Anterior interosseous neuropathy

Background

• Anterior interosseous nerve (C5-T1) is a branch of the median nerve that supplies motor innervation to the deep forearm muscles
• Compression or injury can cause an isolated AIN palsy
• Brachial plexopathy or cervical spine pathologies may present with combined median nerve palsies

Causes

• Peripheral neuropathy
  • Entrapment between forearm muscles or tendons
  • FPL tendon rupture (usually with severe rheumatic disease)
  • Trauma
    • Forearm fracture
    • Pediatric - supracondylar fracture
  • Space occupying lesions
• Post-viral neuritis
  • Parsonage-Turner syndrome
• Iatrogenic

Clinical Features

• May have forearm pain although not as common in other median nerve compression syndromes
• Weakness of finger flexion (FDL), thumb flexion (FPL), and pronation (pronator quadratus)
• Inability to hold a piece of paper between index and thumb fingertips (can't make "OK" sign)
• No sensory loss

Differential Diagnosis

Upper extremity peripheral nerve syndromes

Median Nerve Syndromes

• Carpal tunnel syndrome
• Pronator teres syndrome
• Anterior interosseous neuropathy

Ulnar Nerve Syndromes

• Ulnar neuropathy

Radial Nerve Syndromes

• Radial neuropathy at the spiral groove (ie. "Saturday night palsy")
• Posterior interosseous neuropathy

Proximal Neuropathies

• Suprascapular neuropathy
• Long thoracic neuropathy
• Axillary neuropathy
• Spinal accessory neuropathy
• Musculocutaneous neuropathy

Other

• Brachial plexopathy
  • Parsonage-Turner syndrome
• Cervical radiculopathy

Evaluation

• Clinical diagnosis
• Plain films
• Outpatient EMG or MRI

Management

• Nonoperative: rest, activity modification, NSAIDs, splinting
• Operative decompression: compressive masses

Disposition

• Discharge with orthopedic follow up

References

1. Dang AC et al. Unusual Compression Neuropathies of the Forearm, Part 2: Median Nerve. JHS. 2009;34A:1915-1920.