Anterior Interosseous Nerve Syndrome

Other Names

• Kiloh-Nevin Syndrome
• AIN Compressive Neuropathy
• AINS
• Anterior Interosseus Nerve Syndrome
• Anterior interosseous neuropathy
• Anterior interosseous palsy
• Anterior interosseous nerve palsy

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Background

• This page refers to neuropathy of the Anterior Interosseous Nerve (AIN)

History

• First mentioned by Kiloh and Nevin in 1952

Epidemiology

• Represents <1% of upper extremity neuropathies
• AINS accounts for 18–33% of all neuralgic amyotrophy cases

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Introduction

Illustration of AIN syndrome due to paresis of flexor digitorum profundus, flexor pollicis longus

1a: Drawing showing the origin and course of the AIN in the forearm. 1b: Cadaveric specimen of the right upper limb showing the AIN and its course

Origin and course of anterior interosseous nerve (AIN) in the anterior aspect of the forearm

General

• Typically presents with forearm pain, weakness in the FPL, FDP of the index finger
• Sensation is intact
• Diagnosis is aided by EMG, MRI
• Symptoms are typically self limited, surgery is indicated if symptoms don't resolve spontaneously

Etiology

• Pathoanatomy
  • Most common site of compression is the tendinous edge of the deep head of the Pronator Teres
  • Also must consider
    • Proximal edge of the Flexor Digitorum Superficialis arch (the FDS arcade)
    • Gantzer's muscle (accessory head of the FPL muscle)
    • FDS or FDP accessory muscles
    • Arterial thrombosis (radial or ulnar artery have been implicated)
    • Lacertus fibrous
• Etiology of the disease is controversial
  • Traumatic etiologies:
    • Supracondylar fractures,
    • Penetrating injuries and stab wounds
    • Cast fixation
    • Venipuncture
    • Complication of open reduction and internal fixation of fractures
  • Spontaneous etiologies:
    • Brachial plexus neuritis
    • Compartment syndrome
    • Compression neuropathy
• Martin-Gruber Anastomosis
  • Anatomical variant in which AINM gives off branches to ulnar nerve, patients can then present with symptoms in ulnar nerve patterns

Anatomy of the Anterior Interosseous Nerve

• Terminal motor branch of the Median Nerve
• Innervates
  • Flexor Pollicis Longus
  • Flexor Digitorum Profundus (index, long fingers)
  • Pronator Quadratus

Associated Conditions

• Parsonage Turner Syndrome
  • Be suspicious if patient has bilateral AIN symptoms, shoulder pain, viral prodrome
  • AIN is one of the most commonly affected nerves in PTS, involved in 18–33% of all PTS patients
• Hereditary brachial plexus neuropathy (HBPN)
  • autosomal dominant mutations in the SEPT9 gene cause recurrent episodes of neuralgic amyotrophy, which can present as AINS
• Proximal Radius and Ulna Fractures
• Supracondylar Fracture
  • Pseudo-AIN syndrome by damaging the posteriorly located AIN fascicles within the median nerve trunk at the elbow level

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Risk Factors

• Needs to be updated

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Differential Diagnosis

Differential Diagnosis Wrist Pain

• Fractures
  • Distal Radius Fracture
    • Bartons Fracture
    • Chauffers Fracture
    • Colles Fracture
    • Die-Punch Fracture
    • Radial Styloid Fracture
    • Smiths Fracture
  • Distal Ulna Fracture
  • Carpal Fractures
    • Scaphoid Fracture
    • Lunate Fracture
    • Triquetrum Fracture
    • Pisiform Fracture
    • Trapezium Fracture
    • Trapezoid Fracture
    • Capitate Fracture
    • Hamate Fracture
  • Essex Lopresti Fracture
• Dislocations
  • Wrist Dislocation (Radiocarpal and/or Ulnocarpal)
  • Carpometacarpal Joint Dislocation
  • Distal Radioulnar Joint Dislocation
  • Lunate Dislocation
  • Perilunate Dislocation
• Instability & Degenerative
  • Scapholunate Instability
  • Lunotriquetral Instability
  • Scaphoid Nonunion Advanced Collapse
  • Distal Radial Ulnar Joint Instability
  • Kienbocks Disease
  • Dorsal Wrist Impingement
  • Carpal Instability
• Tendinopathies & Ligaments
  • De Quervain's Tenosynovitis
  • Intersection Syndrome
  • TFCC Injury
  • Wrist Tendinopathies
  • Extensor Carpi Ulnaris Tendinopathy
• Neuropathies
  • Carpal Tunnel Syndrome
  • Pronator Teres Syndrome
  • Anterior Interosseus Nerve Syndrome
  • Posterior Interosseus Nerve Syndrome
  • Guyon Canal Syndrome
  • Wartenbergs Syndrome
• Pediatric Considerations
  • Distal Radial Epiphysitis (Gymnast's Wrist)
  • Torus Fracture
• Arthropathies
  • Wrist Osteoarthritis
  • Scaphotrapeziotrapezoidal Joint Arthritis (STT Arthritis)
  • Rheumatoid Arthritis
  • Gout
• Cartilage
  • Osteochondral Defect
• Vascular
  • Hypothenar Hammer Syndrome
• Other
  • Ganglion Cyst of Wrist
  • Ulnar Impingement Syndrome
  • Infectious Tenosynovitis

