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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

References