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Brachial Plexus

Overview

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Structure: Roots → Trunks → Divisions → Cords → Branches

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1. Roots (Anterior Rami)

• Roots lie between anterior and middle scalene muscles, behind the subclavian artery.
• Each root receives gray rami communicantes from the sympathetic trunk, carrying postganglionic sympathetic fibers.

• Dorsal scapular nerve (C5) - rhomboid major and minor
• Long thoracic nerve (C5, C6, C7) - serratus anterior
• Small contribution to the phrenic nerve (C5)

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2. Trunks

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

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3. Divisions

• Anterior divisions → eventually supply the flexor (anterior) compartments
• Posterior divisions → eventually supply the extensor (posterior) compartments
• No peripheral nerves arise directly from divisions.

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4. Cords (named by relation to the 2nd part of the axillary artery)

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5. Terminal Branches

• Lateral pectoral nerve - pectoralis major
• Musculocutaneous nerve (C5-C7) - coracobrachialis, biceps, brachialis; becomes lateral cutaneous nerve of forearm
• Lateral root of the median nerve

• Medial pectoral nerve - pectoralis minor (and minor)
• Medial cutaneous nerve of the arm
• Medial cutaneous nerve of the forearm
• Ulnar nerve (C8-T1)
• Medial root of the median nerve

• Upper subscapular nerve - subscapularis
• Thoracodorsal nerve - latissimus dorsi
• Lower subscapular nerve - subscapularis and teres major
• Axillary nerve (C5-C6) - deltoid and teres minor
• Radial nerve (C5-T1) - all posterior compartment muscles of arm and forearm

Note: The median nerve (C5-T1) is formed from contributions of both the lateral and medial cords.

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Clinical Injuries

Supraclavicular vs. Infraclavicular Lesions

• Horner syndrome (ptosis, miosis, anhidrosis) - sympathetic chain involvement
• Winged scapula - long thoracic nerve involvement
• Weak rhomboids - dorsal scapular nerve involvement
• Normal sensory nerve action potentials (SNAPs) despite sensory loss on exam (cell bodies in dorsal root ganglia are intact)
• EMG: denervation of paraspinal muscles
• Elevated hemidiaphragm (phrenic nerve involvement)

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Common Injury Patterns

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Investigations

• CT myelography - detects nerve root avulsions (pseudomeningoceles)
• MRI - peripheral neuroma, mass lesions
• EMG/NCS - extent of denervation; must be done 4-6 weeks post-injury (Wallerian degeneration)
• SNAPs - normal in preganglionic lesions despite clinical sensory loss
• Chest X-ray (inspiratory + expiratory) - elevated hemidiaphragm = phrenic nerve involvement
• Cervical spine X-ray: transverse process fracture suggests root avulsion

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Treatment

• Neuroma excision + cable grafting (sural nerve, medial brachial/antebrachial cutaneous nerves as donors)
• Nerve transfers when insufficient proximal axons (avulsions):
  • Oberlin transfer: ulnar nerve fascicle to FCU → musculocutaneous nerve (restores elbow flexion)
  • Spinal accessory (CN XI) → suprascapular nerve (shoulder abduction)
  • Triceps branch of radial nerve → axillary nerve (Leechavengvong procedure)
• Priorities: elbow flexion > shoulder stabilization > hand function
• Best results in young patients treated within 3 months

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Obstetric Brachial Plexopathy (neonatal)

• Risk factors: high birth weight, cephalopelvic disproportion, shoulder dystocia, forceps delivery
• Good prognosis: biceps + deltoid = M1 by 2 months → complete recovery possible
• Poor prognosis: no biceps contraction by 3-6 months
• Nerve grafting results are better in infants than adults; hand intrinsic reinnervation is possible

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Brachial Plexus - Full Anatomy (Exam Prep)

The Big Picture

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Complete Schematic

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Step 1: Roots (Anterior Rami)

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Step 2: Trunks

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Step 3: Divisions

• Anterior divisions → Lateral + Medial cords (supply flexor/anterior compartments)
• Posterior divisions → Posterior cord (supply extensor/posterior compartments)

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Step 4: Cords (named by position relative to the 2nd part of the axillary artery)

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Step 5: Branches (Terminal + Collateral)

From the Lateral Cord (C5-C7)

The musculocutaneous nerve pierces coracobrachialis, runs between biceps and brachialis, and becomes the lateral cutaneous nerve of the forearm. Loss = weak elbow flexion + supination; sensory loss over lateral forearm.

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From the Medial Cord (C8-T1)

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From the Posterior Cord (C5-T1)

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The Median Nerve - Special Case

• Motor: All anterior forearm muscles except FCU and medial FDP; thenar muscles (LOAF - lateral 2 lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis)
• Sensory: Lateral 3.5 fingers on palm side; lateral palm

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Quick Cord-to-Branch Summary Table

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Injury Level - What to Expect

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Key "In the Clinic" Points for Exams

• Cervical spine injuries affect roots
• First rib fracture/severe trauma affects trunks
• Glenohumeral joint dislocation affects divisions and cords
• Axillary nerve is the most commonly injured nerve with anterior shoulder dislocation (tests: sensation over regimental badge area and deltoid power)
• Radial nerve is at risk at the spiral groove of the humerus (mid-shaft fracture = wrist drop)
• Ulnar nerve is at risk at the medial epicondyle (cubital tunnel)
• Median nerve is at risk in the carpal tunnel (CTS) - most common entrapment neuropathy