Etiology

Brachial plexus birth injuries are thought to be caused by traction on the brachial plexus at the time of delivery.[18] During a vaginal delivery complicated by shoulder dystocia, the head and neck can be laterally deviated away from the shoulder, producing traction on the brachial plexus as the nerve roots exit the spinal column. The forces of delivery, either through strong uterine contractions or assistance of delivery, may then exceed the strength of the nerves, causing a stretch injury or rupture. In some cases nerve root avulsions may occur. Rarely, the brachial plexus may be stretched during the initial phase of delivery or due to uterine anomalies (e.g., fibroma, bicornuate uterus).[8] Brachial plexus injury can occur after cesarean delivery, but it is less common.[2][15][16]​​

Identification of pregnancies likely to be complicated by shoulder dystocia is difficult and often impossible until the time of delivery, but maternal diabetes or obesity and an abnormal second stage of labor may indicate problems.[3][19][20] Ultrasound can be inaccurate in estimating fetal size and cannot be used to reliably predict shoulder dystocia.[21]

Pathophysiology

Nerve injury can take several forms.

Neuropraxia

  • The most common type of injury.

  • A stretch injury of the axon that does not cause division of the neural tissue.

  • A conduction block is encountered and is seen clinically as paralysis of the affected muscles.

  • Typically resolves quickly (within several months) and often leads to complete recovery.

Axonotmesis

  • A more severe injury that may involve rupture of the axon with intact nerve sheath.

  • While the recovery may take longer than that with neuropraxia, it proceeds through the process of Wallerian degeneration and repair.[22] Good, if not complete, recovery can be expected.

  • Typically recovery is seen as return of function by 3 to 6 months of age.

Neurotmesis

  • The most severe type of injury.

  • Involves complete division of the nerve, often referred to as a nerve rupture.

  • Leaves no continuity for nerve regeneration and healing to follow.

  • Requires surgical repair to allow any function to occur.

  • Seen clinically as minimal or unsatisfactory return of function beyond 6 months of age. The longer it takes a (denervated) muscle to reinnervate, the more atrophy and less function can be expected to return. For this reason, nerve reconstruction is advocated before 1 year of age if there is a lack of functional recovery. After this timeframe, minimal recovery from nerve surgery can be expected.[22]

Nerve root avulsions

  • Nerve root avulsions from the spinal cord may also occur. This type of injury may be associated with Horner syndrome, indicative of injury to the sympathetic nerves to the face, or injury to the phrenic nerve, causing diaphragmatic paralysis. These injuries will need to be treated with nerve transfers (sewing an adjacent, functioning nerve or part of a nerve into a nonfunctioning nerve in an attempt to restore function in a paralyzed muscle) because there is no neural tissue proximal to the injury that can reliably be grafted.

Classification

Anatomic classification for brachial plexus palsies[5][10][11]

Based on clinical examination, this classification defines the injured levels and directs attention to areas of potential problem after recovery. [Figure caption and citation for the preceding image starts]: Schematic of the cords and roots of the brachial plexusThomas Campbell by commission [Citation ends].com.bmj.content.model.Caption@4cb1cba1

  • C5-C6 involvement (Erb palsy)

  • C5-C7 involvement (extended Erb palsy)

  • C8-T1 involvement (Klumpke palsy)

  • C5-T1 involvement (total plexus palsy).

Narakas classification[12][13][14]​​

Defines the injured nerves and directs attention to potential areas of concern during recovery.

  • Group 1: C5-C6 injury (upper Erb palsy)

    • Paralysis of shoulder and biceps

    • Rate of spontaneous recovery >80%

  • Group 2: C5, C6, C7 injury (extended Erb palsy)

    • Paralysis of shoulder, biceps, and wrist extensors

    • Rate of spontaneous recovery around 60%

  • Group 3: C5, C6, C7, C8, T1 injury (total palsy with no Horner sign)

    • Paralysis of entire limb

    • Rate of spontaneous recovery <50%

  • Group 4: C5, C6, C7, C8, T1 injury (total palsy with Horner sign)

    • Paralysis of entire limb with Horner sign

    • Rate spontaneous recovery around 0%.

Modifications (including an extended Narakas classification with five groups) have been described but are rarely used.[12][14]

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