Working Diagnosis:
Early anterior interosseous neuropathy

Treatment:
Patient encouraged to stretch and ice forearm as needed. Patient also referred to PT for soft tissue massage and/or electric stimulation for symptomatic relief.

Outcome:
Patient reported improved symptoms within 6 weeks.

Author's Comments:
Anterior interosseous nerve syndrome (AINS) is estimated to cause less than 1% of upper extremity neuropathies. Given that the AIN contains no sensory fibers, AINS is a pure motor neuropathy. However, patients such as the above may report "dull" arm pain. Clinical exams sensitive to AINS include the "O.K. test" and the "pinch test", both of which may be abnormal because flexion at the IP joint of the thumb and the DIP joint of the index finger is impaired. Complete AINS results in weakness in the FPL, radial part of the FDP and pronator quadratus muscles. Of note, the clinical findings of AINS can be confounded in up to 25% of patients due to presence of a Martin-Gruber anastomosis where the AIN gives off branches to the ulnar nerve. This results in an atypical motor innervation pattern of the forearm and hand and subsequently effaces clinical symptoms.

The most common etiology of AINS is thought to be compression of the nerve between the pronator teres and proximal FDS, although up to 52% of the population has an anomalous head of the FPL muscle (Gantzer muscle) that potentially contributes to compression. Alternatively, brachial plexus neuritis, idiopathic or secondary to viral infection, has been associated with AINS.

Diagnosis is based on identifying clinical symptoms of forearm and hand weakness as well as imaging with ultrasound and MRI. These modalities can visualize direct causes such as primary nerve sheath tumors, ganglion cysts, osseous spurs and anatomical variants (e.g. Gantzer muscle), or indirect evidence of AINS such as pathological muscle signal patterns. Treatment of AINS is typically conservative with rest and NSARs; corticosteroids have been used in select cases and if symptoms progress, surgery may be considered.

Editor's Comments:
As the author commented AINS presents with mainly motor complaints and dull pain. Medial epicondylitis a much more common diagnosis can present in a similar distribution. It is important to consider diagnoses such as AINS if patients have medial epicondylitis that is not improved with traditional treatments. Rupture of the flexor policis longus tendon can also resemble incomplete anterior interosseous syndrome. It commonly presents with loss of flexion of thumb IP in the setting of a patient with history of rheumatoid arthritis.

References:
1. Berger, R.A., & Weiss, A. P. (2004). Hand surgery. Philadelphia: Lippincott Williams & Wilkins.

2. Miller, T.T., & Reinus, W.R. (2010). Nerve Entrapment Syndromes of the Elbow, Forearm, and Wrist. American Journal of Roentgenology, 195(3), 585-594.

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