Anterior interosseous neuropathy

● weakness of 3 muscles: FDP I & II, FPL, & pronator quadratus. No sensory loss

● loss of flexion of the distal phalanges of the thumb and index finger (pinch sign)

The anterior interosseous nerve is a purely motor branch of the median nerve that arises in the upper forearm. Anterior interosseous neuropathy (AIN) produces no sensory loss and weakness of the 3 muscles supplied by the nerve:

1. flexor digitorum profundus (FDP) I & II: flexion of distal phalanx of digits 2 & 3

2. flexor pollicis longus (FPL): flexion of distal phalanx of thumb

3. pronator quadratus (in the distal forearm): difficult to isolate clinically

Include: idiopathic, amyotrophy, ulna/radius fractures, penetrating injuries, forearm lacerations.

Patients complain of difficulty grasping small objects between the thumb and the index finger. Idiopathic cases may be preceded with forearm aching.

Sensory: no sensory loss.

Strength: digits 1, 2 & 3 are examined individually. The proximal interphalangeal joints are stabi- lized by the examiner and the patient is asked to flex the DIP. With AIN, there is no significant flexion of the DIP. Pinch sign: the patient attempts to forcefully pinch the tips of the index finger and thumb as in making an “OK” sign with AIN the terminal phalanges extend and the pulps touch instead of the tips

In addition to the physical exam, EMG may be helpful. EMG: primarily assesses pronator quadratus & flexor pollicis longus (FDP I & II is difficult on EMG because it has dual innervation with the ulnar nerve innervated portion being more superficial than the median nerve innervated portion). Important to evaluate pronator teres (abnormalities suggest involvement more proximal than forearm).

In the absence of an identifiable cause of nerve injury, expectant management is recommended for 8–12 weeks, following which exploration is indicated, which may reveal a constricting band near the origin.