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Brachial Plexus Study Guide

Anatomical Foundation

The brachial plexus is formed primarily from ventral rami of spinal nerves C5-T1, with occasional contributions from C4 and/or T2, and follows a predictable organizational pattern through five sequential segments: roots, trunks, divisions, cords, and terminal branches. 1, 2

Root Level (C5-T1)

• The ventral rami of C5-C8 and T1 form the nerve roots of the brachial plexus 2, 3
• These roots pass between the anterior and middle scalene muscles alongside the subclavian artery 4
• Anatomical variations can occur, with C4 or T2 occasionally contributing to plexus formation 1, 2

Trunk Level

• Three trunks are formed from the nerve roots: 5
  • Upper trunk: formed by union of C5 and C6 roots
  • Middle trunk: continuation of C7 root alone
  • Lower trunk: formed by union of C8 and T1 roots
• Each trunk subsequently divides into anterior and posterior divisions 2, 5

Division Level

• The three trunks split into six divisions (three anterior, three posterior) 2
• Anterior divisions supply flexor compartments of the upper extremity 5
• Posterior divisions supply extensor compartments 5

Cord Level

• Three cords are formed in the infraclavicular region, traveling with the subclavian artery and vein: 2
  • Lateral cord: formed by anterior divisions of upper and middle trunks
  • Medial cord: formed by anterior division of lower trunk
  • Posterior cord: formed by all three posterior divisions
• The cords are named based on their relationship to the axillary artery 5

Terminal Branch Level

• Five major terminal nerves emerge from the cords at the lateral margin of pectoralis minor: 2, 5
  • Musculocutaneous nerve: from lateral cord
  • Median nerve: from lateral cord (lateral root) and medial cord (medial root)
  • Ulnar nerve: from medial cord
  • Axillary nerve: from posterior cord
  • Radial nerve: from posterior cord

Clinical Relevance and Pathology

Distinguishing Plexopathy from Radiculopathy

• Plexopathy manifests as neuropathic pain, weakness, and sensory loss across multiple peripheral nerve distributions, crossing multiple dermatomes 1, 4
• Radiculopathy follows a single dermatomal distribution with pain radiating in that specific pattern 1
• Complete plexopathy causes flaccid weakness and loss of tendon reflexes in regions innervated by multiple nerves 1, 4
• Clinical diagnosis requires electrodiagnostic studies for confirmation 1, 2

Preganglionic vs. Postganglionic Lesions

• Differentiating between preganglionic (nerve root) and postganglionic (plexus) injuries is critical because treatment approaches differ fundamentally 2, 4
• Preganglionic injuries involve root avulsion and have limited surgical options 6
• Postganglionic injuries may be amenable to nerve transfers or reconstruction 6

Common Etiologies of Brachial Plexopathy

• Traumatic causes: including root avulsion or rupture, significantly impacting quality of life 6
• Neoplastic causes: primary tumors (schwannomas, neurofibromas) or secondary involvement from lung cancer (Pancoast tumors) and breast cancer metastases 1, 4
• Inflammatory/immune-mediated: Parsonage-Turner syndrome (neuralgic amyotrophy), chronic inflammatory demyelinating polyradiculoneuropathy 1, 4
• Radiation-induced plexopathy: delayed complication of radiation therapy 4
• Infectious, hereditary (Charcot-Marie-Tooth), and idiopathic causes 1, 2

Diagnostic Approach

Imaging Modalities

• MRI of the brachial plexus is the imaging modality of choice due to superior soft-tissue contrast and spatial resolution 1, 4
• Dedicated brachial plexus MRI differs from routine neck or spine MRI, requiring orthogonal views through oblique planes of the plexus with T1-weighted, T2-weighted, and fat-saturated sequences 1
• MRI brachial plexus demonstrates 81% sensitivity, 91% specificity, and 88% overall accuracy for detecting plexopathy 1
• MRI provides additional diagnostic information beyond clinical evaluation and electrodiagnostic studies in 45% of patients 1
• MRI with and without IV contrast is useful for detecting and characterizing neoplastic etiologies 1

When to Consider Cervical Spine MRI

• Cervical spine MRI is complementary when clinical uncertainty exists between plexopathy and radiculopathy 1
• Cervical spine MRI is often performed first due to higher prevalence of radiculopathy-related degenerative disease 1
• However, cervical spine MRI is inferior to brachial plexus MRI for evaluating plexopathy because it does not directly visualize the plexus lateral to neural foramina 1

Electrodiagnostic Studies

• Electrodiagnostic studies are essential for confirming clinical diagnosis of plexopathy 1, 2
• These studies show abnormalities across multiple nerve distributions, distinguishing plexopathy from single nerve or root lesions 4

Anatomical Variations and Clinical Implications

Common Variations

• Variations in cord formation and branching patterns occur frequently due to the complex joining and splitting of nerve roots 7
• Prefixed plexus (with C4 contribution) or postfixed plexus (with T2 contribution) can occur 8
• Variations in trunk formation, cord anatomy, and terminal branch patterns have been documented 7, 8

Clinical Significance of Variations

• Awareness of anatomical variations is critical for surgical procedures, nerve blocks, and imaging interpretation 8
• Variations affect the success and safety of interscalene blocks for regional anesthesia 8
• Anomalous anatomy increases risk of iatrogenic injury during neck and shoulder surgery 7
• Understanding variations helps explain atypical clinical presentations and imaging findings 3

Key Learning Points for Clinical Practice

Diagnostic Pitfalls to Avoid

• Do not confuse plexopathy with radiculopathy: plexopathy crosses multiple dermatomes while radiculopathy follows a single dermatomal pattern 1, 4
• Always distinguish preganglionic from postganglionic lesions as this fundamentally changes treatment options 2, 4
• Standard cervical spine MRI is inadequate for evaluating plexopathy; dedicated brachial plexus MRI with specialized sequences is required 1
• CT has limited utility due to poor soft-tissue contrast compared to MRI 1

Treatment Considerations

• Nerve transfers can bypass damaged areas in upper brachial plexus injuries, allowing faster recovery of critical arm functions 6
• Without treatment, patients with upper brachial plexus lesions may lose motor function distal to the injury due to slow nerve regeneration 6
• Treatment approach differs significantly between preganglionic injuries (limited options) and postganglionic injuries (nerve transfer candidates) 6