-Qattan MM, el-Sayed AA, al-Kharfy TM, al-Jurayyan NA. Obstetrical brachial plexus injury in newborn babies delivered by caesar- ean section. J Hand Surg [Br] 1996;21:263-5. [3] SIooff AC, Ubachs JM. Brachial plexus impairment: A birth trauma? Am J Obstet Gynecol 1993;169:230. [4] Alfonso I, Diaz-Arca G, Alfonso DT, et al. Fetal deformations: A risk factor for obstetrical brachial plexus palsy? Pediatr Neurol 2006;35:246-9. Ó 2008 by Elsevier Inc. All rights reserved. 0887-8994/08/$—see front matter Correspondence 371