Updated: 12/17/2018

AIN Compressive Neuropathy

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Introduction
  •  A compressive neuropathy of the AIN that results in 
    • motor deficits only
    • no cutaneous sensory changes
  • Pathoanatomy
    • potential sites of entrapment
      • tendinous edge of deep head of pronator teres 
        • most common cause
      • FDS arcade
      • edge of lacertus fibrosus
      • accessory head of FPL (Gantzer's muscle) 
      • accessory muscle from FDS to FDP
      • abberant muscles (FCRB, palmaris profundus)
      • thrombosed ulnar radial or ulnar artery
    • patient with complete AIN palsy should have no motor function to all muscles innervated by AIN
      • patients with incompletes palsies or with Martin-Gruber anastamoses (anomalous anatomy in 15% of population where axons of AIN may cross over and connect to ulnar nerve and innervate other muscle groups)
        • present with intrinsic weakness
  • Associated conditions
    • Parsonage-Turner Syndrome  
      • bilateral AIN signs caused by viral brachial neuritis
      • be suspicious if motor loss is preceded by intense shoulder pain and viral prodrome
Anatomy
  • AIN is terminal motor branch of median nerve
    • AIN arises from the median nerve approximately 4-6 cm distal to the medial epicondyle
    • Travels between FDS and FDP initially, then between FPL and FDP, then it lies on the anterior surface of the interosseous membrane traveling with the anterior interoseous artery to pronator quadratus
    • Terminal branches innervate the joint capsule and the intercarpal, radiocarpal and distal radioulnar joints. 
  • AIN has principally motor innervation (no cutaneous sensory) and innervates 3 muscles
    • FDP (index and middle finger) 
    • FPL 
    • pronator quadratus 
Presentation
  • Symptoms
    • motor deficits only
    • no complaints of pain, unlike other median compression neuropathies (carpal tunnel syndrome and pronator syndrome)
  • Physical exam
    • weakness of grip and pinch, specifically thumb, index and middle finger flexion
    • patient unable to make OK sign (test FDP and FPL)  
    • pronator quadratus weakness shown with weak resisted pronation with elbow maximally flexed
    • distinguish from FPL attritional rupture (seen in rheumatoids) by passively flexing and extending wrist to confirm tenodesis effect in intact tendon
      • if tendons intact, passive wrist extension brings thumb IP joint and index finger DIP joint into relatively flexed position
Evaluation
  • NCV / EMG
    • helpful to make diagnosis
    • may reveal abnormalities in the FPL, FDP index and middle finger and pronator quadratus muscles
    • assess severity of neuropathy
    • may rule out more proximal lesions
Treatment
  • Nonoperative
    • observation, rest and splinting in 90° flexion 
      • indications
        • in vast majority of patients, unless clear space occupying mass
        • majority will improve with nonoperative management
      • technique
        • elbow splinting in 90 degrees of flexion (8-12 weeks)
  • Operative
    • surgical decompression of AIN 
      • indications
        • if nonoperative treatment fails after several months
        • approximately 75% success rate of surgical decompression
Techniques
  • Surgical decompression of AIN
    • technique
      • release of superficial arch of FDS and lacertus fibrosus
      • detachment of superficial head of pronator teres
      • ligation of any crossing vessels
      • removal of any space occupying lesion
Complications
  • Recurrence
 

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Questions (2)
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(OBQ07.64) A 34-year-old seamstress was diagnosed with Parsonage-Turner brachial neuritis in the right upper extremity 1 month ago. She has weak forearm pronation with the elbow in the flexed position. She denies any current sensory changes. A clinical image of her hands attempting to make a clenched fist are shown in Figure A. Which of the following most likely represents her diagnosis and treatment? Review Topic

QID: 725
FIGURES:
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1

Anterior interosseous nerve syndrome treated with observation

83%

(3043/3680)

2

Posterior interosseous nerve syndrome with release of the Arcade of Frohse

2%

(68/3680)

3

Pronator syndrome with surgical release of the lacertus fibrosis

3%

(119/3680)

4

Anterior interosseous nerve syndrome with surgical release of Gantzer's muscle

9%

(318/3680)

5

Posterior interosseous nerve syndrome treated with observation

3%

(106/3680)

L 2

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