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Anterior Interosseous Nerve (AIN) Sign

Exams and Signs

Historical Overview

  • Anterior interosseous nerve (AIN) syndrome was first reported by Kiloh and Nevin in 1952.1,2 In 1965, the first description of an operation for AIN syndrome was credited to Fearn and Goodfellow.2

Description

  • The AIN sign helps to identify AIN syndrome, which involves partial or complete loss of motor function of the flexor pollicis longus (FPL), the flexor digitorum profundus (FDP), and the pronator quadratus (PQ).

Pathophysiology

  • AIN syndrome often arises from the neuritis, Parsonage-Turner syndrome.3
  • AIN syndrome also may arise from pronator teres syndrome, carpal tunnel syndrome, or peripheral neuropathy.
  • Some patients with AIN syndrome may have suffered a viral illness before experiencing pain in the forearm.3

Instructions

  1. Obtain an accurate and complete patient history. Ask the patient if s/he experiences any difficulty with writing, knitting, or other pinching motions.
  2. Check the patient for weakness in the thumb, index finger, and/or middle finger. 
  3. Examine the patient for lack of distal interphalangeal (DIP) flexion in the index finger and interphalangeal (IP) flexion in the thumb.
  4. Examine the patient for a dull pain in the proximal forearm.2,4
  5. Check the contralateral hand, wrist and forearm.

Variations

  • The AIN sign is also referred to as the “OK” sign.5

Related Signs and Tests

  • Pinch strength2
  • Muscle test: FDP
  • Muscle test: FPL
  • Range of motion (ROM), active
  • Sensory exam by nerve
  • Flexor pollicis manual compression
  • Electrodiagnostic studies6
  • Magnetic resonance imaging (MRI), without contrast

Additional Information

Presentation Photos and Related Diagrams
  • AIN Palsy Sign
    AIN Palsy Sign
  • Note lack of index DIP and thumb IP flexion (arrows) in patient with Anterior Interosseous Nerve Palsy
    Note lack of index DIP and thumb IP flexion (arrows) in patient with Anterior Interosseous Nerve Palsy
Definition of Positive Result
  • A positive result occurs when the patient presents with lack of index DIP and thumb IP flexion.
Definition of Negative Result
  • A negative result occurs when the patient does not present with lack of index DIP and thumb IP flexion.
Comments and Pearls
  • AIN syndrome may resolve spontaneously after continued observation.2,6
  • Three months after an EMG has shown no evidence of reinnervation, operative treatment would include surgical decompression, and a complete release of the median nerve in the proximal forearm.3
  • Magnetic resonance imaging (MRI), without contrast can also be helpful
Diagnoses Associated with Exams and Signs
References
  1. Kiloh LG, Nevin S. Isolated neuritis of the anterior interosseous nerve. Br Med J1952;1(4763):850-1. PMID: 14916168
  2. Gardner-Thorpe C. Anterior interosseous nerve palsy: spontaneous recovery in two patients. J Neurol Neurosurg Psychiatry1974;37:1146-50. PMID: 4374508
  3. Culp R, Jacoby S. Musculoskeletal Examination of the Elbow, Wrist and Hand: Making the Complex Simple. New Jersey: SLACK Incorporated, 2012
  4. Collins DN, Weber ER. Anterior interosseous nerve syndrome. South Med J1983;76(12):1533-7. PMID: 6359461
  5. Rayan G, Akelman E. The Hand: Anatomy, Examination and Diagnosis. Philadelphia: Lippincott Williams & Wilkins, 2012
  6. Rodner CM, Tinsley BA, O’Malley MP. Pronator syndrome and AIN syndrome. J Am Acad Orthop Surg2013;21(5):268-75. PMID: 23637145
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