Compression of the Median Nerve:Forearm

The two types of compressive neuropathy are:
1. Pronator Syndrome
2. Anterior Interosseous Syndrome (AIS)

Median Nerve Anatomy

  • Derived from Medial & Lateral Cords of the Brachial Plexus
  • Runs lateral to the Brachial Artery until proximal to the cubital fossa
    • Crosses to lie medial to the artery
  • Passes under Lacertus Fibrosus (aponeurosis of biceps)
  • Gives off AIN 4 cm distal to the medial epicondyle
  • Passes through the two heads of Pronator Teres
  • Then under the proximal arch of FDS
  • Lies between FDS and FDP in the forearm
  • AIN lies on the interosseous membrane between FDP (index finger) and FPL
  • Anterior Interosseous Nerve (AIN) supplies the muscles it is in contact with:
    • FDP (index), FPL, PQ
  • Median nerve supplies all other flexors and thenar muscles

Pronator Syndrome

  • Compression of the median nerve
  • Not necessarily between heads of Pronator Teres
  • Pain & Paresthesia – often confused with CTS

Possible Compressive Structures

  1. Ligament of Struthers
    • Fibrous tissue between a supracondylar process and medial epicondyle
  2. Lacertus Fibrosus
  3. Pronator Teres Heads
  4. FDS Arch
  5. Space-occupying lesions anywhere on the nerve’s route (e.g., ganglia)

Clinical Findings

  • Weakness of long flexors and thenar muscles
  • More proximal pain/shoulder pain/recent viral infection (brachial neuritis)
  • Thenar paresthesia due to palmar cutaneous branch (preserved in CTS)
  • Phalen’s test negative but history similar to CTS
  • Tinel’s sign positive proximally
  • Often co-exists with Golfer’s elbow

Investigations

  • NCS – May be inconclusive but may rule out CTS
  • MRI – Detects space-occupying lesions
  • X-ray – Detects supracondylar process

Management

Non-Operative

  • Activity modification, splinting, rest

Operative

  • For failed non-operative treatment, when diagnosis confirmed, or motor dysfunction present
  • Release proximal to distal and address all compressive areas
  • Treating Golfer’s elbow may resolve the problem

Anterior Interosseous Syndrome (AIS)

  • Main difference: Weakness but no pain

Compressive Structures

  • Same as Pronator Syndrome plus:
    • Gantzer’s muscle – accessory head of FPL
    • Bicipital Bursa

Other Causes

  • Parsonage-Turner Syndrome (Brachial Neuritis)
  • Supracondylar Fracture in children
  • Compartment Syndrome
  • Space-occupying lesions

Clinical Features

  • Weak pinch
  • Pain-free
  • PQ weakness
    • Resisted pronation in maximal flexion

Investigations

  • NCS – Rarely conclusive
  • MRI – For detecting space-occupying lesions

Management

  • Often recovers spontaneously
    • Splint in elbow flexion to relax AIN
  • Decompression of AIN if non-operative management fails
  • Treat underlying cause
  • Brachial Neuritis is generally self-limiting
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