7.1. Introduction
- General Features:
- Brachial plexus is in close proximity to mobile components of the neck and shoulder which make it susceptible to traction injury
- The brachial plexus has 5 anterior primary rami (APR) that are derived from the C4–T1, primary dorsal foramina between the scalenus anterior and medius muscle
- Trunks are superficial in the supraclavicular fossa
- The lower trunk is adjacent to the subclavian artery and apex of the lung
- Divisions of the plexus (retroclavicular area) are between the clavicle and the first rib
- Infraclavicularly: the cords surround the axillary artery and are close to the proximal humerus and the glenohumeral joint
- Proximal portions of the median, ulnar and radial nerves are adjacent to the proximal humerus and the axillary artery
- Trunks of the plexus form at the lateral border of the anterior and middle scalene muscles:
- C5–C6 APRs coalesce to form the upper trunk
- C8–T1 APRs form the lower trunk
- C7 primary anterior ramus forms the middle trunk
- Trunks split into anterior and posterior divisions:
- Divisions are located deep to the midportion of the clavicle in front of the first rib (during arm adduction)
- Anterior divisions of the upper and middle trunk from the lateral cord; the lower trunk anterior divisions form the medial cord
- Posterior division of the upper trunk, medial trunk and lower trunk fuse to form the posterior cord
- Cords are situated below the clavicle in the axilla and are the longest components of the plexus
- The terminal nerves are the median, ulnar, radial, musculocutaneous and axillary; they take origin in the peripheral axilla
- Supraclavicular plexus is injured more frequently than infraclavicular plexus
- Anterior primary rami (APR) injury occurs in:
- Post median sternotomy incisions
- Cervical rib-band syndrome
- Primary root and mixed spinal nerve involvement occurs in:
- Neuropraxic injury
- Costoclavicular space neoplasm (primary apical lung tumors)
- Root involvement: C5 to T1; C5–C6 most common; C7–T1 rarest
- Middle plexus is rarely affected alone
- Upper plexus is injured by severe trauma more than the lower plexus; an exception might be lower trunk involvement from closed neuropraxic injury (MVA)
- C8–T1 roots are most susceptible to avulsion; C7 is the next most susceptible
- Severe trauma ruptures the upper trunk > the C5–C6 roots lie within the foramina or lateral recess and are covered by fascia. C8–T1 and C7 are not covered by fascia
- Nerves that arise from the brachial plexus above the clavicle:
- Dorsal scapular nerve (C5 anterior ramus):
- Innervates the rhomboid muscle
- A branch is a component of the phrenic nerve
- Long thoracic nerve:
- Originates from the anterior ramus of C5, C6, C7
- Innervate the serratus anterior muscle
- Suprascapular nerve:
- Arises from the C5, C6 APR of the upper trunk
- Traverse the suprascapular notch
- Innervates the supra and infraspinatus muscles
- Nerve to the subclavius derives from the upper trunk
- Nerves that innervate the arm:
- Primarily arise from the cords
- Are below the clavicle
- Lateral cord: musculocutaneous nerves lateral head of the median nerve, lateral pectoral nerve
- Medial cord; ulnar nerve, medial head of the median nerve, medial pectoral nerve, medial brachial and ante brachial cutaneous nerves
- Posterior cord: axillary, radial, thoracodorsal and subscapular nerves
- No exact end between the terminal nerves and the major nerve trunks
Anatomic Variations of the Brachial Plexus
- C4 anterior ramus sends a branch to C5 (a prefixed plexus); T1 component is small or absent
- T2 anterior ramus contributes to T1 (a post fixed plexus); T2 component large and C5 may be absent
- Pre and post fixed plexus occur in approximately 2% of patients
- If C4 contribution to C5 is large, then the caudal portion of the plexus T1 may be attenuated
- Plexus may expand from C4 through T2
Blood Supply of the Brachial Plexus
- Primary artery is the subclavian:
- Supraclavicular components of the plexus are supplied by the ascending cervical, deep cervical, and superior intercostal arteries
- Roots are supplied by branches from the vertebral artery
- Infraclavicularly: cords are supplied by the subclavian, axillary and subscapular vessels
|