Clinical examination of the patient revealed an inability to form an "O" (or "O.K."-sign) with his right thumb and index finger, i.e. weakness of flexion of the distal joints, indicating a lesion of the flexor pollicis longus and digitorum profundus muscles. No other pathological findings were noted. 

An external ultrasound exam of the right forearm had shown intact tendons, but an unspecified "edema of the flexor muscles" (images not available).

The patient presented clinically with a Kiloh-Nevin or anterior interosseus nerve syndrome. MRI confirmed the clinical findings, revealing denervation-induced edema of the flexor pollicis longus (FPL) , flexor digitorum profundus (FDP) and pronator quadratus (PQ) muscles; typically, only the radial aspect of the FDP was affected.

The median nerve is prone to entrapment syndromes, the best-known of which is carpal tunnel syndrome. Two other distinct entities are pronator teres syndrome and anterior interosseous nerve, or Kiloh-Nevin syndrome.

Etiologically, direct trauma, muscle entrapment or entrapment by fibrous bands, aberrant vessels or adjacent pathology ( e.g. callus, soft tissue masses), and idiopathic are the most common causes; primary nerves tumors are a further, rarer consideration. Differential diagnoses are other entrapment syndromes (often with distinct clinical and neurophysiological findings) and brachial neuroplexitis / Parsonage-Turner syndrome.