PIN palsy

Anatomy of Posterior Interosseous Nerve (PIN)

• It is a branch of the radial nerve.
• It passes between the two heads of the supinator and comes to lie intimately with the proximal radius. It then passes over the abductor pollicis longus muscle origin to travel along the posterior interosseous membrane. At the wrist, it passes through the 4th extensor compartment to terminate on the dorsal wrist capsule.

• At the distal end of the supinator (which is innervated by PIN), it bifurcates into two major branches:

  • Recurrent branch or Short branch, which provides motor innervation to
    • Extensor digitorum communis (EDC)
    • Extensor digiti minimi (EDM)
    • Extensor carpi ulnaris (ECU)

   

  • Descending branch or Long branch, which provides motor innervation to

    • Abductor pollicis longus (APL)
    • Extensor pollicis brevis (EPB)
    • Extenspr pollicis longus (EPL)
    • Extensor indicis proprius (EIP); and
    • Sensory innervation to the dorsal wrist capsule

• ECRB is innervated by the radial nerve in 50% of cases and by the PIN in 50% of cases.

Aetiology of PIN Palsy

There are two major groups of patients:

1. Traumatic
   • Fractures around the elbow
   • Lacerations with direct nerve injuries
   • Direct contusions from blunt trauma 

    

2. Atraumatic
   • Entrapment neuropathy
   • Neuralgic amyotrophy (Parsonage-Turner Syndrome)
   • Hourglass constrictions of the PIN
   • Space-occupying lesions (lipoma, ganglion, inflammatory arthropathy)

Among the atraumatic cases, there are certain clinical features that might point towards a particular aetiology:

Presentation

In traumatic situations, the combination of elbow trauma with the following signs should alert the clinician to the possibility of injuries to the PIN:

• weak wrist extension with radial deviation
• extension loss at the metacarpophalangeal joints of all the fingers and thumb
• weak abduction of the thumb

In atraumatic cases, patients may present with

• Aching pain over the proximal dorsoradial forearm
• Tenderness over the radial tunnel
• Motor weakness/paralysis as outlined above
• The presentation may be variable or progressive

Investigations

• Radiographs - necessary in elbow trauma but less useful in atraumatic cases
• MRI - utilised to exclude any space-occupying lesions, may show denervated changes in the muscles
• Neurophysiology - performed after 2-3 weeks of onset of palsy

Treatment

• In traumatic cases,

  • if there is an open wound over the course of the PIN with dysfunction, formal exploration and repair of the nerve are indicated.
  • in elbow fracture-dislocations, the initial treatment should be aimed at restoring normal bony architecture of the joint. In closed injuries (such as elbow dislocation treated with closed reduction), a period of observation is considered. If there is no clinical recovery by 8 weeks, exploration of the nerve is recommended. In more severe trauma or when open surgery on the proximal radius is contemplated, formal exposure of the PIN is recommended.

• In atraumatic cases,

  • Nonoperative

    • Activity modification
    • Rest
    • Painkillers
    • Splinting for the hand

   

  • Operative

    • Surgical decompression (neurolysis) is recommended if there is no clinical recovery by 8 weeks and more expediently if
      • there is progressive weakness
      • there is a space-occupying lesion 

       

    • For delayed presentation, tendon transfer is a reconstructive option to restore fingers and thumb extension.