Question: Our physician noted "anterior interosseous syndrome or focal nerve entrapment" on the patient's chart. What diagnosis should I assign? Michigan Subscriber Answer: Your most accurate choice is probably 354.1 (Other lesion of median nerve). Background: The anterior interosseous nerve (AIN) is a purely motor nerve branch from the median nerve. Anterior interosseous syndrome is a rare compression syndrome with signs and symptoms that include vague discomfort in the proximal forearm, parathesias and numbness. Excessive supination/pronation (such as that seen in a baseball pitcher or hockey/lacrosse player) seems to aggravate the AIN and contribute to the condition.