Upper limb nerve injuries

The five terminal nerves at a glance

Fracture-nerve pairs to remember: surgical neck of humerus and the axillary nerve with the posterior circumflex humeral artery; shaft of humerus and the radial nerve with profunda brachii; supracondylar fracture and the median nerve with the brachial artery; medial epicondyle and the ulnar nerve.

Cord or nerve? Combined lesions

A single nerve cannot explain weakness in two nerve territories. A stab deep in the axilla that paralyses deltoid (axillary) and the elbow, wrist and finger extensors (radial), with numbness over the lower deltoid and back of the forearm, has injured the posterior cord, which gives both nerves. Latissimus dorsi (thoracodorsal) and subscapularis would also be weak.

Axillary nerve and the quadrangular space

• The nerve leaves the posterior cord, passes through the quadrangular space (teres minor above, teres major below, long head of triceps medially, surgical neck of humerus laterally) with the posterior circumflex humeral artery, and winds round the surgical neck.
• Commonest causes of injury: anterior shoulder dislocation and surgical neck fracture; also badly placed deltoid injections and pressure from crutches.
• Quadrangular space syndrome: fibrous bands or hypertrophy in overhead athletes compress the axillary nerve and the posterior circumflex humeral artery: vague posterior shoulder pain, weak abduction, numb lateral upper arm.
• The long head of triceps separates the triangular space (medial) from the quadrangular space (lateral).

Suprascapular nerve

From the upper trunk (C5–6) to supraspinatus and infraspinatus; no cutaneous branch.

The suprascapular artery passes above the transverse ligament and is spared.

Radial nerve along its course

• Wrist extension survives a PIN lesion because ECRL (and often ECRB) is supplied by the radial nerve above its division, while ECU (PIN) is lost.
• Wrist drop also weakens grip: with the wrist flexed, the long finger flexors are slack. A man who wakes after sleeping with his arm over a chair cannot grip a glass for this reason.
• A palsy that appears only after fracture manipulation suggests the nerve is trapped between the fragments and needs exploring.
• Superficial branch of the radial nerve runs with the radial artery in the forearm.

Median nerve along its course

• Recurrent motor branch supplies abductor pollicis brevis, flexor pollicis brevis and opponens pollicis. The function lost when the median nerve is cut at the wrist is opposition of the thumb.
• The palmar cutaneous branch decides the level: numb thenar skin means a lesion above the wrist, so do not release the carpal tunnel.
• Hand of benediction on making a fist; at rest a high median lesion leaves the index finger straighter than the others.

Ulnar nerve along its course

• At the elbow (behind the medial epicondyle, cubital tunnel): FCU and the medial half of FDP are lost too, so clawing is less marked (ulnar paradox); the dorsal ulnar hand is numb as well.
• Tardy ulnar palsy: years after a childhood lateral condyle fracture, a cubitus valgus deformity slowly stretches the nerve, giving numb little finger and wasting of the first dorsal interosseous. A supracondylar malunion gives varus, not valgus.
• At the wrist (Guyon's canal): marked claw of ring and little fingers, wasting of interossei and hypothenar muscles; dorsal hand sensation preserved because the dorsal cutaneous branch leaves 5 cm above the wrist. The superficial branch supplies only palmaris brevis and palmar skin; the deep branch is purely motor.
• Martin–Gruber anastomosis (median-to-ulnar fibres in the forearm, present in a sizeable minority of people) can keep the first dorsal interosseous working after the ulnar nerve is divided at the elbow, so never rely on one muscle test.
• Tests: card test (palmar interossei), Froment sign (adductor pollicis), abduction of the index finger against resistance (first dorsal interosseous).

Remember Wasting of all small hand muscles with sparing of the lateral thenar muscles points to the ulnar nerve; wasting of all, thenar included, points to T1 or combined median and ulnar.

Quick bedside tests

Nerve injury types (Seddon)

• In the first few days, distal axons still conduct even when cut, so early studies cannot separate neurapraxia from axonotmesis. By 2 weeks, normal responses below a block mean neurapraxia (typical of Saturday night palsy).
• A Tinel sign advancing along the nerve shows regenerating axons.

Exam trap A radial nerve lesion alone cannot explain deltoid paralysis: think of the posterior cord.

Exam trap Numb thenar skin rules out pure carpal tunnel syndrome; the median nerve is caught above the wrist.

How FCPS asks about this

• Shoulder dislocation or surgical neck fracture: axillary nerve (with posterior circumflex humeral artery); regimental badge numbness.
• Axillary stab with deltoid and wrist extensor paralysis: posterior cord lesion.
• Distal-third spiral humeral fracture with wrist drop: radial nerve trapped where the lateral intermuscular septum tethers it.
• Finger drop with preserved, radially drifting wrist extension: posterior interosseous nerve; ECRL supplied above the division.
• Median symptoms with numb thenar skin and pain on resisted pronation: pronator syndrome between the heads of pronator teres.
• Ulnar palsy years after a childhood elbow fracture with valgus: cubitus valgus (lateral condyle fracture) stretching the ulnar nerve.
• Wrist drop after sleeping with arm over a chair, conduction blocked only across the groove: neurapraxia; regeneration after axonotmesis is about 1 mm per day.