Anterior Interosseous Nerve Syndrome G56.10 354.1

synonyms: AIN, anterior interosseous nerve syndrome

AIN ICD-10

• G56.10 - Other lesions of median nerve, unspecified upper limb
• G56.11 - Other lesions of median nerve, right upper limb
• G56.12 - Other lesions of median nerve, left upper limb

AIN ICD-9

• 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

• Axillary artery rupture / injury
• Burner Syndrome
• Thoracic Outlet Syndrome
• Paget Schroetter
• Parsonage-Turner Syndrome
• Carpal tunnel syndrome
• Brachial plexus injury
• FDP avulsion
• C8 Radiculopathy

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