Anatomy of the ulnar nerve
 Course in the arm
The larger of the two terminal branches of the medial cord of the brachial plexus continues as the ulnar nerve, while the smaller terminal branch forms the medial root of the median nerve. In the axilla and upper arm, the ulnar nerve runs inferior and medial to the axillary artery. It passes anterior to the triceps muscle and enters the ulnar groove between the medial epicondyle of the humerus and the olecranon.
Posterior to the medial epicondyle of the humerus, the ulnar nerve is superficial and easily palpable. Before entering the forearm, the ulnar nerve sends articular branches to the elbow joint.
Course in the forearm
In the proximal forearm, the ulnar nerve passes between the two heads of the flexor carpi ulnaris. It descends deep to this muscle on the surface of the flexor digitorum profundis accompanied by the ulnar artery. It then passes medial to the artery and lateral to the flexor carpi ulnaris.
In the distal part of the forearm, the ulnar nerve becomes relatively superficial, covered only by fascia and skin. Near the pisiform bone, it passes superficial to the flexor retinaculum and ends by dividing into superficial and deep branches.
Branches
The branches of the ulnar nerve in the forearm include articular branches which pass to the elbow joint, while the nerve is in the groove between the olecranon and the medial epicondyle, muscular branches to the flexor carpi ulnaris and the medial half of the flexor digitorum profundus, a palmar cutaneous branch which arises near the middle of the forearm and supplies the skin on the medial part of the palm, and the dorsal cutaneous branch, which arises in the distal half of the forearm and passes between the ulnar and the flexor carpi ulnaris to supply the dorsal surface of the medial part of the hand. The ulnar nerve ends by dividing into a superficial and a deep branch. The superficial branch of the ulnar nerve supplies cutaneous fibres to the anterior surfaces of the medial one and one-half digits. The deep branch supplies motor fibres to the hypothenar muscles, the medial two lumbrical muscles, the adductor pollicis muscle and all of the interossei. The deep branch also supplies several joints (wrist, intercarpal, carpometacarpal and intermetacarpal). The muscles innervated by the ulnar nerve are mainly concerned with fine movements of the hand.
Movements
- Adduction and abduction of the fingers (interossei)
- Adduction of the thumb (adductor pollicis)
- Flexion and adduction at the wrist (interossei)
Ulnar nerve lesions
1. Axilla & upper arm
Causes:
Compression: tourniquets, pressure from external arm restraints.
Clinical effects:
1) Involvement of forearm muscles: Flexor carpi ulnaris and Flexor digitorum profundus
2) Associated damage to median and ulnar nerves.
2. Damage at the elbow
Causes:
Trauma: acute, chronic. The nerve may be damaged by stretcher poles hitting the elbow, or prolonged pressure from external arm restraints.
Clinical effects:
Injury results in loss of cutaneous sensation on the ulnar 1.5 fingers and the ulnar side of the hand. Paralysis of the small muscles of the hand results in clawing.
Sensory innervation of ulnar nerve:
Pressure with elbow flexed:
- Soft tissue masses: in condylar groove or cubital tunnel
- Anomalous muscle; 2% of ulnar neuropathies
- Requires surgical decompression of nerve
- Ulnar nerve prolapse
- Nerve rolls out of ulnar groove, predisposes to repetitive trauma
Clinical features:
- Pain: maximal at elbow
- Paresthesias and numbness, evoked by Tinel's sign (light tapping over the elbow)
- Ulnar-innervated hand muscles weak: especially 1st dorsal interosseus
- Forearm muscles relatively spared
- Forearm trauma: haematoma in forearm muscles e.g. haemophiliacs
- Dialysis shunts may cause ischaemia
3. Damage at the wrist and hand
Four sites:
a) Guyon's canal:
- Sensory loss and weakness in all ulnar hand muscles
- Secondary Mass (ganglion, lipoma, synovial cyst), external pressure
b) Distal to Guyon's canal:
- Weakness in all ulnar hand muscles; normal sensation
- Secondary external pressure/compression: ligament; tumour
c) Hook of hamate:
- Spares hypothenar eminence; normal sensation
- Secondary external pressure/compression: ligament, ganglion
d) Superficial terminal branch:
- Sensory loss only
- Fracture of hook of hamate (ununited); ulnar artery aneurysm
Reference
Nerve injury associated with anesthesia.
Kroll DA et al.
Anesthesiology 1990; 73: 202-7.
Abstract
The authors examined the American Society of Anesthesiologists Closed Claims Study database to define the role of nerve damage in the overall spectrum of anesthesia-related injury that leads to litigation. Of 1,541 claims reviewed, 227 (15%) were for anesthesia-related nerve injury. Ulnar neuropathy represented one-third of all nerve injuries and was the most frequent nerve injury. Less-frequent sites of nerve injury were the brachial plexus (23%) and the lumbosacral nerve roots (16%). In a large proportion of cases, the exact mechanism of injury was unclear despite evidence of intensive investigation in the claim files. Median payment for nerve damage claims involving disabling injury was $56,000, which was significantly lower than the $225,000 median payment for claims for disabling injury not involving nerve damage (P less than 0.01). The closed claims reviewers judged that the standard of care had been met significantly more often in claims involving nerve damage than in claims not involving nerve damage. The authors conclude that nerve damage is a significant source of anesthesia-related claims but that the exact mechanism of nerve injury is often unclear. In particular, ulnar nerve injuries seemed to occur without identifiable mechanism.
ArticleDate:20040414
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