Upper Limb - Ulnar Nerve
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Ulnar nerve
- Innervates the medial flexors below the elbow.
- The C8, T1 nerve roots supply all of the ulnar nerve-innervated muscles (so we omit the roots with each group; they are all C8, T1).
Forearm muscle group
- Comprises flexor carpi ulnaris (FCU) and flexor digitorum profundus 4 and 5 (FDP 4, 5).
- As the ulnar nerve enters the hand, it splits into the superficial sensory division, which is purely sensory except that it provides motor innervation to palmaris brevis, and the deep branch, which is purely motor and innervates muscle groups across the hand.
The deep branch innervates:
The hypothenar group:
- Comprises abductor digiti minimi (ADM), opponens digiti minimi (ODM), and flexor digiti minimi (FDM).
The intrinsic hand group
- Comprises lumbricals 3 and 4, the palmar interossei, and dorsal interossei muscles; notably, the first dorsal interosseous provides finger abduction of the second digit and is easy to test and inspect.
The thenar group
- Comprises flexor pollicis brevis (FPB) and adductor pollicis (AP).
Ulnar neuropathies
Proximal
- Cubital tunnel — the most common ulnar nerve entrapment site.
- Supracondylar fracture or medial epicondylar compression.
- All of the motor and sensory components of the ulnar nerve lie distal to the distal humerus and cubital tunnel; therefore, all of the components of the ulnar nerve are affected.
Distal
- At the wrist - Guyon's canal (aka Guyon's tunnel) formed from the palmar carpal ligament.
- The vast majority of Guyon canal entrapments only affect the deep, motor branch, which passes between the hook of the hamate and the pisiform bone.
- Proximal to Guyon's canal, the ulnar nerve derives the palmar and dorsal ulnar cutaneous nerves, which are unaffected in Guyon canal entrapment.
- Fracture of the hook of the hamate
Clinical Effect of Ulnar Neuropathies
- Weakness of the medial flexors and a more restricted clawing of the hand, so-called ulnar claw, with clawing of digits 4 and 5.
- Versus Klumpke's palsy, which affects both median and ulnar nerve distal musculature.
- 4th and 5th digit clawing forms the shape of a Pope's blessing, the so-called: Benediction sign.