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
- 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 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