Brachial Plexus

Overview

Anatomy: Layer by Layer

1. Roots (C5-T1)

• The anterior rami of C5-T1 form the roots
• They pass between the anterior and middle scalene muscles (interscalene space)
• Receive gray rami communicantes from the sympathetic trunk (postganglionic sympathetic fibers)
• Branches off the roots:
  • Dorsal scapular nerve (C5) - rhomboid major & minor
  • Long thoracic nerve (C5-C7) - serratus anterior
  • Contribution to phrenic nerve (C5)

2. Trunks (3 trunks)

• Suprascapular nerve (C5, C6) - supraspinatus, infraspinatus
• Nerve to subclavius (C5, C6) - subclavius

3. Divisions

• Each trunk splits into an anterior and a posterior division (6 divisions total)
• No peripheral nerves arise directly from divisions
• Anterior divisions - give rise to nerves of the anterior (flexor) compartments
• Posterior divisions - give rise to nerves of the posterior (extensor) compartments

4. Cords

Branches

From the Lateral Cord (C5-C7)

• Lateral pectoral nerve - pectoralis major & minor
• Musculocutaneous nerve - biceps, brachialis, coracobrachialis; sensory to lateral forearm
• Lateral root of median nerve

From the Medial Cord (C8, T1)

• Medial pectoral nerve - pectoralis major & minor
• Medial cutaneous nerve of arm - medial distal arm
• Medial cutaneous nerve of forearm - medial forearm
• Ulnar nerve (C8, T1)
• Medial root of median nerve

The median nerve is formed by the union of the lateral and medial roots, creating the classic "M" shape over the axillary artery with the musculocutaneous and ulnar nerves.

From the Posterior Cord (C5-T1)

• Superior subscapular nerve (C5, C6) - subscapularis (upper)
• Thoracodorsal nerve (C6-C8) - latissimus dorsi
• Inferior subscapular nerve (C5, C6) - subscapularis (lower), teres major
• Axillary nerve (C5, C6) - deltoid, teres minor; sensory to upper lateral arm
• Radial nerve (C5-T1) - all posterior compartment muscles of arm and forearm

Clinical Correlates

Erb-Duchenne Palsy (Upper Brachial Plexus Injury)

• Roots injured: C5, C6 (superior trunk)
• Cause: Forceful widening of the angle between the neck and shoulder (shoulder dystocia in neonates, motorcycle accidents in adults)
• Muscles affected: Deltoid, biceps, brachialis, brachioradialis, supinator
• Posture: Arm internally rotated, extended at elbow, pronated forearm - the classic "policeman receiving a tip" (waiter's tip) deformity
• Sensory loss: Lateral arm and upper lateral forearm
• Prognosis: Best prognosis; >90% of neonatal cases resolve without intervention

Klumpke's Palsy (Lower Brachial Plexus Injury)

• Roots injured: C8, T1 (inferior trunk)
• Cause: Forceful hyperabduction (e.g., grasping overhead during a fall, breech delivery with arms above head)
• Muscles affected: Intrinsic hand muscles (interossei, lumbricals, thenar, hypothenar)
• Posture: Claw hand (combined median and ulnar intrinsic muscle paralysis)
• Sensory loss: Medial forearm, hand, and inner 1.5 fingers (C8, T1)
• Horner syndrome: Ptosis, miosis (pupil constriction), enophthalmos, anhidrosis - due to damage to sympathetic fibers from T1

Total (Complete) Brachial Plexus Palsy (C5-T1)

• Worst prognosis
• Complete paralysis and anesthesia of the upper limb (except upper medial arm, supplied by T2 intercostobrachial nerve)

Pancoast Tumor

• Apical lung cancer that invades the inferior trunk (C8, T1), causing Klumpke-like palsy plus Horner syndrome

Thoracic Outlet Syndrome

• Compression of the medial, lateral, and posterior cords between the first rib and clavicle (or under pectoralis minor)

Key Memory Aids