Pathophysiology of Neonatal Brachial Plexus

Pathophysiology of Neonatal Brachial Plexus
The first known description of neonatal brachial plexus palsy (BPP) dates from 1779 when Smellie reported the case of an infant with bilateral arm weakness that resolved spontaneously within a few days after birth. In the 1870s, Duchenne and Erb described cases of upper trunk nerve injury, attributing the findings to traction on the upper trunk, now called Erb’s palsy (or Duchenne-Erb’s palsy). [1]In 1885, Klumpke described injury to the C8-T1 nerve roots and the nearby stellate ganglion that now bears her name.

Many cases of BPP are transient, with the child recovering full function in the first week of life. A smaller percentage of children continue to have weakness leading to long-term disability from the injury. The mainstay of treatment for these children is physical and/or occupational therapy in concert with a regular home exercise program. A select few patients may benefit from surgical intervention in the early stages to improve innervation of the affected muscles. Others benefit from tendon transfers performed later to improve shoulder and (sometimes) elbow function. 

Numerous other nonsurgical treatments, including electrical stimulation and botulinum toxin injections, also may prove effective in the treatment of children with BPP. In view of the variability in presentation, treatment options, and outcome measures, a multidisciplinary approach to the care of the infant with BPP is recommended.

Pathophysiology

  • To understand the clinical presentation of brachial plexus palsy (BPP) and provide anticipatory guidance for families affected by the condition, the clinician must first know basic anatomy. As seen in the image below, the brachial plexus consists of nerves (the ventral rami) from C5-T1.
  • C5 and C6 join to form the upper trunk, C7 travels alone as the middle trunk, and C8-T1 join as the lower trunk. Each trunk divides into anterior and posterior divisions to create the cords, which then subdivide further into branches that supply the muscles of the arm. Injuries of the brachial plexus may be mild, with only temporary sequelae, or devastating, leaving the child with a flaccid, insensate arm.

Severity depends on the number of nerves involved and the degree to which each level is injured. The basic types of BPPs include the following:

  • Erb’s palsy affects nerves arising from C5 and C6.
  • Upper-middle trunk BPP involves nerve fibers from C5, C6, and C7 levels.
  • Klumpke palsy results in deficits at levels C8 and T1, although many clinicians agree that pure C8-T1 injuries do not occur in infants and may be indicative of spinal cord injury (SCI).
  • Total BPP affects nerves at all levels (C5-T1).
  • Bilateral BPP demonstrates bilateral involvement.
  • When defining the severity of a peripheral nerve injury, differentiation between neurapraxic, axonotmetic, and neurotmetic lesions is helpful.

Purely neurapraxic lesions do not affect the axon itself. These lesions generally are reversible and do not leave sequelae.

Axonotmetic lesions involve disruption of the myelin sheath and the axon, leading to degeneration of the axon distal to the injury. The connective tissue across the lesion remains intact. These injuries improve gradually over 4-6 months, depending on the level of the lesion.

Neurotmetic lesions are the most severe, destroying not only the axon and myelin, but also the supporting structures across a nerve. As the proximal end of the nerve attempts to regenerate without this supportive connective tissue, a neuroma may develop. The extent of improvement in the patient’s condition depends on the ultimate number of nerve fibers that reconnect distal to the neuroma. Muscle atrophy from a neurotmetic lesion begins 3-6 months after injury and by 1.5-2 years is irreversible.

Although the traditional mechanism of injury is lateral neck flexion, the upper rootlets (C5-C7) are 25% as likely to be avulsed as the lower roots (C7-T1). The upper roots (C5-C6), however, are far more likely to be ruptured (88%) because of the anatomy of the transverse processes and the degree of flexibility at that level.

The clinician must also distinguish neonatal BPP from traumatic BPP in older children and adults. The damage in neonates usually results from slow traction injuries, unlike the high-energy shearing type of trauma seen in older individuals. Not only are the latter injuries often more severe, but with similar injuries, infants show a better functional outcome.

This clinical observation is confirmed by Vredeveld and colleagues, who studied 14 infants and 19 adults with surgical evidence of complete avulsion of the C5-C6 roots or upper trunk.  Electromyography (EMG) showed normal recruitment of biceps and deltoid in the infants and complete denervation in the older individuals. When C7 also was torn, the infants demonstrated complete denervation. Vredeveld and coworkers attributed this observation to neonatal C7 innervation of the biceps and deltoid that subsequently was lost if the C5-C6 roots were functional.

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