Fascicular torsion in the median nerve within the distal third of the upper arm: three cases of nontraumatic anterior interosseous nerve palsy.

Three patients with nontraumatic anterior interosseous nerve palsy are presented. All patients also had paralysis of the pronator teres, flexor carpi radialis, and/or palmaris longus. One patient also had sensory disturbance and palsy of the thenar muscles. An hourglass-like constriction was seen within a 7-cm section of the nerve fascicles (2-9 cm proximal from the medial epicondyle of the humerus) in the median nerve trunk. All constrictions exhibited approximately 30 degrees of fascicular torsion. Because this nerve section is anatomically proximal to the branching point for the earlier mentioned motor branches and the anterior interosseous nerve, the nerve fascicles may have been structurally twisted before the onset of palsy. Structural abnormalities causing inflammation and edema of nerve fascicles as well as factors such as compression from surrounding small vessels may have maximized torsion, resulting in the formation of constrictions.