AIN Compressive Neuropathy

Author: Mark Vitale

Topic updated on 03/14/13 6:47pm

Introduction

A compressive neuropathy of the AIN that results in
- motor deficits only
- no 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)
  - thrombosed ulnar radial or ulnar artery
- patient with complete AIN palsy should have no motor function to all 4 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 to innervate other muscle groups) may present differently
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 cm distal to the medial epicondyle where it passes into the anterior interosseous membrane to sites of innervation
AIN has motor innervation only (no sensory) and innervates 4 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

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

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Qbank (1 Questions)

TAG

(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? Topic

Review Topic

FIGURES: A

1. Anterior interosseous nerve syndrome treated with observation
2. Posterior interosseous nerve syndrome with release of the Arcade of Frohse
3. Pronator syndrome with surgical release of the lacertus fibrosis
4. Anterior interosseous nerve syndrome with surgical release of Gantzer's muscle
5. Posterior interosseous nerve syndrome treated with observation

PREFERRED RESPONSE ▶

DISCUSSION: This patient presents with anterior interosseous nerve (AIN) syndrome and is often seen in conjunction with brachial neuritis (Parsonage-Turner Syndrome).

AIN sydrome leads to motor palsies of the flexor pollicis longus and the two radial profundus tendons leading to the clincal image shown in Figure A. The pronator quadratus is also involved and can be tested with the elbow held in a flexed position to neutralize the humeral head of the pronator teres muscle. No sensory changes occur and Electromyographic (EMG) and nerveconduction (NCV)studies are often helpful in establishing the diagnosis. Anterior interosseous nerve syndrome usually resolves with time, particularly if the lesion is secondary to neuritis. Observation for 3 to 6 months with splinting at 90 degrees is favored before surgical treatment.

REFERENCES:

1. Green DP, Hotchkiss RN, Pederson WC (eds): Entrapment and compression neuropathy, in Green’s Operative Hand Surgery, ed 4. Phila, PA, 1999. pp 1404-1447

Question Authors:

Michael Hughes MD, Ben Taylor MD,

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Videos

Flexor Digitorum Superficialis to Anterior Interosseous Nerve Transfer - Dr. Mackinnon

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Groups

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Evidence & References Show References

Textbooks

Review of Orthopaedics, 6th Edition, Mark D. Miller MD, Stephen R. Thompson MBBS MEd FRCSC, Jennifer Hart MPAS PA-C ATC, an imprint of Elsevier, Philadelphia, Copyright 2012
AAOS Comprehensive Orthopaedic Review, Jay R. Leiberman. Published by American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2009
Orthopaedic Knowledge Update 10, John M Flyn. Published by American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2011
Hoppenfeld SP. Surgical Exposures in Orthopaedics: The Anatomic Approach. Lipponcott, Williams, and Wilkins, Philadelphia, PA, Copyright 2009
Orthopaedic In-training Examination (OITE) Questions 2004-2012, American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2004-2012
Self-Assessment Examination (SAE) Questions 2004-2012, American Academy of Orthopaedic Surgeons, Rosemont IL. Copyright 2004-2012

Undefined

Green DP, Hotchkiss RN, Pederson WC (eds): Entrapment and compression neuropathy, in Green’s Operative Hand Surgery, ed 4. Phila, PA, 1999. pp 1404-1447

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