brachial plexus

Function

The brachial plexus is responsible for cutaneous and muscular innervation of the entire upper limb, with two exceptions: the trapezius muscle innervated by the spinal accessory nerve and an area of skin near the axilla innervated by the intercostobrachialis nerve.

Therefore, lesions of the plexus can lead to severe functional impairment.

Anatomy

Path

One can remember the order of brachial plexus elements by way of the mnemonic, "Randy Travis Drinks Cold Beer" - Roots, Trunks, Divisions, Cords, Branches^[1]

• The five roots are the five anterior rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck.

• These roots merge to form three trunks:
  • "superior"^[2] or "upper" (C5-C6)
  • "middle"^[3] (C7)
  • "inferior"^[4] or "lower" (C8-T1)

• Each trunk then splits in two, to form six divisions:
  • anterior division^[5] of the superior, middle, and inferior trunks
  • posterior division^[6] of the superior, middle, and inferior trunks

• These six divisions will regroup to become the three cords. The cords are named by their position in respect to the axillary artery.
  • The posterior cord is formed from the three posterior divisions of the trunks (C5-T1)
  • The lateral cord is the anterior divisions from the upper and middle trunks (C5-C7)
  • The medial cord is simply a continuation of the lower trunk (C8-T1)

• The branches are listed below. Most branch off of the cords, but a few branch (indicated in italics) directly off of earlier structures. The five in bold are considered "terminal branches".

Specific branches

Diagram

Injuries

Brachial plexus lesions are classified as traumautic or obstetric. These typically result from excessive stretching and avulsion injury. Traumatic injuries are often caused by high-velocity motor vehicle accidents, especially in motorcyclists. Injury from a direct blow to the lateral side of the scapula is also possible.

Most commonly, forceps delivery or falling on the neck at an angle causes upper plexus lesions (Erb's Palsy). This type of injury produces a very characteristic sign called Waiter's tip deformity due to loss of the lateral rotators of the shoulder, arm flexors, and hand extensor muscles.

Much less frequently, sudden upward pulling on an abducted arm (as when someone breaks a fall by grasping a tree branch) produces a lower plexus injury. This results in the sign known as clawed hand due to loss of function of the ulnar nerve and the intrinsic muscles of the hand it supplies.

The cardinal signs of brachial plexus avulsion are:

• a weakness in the arm
• diminished reflexes
• corresponding sensory deficits

In most cases the nerve roots are stretched or torn from their origin, since the meningeal coverings of the nerve roots are thinner than the sheaths enclosing the peripheral nerves. The epineurium of the peripheral nerve is contiguous with the dural mater, providing extra support to the peripheral nerves. In cases where the nerve roots have been torn, recovery is unlikely without invasive experimental surgical techniques ^{citation needed]}.

The diagnosis may be confirmed by an EMG examination in 5-7 days. The evidence of denervation will be evident. If there is no nerve conduction 72 hours after the injury, then avulsion is most likely.

Additional images