What is the brachial plexus?

The Brachial Plexus: Anatomy and Clinical Significance

The brachial plexus is a complex network of nerves formed from the ventral rami of C5-T1 spinal nerves (with occasional contributions from C4 and/or T2) that provides motor, sensory, and sympathetic innervation to the upper extremity. 1, 2

Anatomical Organization

The brachial plexus follows a structured organization pattern:

1. Roots: Formed by ventral rami of spinal nerves C5-T1

   • The roots pass between the anterior and middle scalene muscles alongside the subclavian artery 1, 3

2. Trunks: The roots combine to form three trunks

   • Upper trunk (C5-C6)
   • Middle trunk (C7)
   • Lower trunk (C8-T1)

3. Divisions: Each trunk divides into anterior and posterior divisions

4. Cords: The divisions regroup to form three cords, named for their relationship to the axillary artery

   • Lateral cord
   • Medial cord
   • Posterior cord

5. Terminal branches: The five major terminal nerves

   • Musculocutaneous nerve (from lateral cord)
   • Median nerve (from lateral and medial cords)
   • Ulnar nerve (from medial cord)
   • Axillary nerve (from posterior cord)
   • Radial nerve (from posterior cord) 4

Functional Organization

The brachial plexus has a clear functional organization:

• C5-C6: Primarily innervate the shoulder and elbow
• C7: Innervates shoulder, elbow, wrist, and hand
• C8-T1: Primarily innervate the hand 5

Specific functional contributions include:

• C5: Primarily forms the axillary nerve (deltoid muscle)
• C6: Primarily contributes to the musculocutaneous nerve (biceps muscle)
• C7: Primarily contributes to the radial nerve (triceps muscle)
• C8: Primarily contributes to the median nerve (finger flexors)
• T1: Primarily contributes to the ulnar nerve (intrinsic hand muscles) 5

Clinical Significance

Plexopathy vs. Radiculopathy

• Plexopathy: Affects multiple peripheral nerve distributions with neuropathic pain, dysesthesia, and/or burning/electric sensations
• Radiculopathy: Pain radiates in a dermatomal distribution with sensory/motor loss reflecting spinal nerve root innervation 1, 2

Diagnostic Approach

MRI of the brachial plexus is the imaging modality of choice for suspected brachial plexus pathology 1, 2. Key imaging considerations:

• Dedicated brachial plexus MRI differs from routine neck, chest, or spine MRI
• Should include orthogonal views through the oblique planes of the plexus
• Sequences should include T1-weighted, T2-weighted, fat-saturated T2-weighted or STIR, and may include fat-saturated T1-weighted post-contrast sequences 1

Electrodiagnostic studies are essential to confirm diagnosis and determine the extent of nerve damage 2.

Common Pathologies

The brachial plexus can be affected by:

• Traumatic injuries
• Neoplastic processes (primary nerve sheath tumors or extrinsic compression)
• Inflammatory conditions (Parsonage-Turner syndrome/neuralgic amyotrophy)
• Chronic inflammatory neuropathies
• Hereditary neuropathies
• Infectious processes 1, 6

Anatomical Variations

Variations in the brachial plexus are common and can include:

• Prefixed plexus: Greater contribution from C4
• Postfixed plexus: Greater contribution from T2
• Variations in length and caliber of components 7, 4

Understanding these variations is crucial for accurate diagnosis and treatment of brachial plexus disorders, as well as for surgical approaches to the region.