This is Dr. Wang, and today I'm speaking to you about the examination of the anterior interosseous nerve. This nerve is an important nerve to be able to evaluate, especially when differentiating median neuropathy at the wrist versus a more proximal median neuropathy. A little bit of anatomy, the anterior interosseous nerve is a branch of the median nerve in the proximal forearm. It's a pure motor nerve, except some sensory branches to the interosseous membrane between the radius and the ulna, as well as the wrist joint. It carries contributions from the C7, C8, and T1 nerve roots. It innervates the flexor pollicis longus, pronator quadratus, flexor digitorum profundus to the second and third digits. AIN neuropathy is usually associated with some kind of trauma, but in a non-traumatic setting can occur with brachial neuritis. When isolating a muscle for testing, it's most important to isolate that joint, so to fixate the portion of the limb or digit above and below that joint. So for testing the flexor pollicis longus, begin with a hand in a supinated position, resting comfortably on a table. You want to isolate above and below the joint that you are testing the strength of. First you want to secure your own fingers on the proximal phalanx of the thumb, and then have the patient bend the tip, the distal phalanx, against resistance. Testing the pronator quadratus, again have the patient in a comfortable seated position with their forearms supinated and the elbow in flexion. The pronator quadratus is difficult to isolate completely from the pronator teres, but having the elbow more in flexion than extension can help isolate some of those fibers. So with their elbow in flexion, you want to isolate the proximal forearm and ask the patient to pronate against resistance. Testing the flexor digitorum profundus, again have the patient with their forearm and hand in a supinated position. Isolate the middle or intermediate portion of the phalanx. Once this has been isolated and stabilized, have the patient bend their distal phalanx against resistance. A common pitfall is isolating the proximal phalange instead of the intermediate portion of the phalanx. When this is done, the flexor digitorum superficialis muscles may fire to compensate for weakness of the flexor digitorum profundus muscle. The OK sign, this sign evaluates the patient's ability to bring together the distal portion of their first and second digits, essentially their ability to bend the distal interphalangeal joint of both fingers. If this is present and the patient has an anterior interosseous neuropathy, they will usually compensate by bringing together the pads of their fingers with those distal interphalangeal joints in extension.

examination

anterior interosseous nerve

AIN

median neuropathy

motor nerve