Anterior Interosseous Branch of Median Nerve

- Discussion:
      - arises from median nerve, 5 cm above medial epicondyle;
      - runs on volar surface of FDP and along interosseous membrane between ulna & radius;
      - supplies FPL, lateral half of FDP, & pronator quadratus;
      - may supply sensory branches to distal ru and carpal joints;
      - variations:
              - martin gruber anastomosis occurs in 10-15% of all forearms and in half of these cases, the nerve communication arises from the AIN branch;
               - hence palsy of the AIN could lead to palsy of the hand intrinsics normally supplied to the ulnar nerve; 
     - diff dx: FDP avulsion

- Anatomy:

     - AIN is principally a motor nerve;
     - it arises from median nerve at a variable point as it passes between two heads of the pronator teres, descends vertically in front of interosseous membrane between
             FDP & FPL, supply these 2 muscles, & finally terminates in pronator quadratus near wrist joint;
    - it divides from the median nerve 4 to 6 cm below the elbow;
    - points of compression:
               - as it passes between 2 heads of pronator teres;
               - deep head of   pronator teres
               - as it descends vertically in front of interosseous membrane between FDP & FPL;
               - orgin palmaris profundis
               - gantzer's muscle (accessory head to FPL
               - origin FDS
               - origin of FCR
               - thrombosis of ulnar collateral vessels;
               - median artery

- Exam:
    - principal weakness: difficulty moving index & middle fingers;
    - weakness in flexors of ip joint of thumb (FPL) & dip joints of index and middle fingers - FDP;
    - this can be observed by observing pitch attitude of the hand;
    - normally when individual pinches something between index finger & thumb, MP & IP joints of thumb and index finger are flexed;
          - w/ nerve deficit, terminal phalanges of thumb and index finger are extended or hyperextended;
    - note: 
          - unusual innervation patterns of hand will confuse picture;
          - median nerve hand (martin gruber) anastomosis: 
          - entire hand is innervated by the median nerve 
          - cross over ulnar innervations  of FDP 
          - superficial innervations by anterior interosseous nerve; 

- EMG: needle examination is difficult because of the deep location; 

- Causes of anterior interosseous nerve compression:
         - tendinous origin of deep head of pronator teres  (most common);
         - enlarged bicipital tendon bursa may impinge AIN;
         - aberrant or thrombosed radial  artery in midforearm;
         - thrombosed ulnar artery;
         - fascial band at the origin of FDS;
         - compression w/in deep palmar compartment from aberrant accessory muscles such as FPL (gantzer's) muscle, palmaris profundus mass, or enlarged ? FCR brevis;

- Diff Dx:
    - lateral cord lesion;
    - FDP avulsion  or avulsion of index profundus tendons;
         - tendon ruptures are noted by placing digits in different positions and applying tension to the flexor tendons;
         - electrical stimulation may indicates whether muscle belly is partially denervated;
         - succinylcholine test: which may demonstrate more fasciculations of FPL if there is partial or complete denervation;
    - in pts w/ low ulnar nerve injury, some interosseous muscle intrinsic function may be maintained due to martin gruber anastomosis between AIN nerve and unlnar nerve;
    - C-8 radiculopathy:
          - rare finding;
          - the correct diagnosis is made by determining the function of the muscles innervated by the C-8 portion of the ulnar nerve;
    - Parsonage-Turner Syndrome

A simple clinical test to differentiate rupture of flexor pollicis longus and incomplete anterior interosseous paralysis.

The anterior interosseous nerve syndrome.

An anomaly of the median artery associated with the anterior interosseous nerve syndrome.

Wrist Denervation and the Anterior Interosseous Nerve: Anatomic Considerations.

Neurolysis is not required for young patients with a spontaneous palsy of the anterior interosseous nerve: retrospective analysis of cases managed non-operatively 

Brachial neuritis presenting as anterior interosseous nerve compression--implications for diagnosis and treatment: a case report

Anterior interosseous nerve syndrome: retrospective analysis of 14 patients

Original Text by Clifford R. Wheeless, III, MD.

Last updated by Clifford R. Wheeless, III, MD on Monday, April 6, 2015 9:04 pm