Anterior Interosseous Nerve Syndrome G56.10 354.1


synonyms: AIN, anterior interosseous nerve syndrome



  • 354.1 (other lesion of median nerve)

AIN Etiology / Epidemiology / Natural History

  • vague forearm pain with loss of function
  • loss of FPL and Index finger FDP with no sensory changes
  • AIN palsy = inability to flex the thumb IP joint and  distal interphalangeal joint of the index finger because of weakenss and/or paralysis of the FDP to the index finger.
  • may be peripheral compression or neuritis

AIN Anatomy

  • Origin: branches from the the median nerve 4-6cm below the elbow.
  • Affects muscles of the deep compartment of the volar forearm.
  • AIN innervated muscles=radial 2 FDP, Flexor pollicis longus, Pronator quadratus. Provides sensation and pain to volar carpus.
  • Sources of impingement: 1-pronator quadratus fibrous bands, 2-FDP origin, 3-lacertus fibrousus, 4-Gantzer's muscle, 5-Enlarged vessels/bursa/tumor.
  • Excessory head of FPL (Gantzer's muscle) anatomic variant may cause AIN syndrome

AIN Clinical Evaluation

  • A-OK signs tests FDP and FPL. Patients demonstrate weakness in pinch and grip.
  • No active thumb IP joint and index DIP joint flexion. 
  • May note vague forearm pain.
  • No sensory deficit.
  • must rule out viral brachial neuritis(Pasonage-Turner syn) if bilateral

AIN Xray / Diagnositc Tests

  • Plain films normal
  • Electrodiagnostic studies (EMG/NCS):  establisthes diagnosis, with the affected muscles exhibiting fibrillations, sharp waves, abnormal latencies, and abnormal compound motor action potentials. EMG can be difficult due to deep location of AIN.
  • MRI: may demonstrate a specific compressive process.

AIN Classification / Treatment

  • AIN Palsy Treatment: maintain ROM, observation for 6 months as most will resolve by 6 months.  Surgical decompression if no improvement at 6-12 months.

AIN Associated Injuries / Differential Diagnosis

AIN Complications

  • Loss of AIN function

AIN Follow-up Care

  • Consider repeat EMG to evaluate for nerve recovery.
  • Clinical improvement can occur for up to 18 months.

AIN Review References

  • Miller-Breslow, J Hand Surg 15A:493:1990
  • Seki M, Nakamura H, Kono H. Neurolysis is not required for young patients with a spontaneous palsy of the anterior interosseous nerve: retrospective analysis of cases managed non-operatively. J Bone Joint Surg Br. 2006 Dec;88(12):1606-9. PubMed

  • Rodner CM, Tinsley BA, O'Malley MP. Pronator syndrome and anterior interosseous nerve syndrome. J Am Acad Orthop Surg. 2013 May;21(5):268-75. doi: 10.5435/JAAOS-21-05-268. Review. PubMed

  • Nzeako OJ, Tahmassebi R. Idiopathic Anterior Interosseous Nerve Dysfunction. J Hand Surg Am. 2015 Nov