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Anterior Interosseous Nerve Syndrome Redefined
Scott W Wolfe, MD1, Darryl B Sneag, MD2, Ogonna K. Nwawka, MD2, Zsuzsanna Aranyi, MD, Ph.D.3, Esther Zusstone, BS2, Steve K Lee, MD4 and Joseph H Feinberg, MD2, (1)Hand and Upper Extremity Service, Hospital for Special Surgery, New York, NY, (2)Hospital for Special Surgery, New York, NY, (3)Semmelweis Egyptem, Budapest, Hungary, (4)Orthopedic Surgery, Hospital for Special Surgery, New York, NY

Introduction

The etiology of anterior interosseous nerve syndrome (AINS) is controversial, and ranges from compressive neuropathy in the forearm to immune-mediated neuritis. Several authors have reported hourglass constriction (HGC) of upper extremity nerves, including the AIN and posterior interosseous (PIN) nerves. A link between HGC and AINS has not been established. We hypothesize that AINS is not a compressive or inflammatory lesion of the forearm, but rather a proximal intraneural disease of the postero-medial fascicle of the median nerve.



Methods

This is an IRB-approved dual-site retrospective study of a consecutive series of 45 patients with AINS in 2 academic centers in the United States and Hungary. The diagnosis of AINS was strictly defined by clinical and electromyographic palsy of one or more muscles of the AIN. Each patient was evaluated with MRI, ultrasound or both by a neurodiagnostic imaging specialist. Seven patients with recalcitrant disease were treated with micro epi- and peri-neurolysis for failure to recover clinically or electrically after 12.4 months (range, 10 to 16 months).



Results

29 patients had a characteristic pain prodrome of neuralgia amyotrophy (NA) that immediately preceded the onset of motor palsy. In 15 cases, the AIN was associated with palsy of additional median-innervated muscles, including the flexor carpi radialis, pronator, and/or palmaris longus. MRI identified HGCs of the nerves in 22 of 22 limbs studied. Ultrasound identified swelling or HGC's of the anterior interosseous fascicle in 20 of 23 limbs. In all cases, the constriction was proximal to the ulno-humeral joint line (mean 5.4cm, from 0 to 18cm). No extrinsic site of compression was identified in the forearm. Surgical exploration in 7 patients precisely corroborated imaging findings. 5 of the 7 patients are greater than six months from surgery and have recovered MRC grade 4 strength of FPL flexion and EDX confirmation of re-innervation.



Discussion

Preoperative evaluation of AINS requires electromyographic confirmation, but advanced imaging has been discouraged (1). Recent international studies have confirmed the value of MRI and ultrasonography to identify abnormalities of nerves affected by neuralgic amyotrophy (NA) (2, 3). These data in 45 patients suggest that AINS is a subtype of NA, and confirm that AINS is a non-compressive neuropathy characterized by fascicular constrictions of the median nerve in the arm, frequently also involving pronator teres/flexor carpi radialis fascicles. Microsurgical perineurolysis of hourglass constrictions of the median nerve above the elbow is an effective treatment for AINS.


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