By Chris Faubel, MD —
I’ve made it the “A-OK” sign, instead of just the “OK” sign, because the “A” will help to remind you about its innervation…the Anterior interosseous nerve (AIN)
- the AIN is a pure motor branch of the median nerve; just like the posterior interosseous nerve is a pure motor continuation of the deep branch of the radial nerve.
So which muscles allow you to make this A-OK sign, and what is the clinical significance?
First you must think about which actions are being performed to make this sign:
- the forearm is pronated = pronator quadratus
- the distal phalanx of the thumb is flexed = flexor pollicis longus
- the index/middle fingers are flexed at the DIP = lateral half of the flexor digitorum profundus
Now, what happens if the anterior interosseous muscle is damaged? Inability to pronate the hand as much (but still have the pronator teres). Inability to flex the distal phalanx of the thumb, and the DIJs of the index and middle fingers.
Clinical significance:
- Anterior interosseous syndrome
- Hx: Acute poorly localized proximal forearm pain, and pain in the cubital fossa and elbow
- PE: Difficulty picking up a coin from a flat surface; weakness in the A-OK muscles only;no sensory deficits
- Note: since the Martin-Gruber anastomosis frequently goes from the AIN to the ulnar nerve, there may also be some weakness (or EMG denervation) in some ulnar-innervated muscles of the hand
- EDx:
- Fibs/PSWs in the A-OK muscles (and possibly some ulnar muscles if the AIN is part of the MG anastomosis).
- Sensory NCSs are normal, and therefore r/o a pronator teres syndrome of more proximal median nerve injury
- need to r/o Parsonage-Turner (neuralgic amyotrophy) by looking at other muscles from other brachial plexus nerves
- Trmt:
- physical therapy for massage, stretch, nerve mobilization
- injection of local anesthetic into suspected area of compression
- surgical decompression
- Note: the AIN branches off from the median nerve AFTER traveling through the pronator teres
- therefore, the AIN muscles will be affected in pronator teres syndrome
