Hand Series: Peripheral Nerve Injuries - The Essentials

Peripheral nerve injuries represent a high-yield topic for the FRACS exam because they test three core domains simultaneously: detailed anatomy, precise clinical examination, and complex surgical decision-making. Whether it's a "Saturday Night Palsy" or a catastrophic brachial plexus injury, the principles remain the same. This guide breaks down the essentials from the basic science of the neuron to the nuances of tendon transfers.

Visual Element: A cross-sectional diagram of a peripheral nerve showing the hierarchy: Epineurium, Perineurium (blood-nerve barrier), Endoneurium, and the Axon/Myelin sheath.

Nerve Anatomy Essentials

Microanatomy

Understanding the structure is vital for understanding repair techniques.

1. Epineurium: The outer sheath. Contains the vasa nervorum (blood supply). This is what you hold with forceps during repair.
2. Perineurium: Surrounds fascicles (bundles of axons). It provides the blood-nerve barrier and high tensile strength.
3. Endoneurium: The delicate connective tissue surrounding individual axons.
4. Axon & Schwann Cell: The functional unit.

Fibre Types and Prognosis

Not all fibers recover at the same rate.

• A-alpha (Motor/Proprioception): Large, myelinated. Most susceptible to compression (ischemia).
• A-beta (Touch): Large, myelinated.
• A-delta (Pain/Temp): Small, myelinated.
• C fibers (Pain): Unmyelinated. Most resistant to hypoxia.

Wallerian Degeneration

When a nerve is cut, the distal segment dies.

1. Distal Degeneration: Occurs within 24-48 hours.
2. Macrophage Clearance: Debris is cleared over 2-3 weeks.
3. Regeneration: The proximal axon sprouts and seeks the distal endoneurial tube.
4. Rate: 1mm per day (approx. 1 inch per month). This is the "Rule of 1".

Classification Systems

Seddon (1943) vs Sunderland (1951)

You must know both and how they correlate.

The Three Major Nerves: Clinical Patterns

1. Median Nerve ("Eye of the Hand")

• Sensation: Thumb, Index, Middle, Radial half of Ring finger (volar aspect).
• Motor (LOAF): Lumbricals (1,2), Opponens pollicis, Abductor pollicis brevis (APB), Flexor pollicis brevis (superficial).

Injury Levels:

• Low (Wrist): Loss of sensation + Thenar wasting. "Ape Hand Deformity" (loss of opposition).
• High (Elbow): All of the above PLUS loss of FPL (IP flexion thumb), FDP (index/middle), FCR (wrist flexion), Pronator Teres. "Hand of Benediction" (cannot make a fist).

2. Ulnar Nerve ("The Power Nerve")

• Sensation: Little finger, Ulnar half of Ring finger.
• Motor: All Interossei, Lumbricals (3,4), Adductor Pollicis, Hypothenar muscles, FPB (deep).

Injury Levels:

• Low (Wrist): Loss of sensation + Intrinsic loss. Marked Clawing.
• High (Elbow): Above PLUS loss of FDP (ring/little) and FCU. Less Clawing.

The Ulnar Paradox: "The higher the lesion, the better the hand looks."

• Explanation: In a high lesion, the FDP is paralyzed, so the fingers do not curl (claw). In a low lesion, the FDP works but the intrinsics don't, creating a muscle imbalance that causes severe clawing.

3. Radial Nerve ("The Extensor Nerve")

• Sensation: Dorsal webspace (First Web).
• Motor: All extensors of arm, wrist, and fingers.

Injury Levels:

• High (Spiral Groove/Humerus): Wrist Drop + Finger Drop + Sensory loss. Triceps usually spared (innervated higher).
• Low (PIN - Posterior Interosseous Nerve): Finger Drop + Thumb Drop. Wrist Extension Preserved (ECRL branches above PIN). No sensory loss (Superficial Radial is separate).

Examination Approach

1. Look: Wasting (First dorsal interosseous = Ulnar; Thenar = Median), Scars, Dry skin (anhidrosis = autonomic loss), Clawing.
2. Feel: Pulses (neurovascular bundles often injured together).
3. Move (Power):
   • Median: "Point your thumb to the ceiling" (APB - best test).
   • Ulnar: "Cross your fingers" or "Hold paper between ring/little finger." Froment's Sign: When pinching paper, thumb IP flexes (FPL compensates for weak Adductor Pollicis).
   • Radial: "Thumbs up" (EPL) and "Wrist up".
4. Sense: Two-point discrimination. Normal is <6mm. <15mm is functional.

Management Principles

Timing of Repair

• Clean/Sharp Laceration: Primary Repair (within days). Best outcome.
• Blunt/Crush/Avulsion: Delayed Repair (3 weeks). Allows the zone of injury to demarcate (scar vs healthy tissue) so you don't sew dead nerve to dead nerve.
• Closed Injury (e.g., Humeral shaft fracture): Observation. 85% of radial nerve palsies recover spontaneously. Monitor with EMG at 6-12 weeks.

Surgical Ladder

1. Primary Direct Repair: Tension-free. Epineural sutures (9-0 Nylon).
2. Nerve Graft: If there is a gap. Gold standard is Sural Nerve autograft. Cable graft technique.
3. Nerve Conduit: For small gaps (<2cm) in small diameter nerves. Vein or synthetic.
4. Nerve Transfer: "Robbing Peter to pay Paul." Taking a redundant motor branch (e.g., branch to pronator teres) and plugging it into a critical denervated nerve (e.g., ulnar motor branch) close to the target muscle. Faster recovery as distance is short.
5. Tendon Transfer: If nerve recovery fails.

Visual Element: Schematic of a Nerve Transfer (e.g., Oberlin transfer) vs a Nerve Graft, showing the difference in regeneration distance to the target muscle.

Tendon Transfers: The Bailout

When the nerve is gone forever, move tendons to restore function.

• Radial Nerve Palsy (Jones Transfer):
  • Pronator Teres → ECRB (Restores wrist extension).
  • FCR → EDC (Restores finger extension).
  • Palmaris Longus → EPL (Restores thumb extension).
• Ulnar Nerve Palsy:
  • "Lasso" procedures (FDS passed through sheath) to prevent clawing.
• Median Nerve Palsy:
  • Opponensplasty (e.g., EIP to APB) to restore opposition.

Evidence Corner

• Oberlin Transfer: Transferring a fascicle of the Ulnar nerve (to FCU) to the Musculocutaneous nerve (biceps). High success rate for restoring elbow flexion in brachial plexus injuries.
• Early vs Late: Outcomes degrade significantly if repair is >6 months post-injury due to degradation of the motor endplates in the muscle. After 12-18 months, nerve repair to muscle is futile (muscle is fibrosed).

Summary Checklist for Exams

• Can you draw the brachial plexus? (The ultimate anatomy question).
• Can you differentiate High vs Low palsies for all 3 nerves?
• Do you know the order of recovery (Pain -> Touch -> Motor)?
• Can you name one tendon transfer for each nerve palsy?

Related Topics:

• Brachial Plexus Injuries
• Carpal Tunnel Syndrome
• Cubital Tunnel Syndrome
• Tendon Transfer Principles