Which nerve's innervated muscles will be paralyzed if the inferior trunk of the brachial plexus (BP) is severed?

Ulnar Nerve Paralysis Following Inferior Trunk Brachial Plexus Injury

If the inferior trunk of the brachial plexus is severed, all muscles innervated by the ulnar nerve will be paralyzed.

Anatomical Basis for This Answer

The brachial plexus is organized into roots, trunks, divisions, cords, and terminal branches. Understanding this organization is crucial to determining which muscles will be affected by specific injuries:

• The brachial plexus is formed from ventral rami of C5-T1 spinal nerves (occasionally with contributions from C4 and/or T2) 1
• These roots combine to form three trunks:
  • Superior trunk (C5-C6)
  • Middle trunk (C7)
  • Inferior trunk (C8-T1)

Functional Motor Innervation Pattern

Electrophysiological studies have demonstrated specific patterns of functional motor innervation in the brachial plexus 2:

• 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 (flexor digitorum muscles)
• T1 primarily forms the ulnar nerve (intrinsic hand muscles)

Why Ulnar Nerve is the Answer

The inferior trunk of the brachial plexus is formed by the C8 and T1 nerve roots 1, 2. When this trunk is severed:

1. The ulnar nerve, which derives primarily from the T1 contribution to the inferior trunk, will be completely affected 2
2. While the median nerve receives some contributions from C8 (part of the inferior trunk), it also receives significant contributions from more superior roots (C6-C7), meaning not all median-innervated muscles would be paralyzed
3. The radial nerve primarily derives from C7 (middle trunk), so its function would be largely preserved
4. The axillary nerve derives from C5-C6 (superior trunk), so it would be unaffected
5. The musculocutaneous nerve derives from C5-C6 (superior trunk), so it would be unaffected

Clinical Implications of Ulnar Nerve Paralysis

When the ulnar nerve is paralyzed due to inferior trunk injury, the following muscles are affected:

• Intrinsic hand muscles:

  • Most interossei muscles (palmar and dorsal)
  • Hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi)
  • Deep head of flexor pollicis brevis
  • Adductor pollicis
  • Third and fourth lumbricals

• Forearm muscles:

  • Flexor carpi ulnaris
  • Medial half of flexor digitorum profundus (affecting ring and little fingers)

Diagnostic Considerations

In cases of suspected brachial plexus injury affecting the inferior trunk:

• MRI of the brachial plexus is the imaging modality of choice 1
• Electrodiagnostic studies are essential to confirm the diagnosis and determine the extent of nerve damage 1
• Conduction block of the ulnar nerve may be observed in the acute phase 3

Common Pitfalls to Avoid

1. Mistaking radiculopathy for plexopathy: Radiculopathy presents with dermatomal distribution of symptoms, while plexopathy affects multiple peripheral nerve distributions 1, 4

2. Overlooking partial median nerve involvement: While the ulnar nerve is completely affected by inferior trunk injuries, the median nerve may be partially affected, particularly in muscles innervated by its anterior interosseous branch 5, 6

3. Confusing superior trunk injuries with inferior trunk injuries: Superior trunk injuries affect shoulder and elbow function (C5-C6), while inferior trunk injuries primarily affect hand function (C8-T1) 2

4. Assuming all brachial plexus injuries are the same: The brachial plexus is functionally divided into three groups: C5-6 (shoulder/elbow), C7 (mixed), and C8-T1 (hand) 2