Differential Diagnosis Forearm Pain

• Fractures
  • Distal Radius Fracture
    • Barton's fracture
    • Chauffer's Fracture
    • Colles' Fracture
    • Die-Punch Fracture
    • Radial Styloid Fracture
    • Smith's Fracture
  • Radius Ulna Fracture (Both Bone)
  • Proximal Radius Fracture (Head, Neck)
  • Isolated Ulna Fracture
  • Monteggia Fracture
  • Galeazzi Fracture
  • Distal Ulna Fracture
  • Essex Lopresti Fracture
• Pediatric Specific Fractures
  • Torus Fracture
  • Greenstick Fracture
  • Salter Harris Fracture
  • Plastic Deformation
• Dislocations & Instability
  • Distal Radioulnar Joint Dislocation
  • Proximal Radioulnar Joint Instability
• Soft Tissue Trauma
  • Acute Compartment Syndrome
• Tendinopathies
  • De Quervain's Tenosynovitis
  • Intersection Syndrome
• Neuropathies
  • Radial Tunnel Syndrome
  • Posterior Interosseus Nerve Syndrome
• Pediatric Considerations
  • Distal Radial Epiphysitis

Other Considerations

• Thoracic Outlet Syndrome
• Cervical Radiculopathy
• Parsonage Turner Syndrome

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Clinical Features

Abnormal pinch grip in the right hand

Pronator Quadratus Isolation Test

History

• Poorly localized pain from proximal volar forearm to anticubital fossa
  • Present in approximately 85% of cases
  • Pain may radiate proximally toward the shoulder or distally
  • Pain typically precedes weakness by hours to weeks
• The patient will not report any parasthesias, numbness or tingling
  • This is a critical distinguishing feature from other forearm/hand neuropathies
• Motor complaints
  • Patients report difficulty with pinching, deterioration of handwriting, and loss of fine motor dexterity
  • Inability to make "OK" sign
• Onset is typically spontaneous, but may follow changes in activity or trauma
• Dominant hand is most commonly affected

Physical Exam: Physical Exam Wrist

• This disease is characterized by motor deficits only, there are no sensory deficits
• Weakness of index and thumb pincer movement
  • Weakness of the flexor pollicis longus, flexing the thumb IPJ against resistance
  • Weakness of FDP in the index finger at PIP and DIP joints
  • Pronator quadratus weakness, tested by forearm pronation with the elbow fully extended
• Sensory exam should be completely normal

Special Tests

• Pinch Test: Patient unable to hold sheet of paper between thumb and index finger
• Pronator Quadratus Isolation Test: Resisted pronation with the elbow maximally flexed

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Evaluation

Axial images of this MRI reveal significant hyperintense signal changes of FPL, FDP and PQ

A Transverse ultrasound image of the mid left forearm demonstrating an abnormally thickened anterior interosseous nerve (arrow) adjacent to its accompanying vessel (red color). B Comparison of the unaffected right forearm with normal AIN and barely perceptible hypoechoic fascicles adjacent to its accompanying vessel (black color).

A Axial T2 weighted fat suppressed images of the left forearm preoperatively and B three months following decompression. The median nerve (white arrow) is abnormally T2 hyperintense on the preoperative image, indicating neural edema/neuropathy. The nerve signal normalizes on the postoperative image. The AIN was not well seen. Note the geographicAQ9 muscle denervation edema in some of the flexor musculature (arrowhead).

