October 2003 Case of the Month

History: 26 year old professional baseball player with increased arm weakness progressing over several weeks.

Compiled by: Steven R. Nudo, MD

Technical Factors: Long and short axis fat and water weighted sequences were performed.

Findings: Sagittal STIR and T1 sequences demonstrate increased signal and atrophic change involving the flexor digitorum profundus (blue arrows).

Findings: Coronal T2 weighted images demonstrate increased signal and atrophic change involving portions of the flexor pollicis longus (blue arrows).

Findings: Axial STIR sequence demonstrates abnormal hyperintense signal within the anterior interosseous nerve (yellow arrow). Increased signal is also demonstrated within portions of the flexor pollicis longus and flexor digitorum profundus (red arrow).

Findings: Axial T1 weighted sequence demonstrates increased caliber of the anterior interosseous nerve (yellow arrow). The Tunnel of Kiloh-Nevin is labeled with the (red arrow).

DIAGNOSIS: Syndrome of Kiloh-Nevin (anterior interosseous nerve syndrome) DISCUSSION: The anterior interosseous nerve is a branch of the median nerve that originates in the cubital region and is exclusively a motor nerve. The nerve courses along the interosseous membrane between the flexor pollicis longus laterally and the flexor digitorum profundus medially. The nerve ends in the pronator quadratus and serves as motor innervation for these three muscles. The Syndrome of Kiloh-Nevin, also known as anterior interosseous nerve syndrome, is a rare entity that may affect athletes involved in upper extremity sports. Compression of the nerve may occur following aggressive forearm exercise. Compression by fibrous bands of the deep head of the flexor digitorum superficialis or pronator teres, aberrant or anomalous vessels/muscles, or enlarged bicipital-radial bursa are also potential etiologies. Of these, compression by the flexor digitorum superficialis is the most common. The typical clinical presentation is motor weakness. Weakness in the distribution of the flexor pollicis longus and flexor digitorum profundus may involve the 2nd and 3rd digits. The classic physical finding is the inability to form the "OK" sign with the thumb and 2nd digit. Also, inability to flex the wrist or clench the fist may be seen. MRI findings include increased T2 or STIR signal and/or atrophy within the flexor pollicis longus, pronator quadratus, or flexor digitorum profundus muscles. Abnormal signal may be present within the anterior interosseous nerve itself and there may be visible encroachment upon the tunnel of Kiloh-Nevin. Treatment consists of conservative therapy for 6 months with anti-inflammatory medication followed by surgical decompression if initial therapy is unsuccessful. REFERENCES: 1. Pomeranz, Stephen J: Gamuts and Pearls in MRI, 2nd Ed. MRI-EFI Publications 1993 2. Nerve Entrapment in Athletes. Aldridge JW - Clinics of Sports Medicine - 01-Jan-2001; 20(1):95-122 3. Nerve Injuries of the Elbow. Wrist, and Hand in Athletes. Izzi J - Clinics of Sports Medicine - 01 - Jan - 2001; 20(1): 203-17 4. Kaplan, Helms, Dussault, Anderson, and Major: Musculoskeletal MRI. PA: Saunders, 2001.