The etiology of brachial plexus injuries is complex and subject to debate. The prevailing etiologic theory relates to stretching of the nerves in an infant who is large for gestational age and who has sustained a “difficult” vaginal delivery.
Many injuries to the brachial plexus in neonates are presumed to have a traumatic origin resulting from a difficult delivery. Signs and symptoms of such injuries include abnormal presentation, fetal depression, high incidences of macrosomia and shoulder dystocia, and prolonged or augmented labor.
11 A connection between injuries and difficult deliveries is consistent with the presumed pathogenesis of some brachial plexus lesions, which involves stretching of the nerve roots from traction or, in more severe lesions, severing of the nerve sheath. Other brachial plexus injuries may be related to compression of the nerve.
The possibility that some injuries of the brachial plexus are caused by intrauterine malpositioning rather than traumatic delivery or compression has been proposed by some authors.
12,13 This possibility is supported by the high percentage of abnormal presentations at birth (eg, breech, occiput posterior, occiput transverse) among neonates with brachial plexus injuries.
13,14 The possibility is also supported by the occurrence of brachial plexus injuries in infants who were born prematurely or delivered by cesarean section—in cases in which there was no shoulder dystocia and in cases in which there was shoulder dystocia but the posterior arm was the affected extremity.
11,13,14 One study
15 described Erb's palsy in several infants of normal weight who did not experience traumatic delivery. The presence of either abnormal dermatoglyphics, muscle atrophy, undersized extremities, or deformation of the ribs or neck are useful in indicating prenatal onset caused by intrauterine malpositioning.
16,17 In addition, compression of the plexus against the walls of a malformed uterus or uterine fibroma, exostosis of the first rib, an amniotic band, and the umbilical cord have been implicated as etiologic factors.
13
Electrophysiologic studies have demonstrated evidence of denervation (ie, nerve injury) within days after birth—notwithstanding the fact that it usually takes at least 10 to 14 days after injury for denervation to be detected with EMG.
12,13 Thus, an EMG result that indicates denervation within the first several days of life suggests that an injury is likely to be prenatal in onset—though recent data collected from animal subjects have raised questions about the use of EMG to time the onset of brachial plexus injuries in infants.
7 Other studies have noted that EMG changes consistent with denervation may be found in the normal infant and, therefore, cannot be relied upon to determine etiology unless the findings are present only in the affected extremity.
16
Nontraumatic, hereditary origins, though rare, should also be considered. An inherited autosomal-dominant brachial plexopathy (often referred to as hereditary neuralgic amyotrophy) has been identified.
18 Although this disorder typically affects individuals in the second or third decade of life, it has been rarely reported in neonates.
18,19 Both adult and infant patients may have mild dysmorphisms (eg, cleft palate, epicanthal folds, hypotelorism, short stature, syndactyly). The cranial nerves and the nerves of the lower extremities may also be affected. Episodes of weakness are recurring, and, during an episode, symptoms progress over a period of hours or days. Recovery usually occurs within 30 days, but some weakness and atrophy may persist.
A gene for hereditary neuralgic amyotrophy has been localized to chromosome 17q24-q25.
18 The key to diagnosis is a family history that shows a similar clinical syndrome. There is a single report of a patient with an apparent bilateral brachial plexopathy associated with agenesis of the biceps muscle.
15 It is likely that other nontraumatic hereditary origins of brachial plexus injuries will be reported in newborns.