Anterior interosseous nerve syndrome is an uncommon entrapment neuropathy of the anterior interosseous branch of the median nerve below the elbow that is caused by compression of the anterior interosseous branch of the median nerve by a variety of anatomic abnormalities including (1) the accessory head of flexor pollicis longus (Gantzer muscle); (2) the deep head of the pronator teres muscle; (3) a fibrous arch in the flexor digitorum superficialis of the middle finger; (4) posttraumatic hematoma; (5) soft tissue masses, for example, lipomas; (6) prolonged external compression from crush injuries or tourniquet; (7) fractures of the radius; (8) aberrant origin of the flexor carpi brevis radialis muscle; (9) aberrant origin of the palmaris profundus muscle; (10) an inflammatory neuropathy analogous to Parsonage-Turner syndrome; and (11) thrombosis of the ulnar collateral vessels (Table 62.1; Fig. 62.1). These anatomic abnormalities may work alone, together, or in conjunction with other entrapments of the fibers that ultimately comprise the anterior interosseous nerve as it passes from the neck through the brachial plexus and to the forearm toward its most common site of entrapment, the carpal tunnel.

Patients suffering from anterior interosseous nerve syndrome will experience a dull, aching pain with movementinduced dysesthesias radiating from the site of compression both proximally to the elbow and distally to the anterior forearm. Patients suffering from anterior interosseous nerve syndrome frequently complain of a heavy or tired sensation in the muscles of the forearm and clumsiness of the affected extremity. Unlike carpal tunnel syndrome, nighttime symptomatology is uncommon. The onset of anterior interosseous nerve syndrome can be acute following twisting injuries to the elbow or as a result of direct trauma to the area overlying the anterior interosseous branch of the median nerve. More commonly, the onset of anterior interosseous nerve syndrome is insidious and is usually the result of misuse overuse of the elbow joint and proximal forearm from repetitive activities like chipping ice or shoveling snow. If this entrapment neuropathy is not treated, pain and functional disability may become more severe, and ultimately, permanent weakness of the deep muscles of the forearm and hand may occur.

Physical findings in patients suffering from anterior interosseous nerve syndrome will exhibit weakness of the intrinsic muscles of the forearm and hand innervated by the anterior interosseous branch of the median nerve. The anterior interosseous branch median nerve will often be tender to palpation at the site of entrapment, and occasionally, a Tinel sign 6 to 8 cm below the elbow may be present. Patients suffering from anterior interosseous nerve syndrome frequently will exhibit positive functional muscle testing that can help localize the site of median nerve entrapment to the anterior interosseous branch of the median nerve. Patients suffering from anterior interosseous nerve syndrome will exhibit a positive Playboy and Spinner sign. The Playboy bunny sign is positive when the patient is unable to form the A-OK sign with his or her thumb and index finger due to weakness of the flexor pollicis longus and the flexor digitorum profundus muscles causing extension of the distal interphalangeal joint and thumb interphalangeal joint to form the elongated nose of a Playboy bunny (Fig. 62.2). The Spinner sign is positive when the index finger of the affected extremity cannot achieve full flexion to the palmar crease as the middle, ring, and little fingers can when making the thumbs up sign (Fig. 62.3).

Electromyography and nerve conduction velocity testing will aid in determining the exact location of median nerve entrapment in patients who present clinically with

signs and symptoms consistent with anterior interosseous nerve syndrome. These electrodiagnostic tests will also help distinguish the various causes of anterior interosseous nerve syndrome from isolated entrapment of other motor branches of the median nerve. Furthermore, it should be remembered that cervical radiculopathy and median nerve entrapment may coexist as the so-called “double crush” syndrome. The double crush syndrome is seen most commonly with median nerve entrapment at the wrist or with carpal tunnel syndrome but has been reported with the median nerve.