The lesion of anterior interosseous nerve syndrome is not in the forearm

The etiology of anterior interosseous nerve syndrome (AINS) is controversial, and ranges from compressive neuropathy in the forearm to spontaneous neuritis. Since the early 1990u0027s, several series that report hourglass constriction (HGC) of the nerves in the upper extremity, including the anterior interosseous nerve (AIN) have been reported. The link between HGC and AINS has not been definitively resolved. 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. This is a retrospective radiologic review of a consecutive series of 42 limbs in 40 patients with AINS in 2 academic centers in the United States and Hungary. The diagnosis of AIN palsy was strictly defined by complete clinical and electromyographic palsy of one or more muscles of the AIN. Each patient was evaluated with MRI, ultrasound or both by an expert neurodiagnostic imaging specialist. Five patients with recalcitrant disease were treated with interfascicular epi- and peri-neurolysis for failure to recover clinically or electrically after 12.4 months (range, 10 to 16 months). Twenty-nine 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. In 26 cases an ultrasound was performed and in 20 cases a MRI was performed. At least one hourglass constriction was identified in 25 patients, with a total of 42 constrictions reported. A fascicular swelling assessed with ultrasound or hyperintensity of the posteromedial fascicular bundle of the median nerve on MRI imaging was reported in eleven patients. In 71% of cases, the constriction was proximal to the medial epicondyle (mean 1.6 cm, from 0 to 18 cm) and in 29% the constriction was distal to the medial epicondyle (mean 1.3cm, from 0 to 3.5 cm). No extrinsic compression was identified in the forearm. Surgical exploration in 5 patients precisely corroborated imaging findings. 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 NA [2] , [3] . These data of 42 cases of AINS confirms that the disease is an intra-neural affectation of the median nerve (neuralgia amyotrophy) and not a compressive or inflammatory lesion of the AIN in the forearm. Intraneural disease of the posteromedial fascicular group of the median nerve is the primary pathology of AINS. We recommend that patients with AINS should undergo advanced imaging of the median nerve above and below the elbow by a specialist trained in imaging of the peripheral nerve.