Radiographs

• Standard Radiographs Wrist
  • 3 View radiographs typically acquired to evaluate for other etiologies
  • No role direct role in the diagnosis of AIN syndrome
• Findings
  • Atraumatic AIN syndrome, XR are typically normal
  • Rarely, they can detect masses, accessory ossicles, or hardware that might contribute to extrinsic compression

CT

• CT is not part of the standard workup for spontaneous AINS
• Limited for direct nerve evaluation but can be useful in post-traumatic cases
• Potential findings including fracture fragments, callus formation, or heterotopic ossification

EMG/NCS

• General
  • Cornerstone for confirming the diagnosis
  • Can determine severity and monitor recovery
  • Sensory nerve conduction studies of median nerve should be normal
  • Can help differentiate neuralgic amyotrophy verse a compression neuropathy
• Key findings
  • Fibrillations
  • Reduced recruitment in FPL/FDP/PQ
  • Normal median sensory NCS
• Limitations
  • 2–3 week delay for denervation changes
  • needle recording needed for selective AIN NCS

MRI

• General
  • High-resolution Magnetic Resonance Neurography at 3T — is the most informative imaging modality for AINS
• Potential findings
  • Muscle denervation changes, increased signal/ edema
  • Localization of fasicular lesion
  • Fasicular constrictions
  • Broader denervation pattern
• Identifying compression of the AIN on MRI can be difficult
  • Entrapment may not be visualized
  • Enhancement within the muscles affected by the nerve root entrapment

Ultrasound

• High-frequency neuromuscular ultrasound is a complementary modalit
  • Accessible, dynamic, and can be performed at the point of care
• Potential findings
  • Fasicular constrictions (39% sensitive compared to 100% sensitivee in MRN)
  • Nerve morphology
  • Muscle changes

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Classification

• N/A

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Management

Consider immobilization in a posterior long arm splint

Nonoperative

• General
  • Most patients improve with 6-12 weeks of relative rest and conservative care
  • Follows a stepwise approach with conservative care, surgery reserved for patients who do not show improvement
• Medications including
  • In the early prodromal phase, pain can be severe
  • NSAIDS, Acetaminophen
  • Can also consider neuropathic pain agents (gabapentin, amitriptyline, carbamazepine)
  • Rarely a short course of opioids
• Corticosteroids
  • A retrospective study showed reduction in length of symptoms, higher rates of full recovery
  • No randomized controlled trials exist
  • Recommended regimen is prednisone 60 mg daily for 7 days followed by a 10 mg/day taper
• IVIG has been considered
  • Evidence is limited to case reports
• Observation and monitoring
  • Relative rest, especially from provocative activities
  • Splint elbow at 90° flexion
  • Physical Therapy
  • Activity modification

Operative

• Indications
  • Failure of conservative measures
  • No consensus on timeline for surgery, ranging from 3 months to 12+ months
• Procedure
  • Surgical decompression of AIN
  • Interfascicular neurolysis (fascicular microneurolysis
  • Nerve transfers
  • Nerve grafting
  • Salvage: tendon transfer

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Rehab/ Return to Play

Rehabilitation exercises

Rehabilitation Phase I: Acute/Protective (Weeks 0–6)

• Avoid repetitive pronation/supination and forceful gripping
• Splint wrist/hand in functional position to prevent compensatory overuse
• Edema control: elevation, retrograde massage, compression
• Passive/active-assisted ROM of wrist, fingers (DIP of index, IP of thumb), forearm
• Median nerve gliding exercises: 6 positions, 3–5x/day, 5–10 reps per position
• Maintain cardiovascular fitness and strength of uninvolved extremities/core

Rehabilitation Phase II: Motor Re-education (Weeks 6 to ~6 Months)

• Monthly MMT of FPL, FDP, PQ
• Repeat EMG/NCS at 3 and 6 months to detect reinnervation
• Once nascent reinnervation detected (MRC 1–2):
  • EMG biofeedback and NMES
  • Active-assisted exercises in gravity-eliminated positions
  • Isolated thumb IP flexion (FPL), index DIP flexion (FDP), forearm pronation (PQ)
• Continue median nerve gliding, progress as tolerated
• Tendon gliding exercises: 5 positions (straight → hook → full fist → tabletop → straight fist)
• Teach compensatory pinch patterns (e.g., pulp-to-pulp using FDS substitution)
• Decision point at 6 months: no improvement → surgical referral

Rehabilitation Phase III: Strengthening (Months 4–8, When MRC ≥3)

• Progress from gravity-eliminated to antigravity exercises
• Progressive resistive exercises (MRC 3+–4):
  • Theraputty gripping/pinching (graded resistance)
  • Thumb IP flexion against resistance (rubber bands, light weights)
  • Index DIP flexion against resistance
  • Forearm pronation with progressive resistance (dumbbell, bands)
  • Tip pinch and key pinch strengthening
• EMG biofeedback to correct compensatory substitution patterns
• Serial grip (Jamar) and pinch gauge dynamometry to track progress

Rehabilitation Phase IV: Functional/Sport-Specific (Months 6–12+)

• Sport-specific task training:
  • Throwing athletes: progressive throwing program, grip control emphasis
  • Racquet sports: graded racquet handling, grip endurance drills
  • Climbing/gymnastics: progressive grip loading, hang board training
  • Contact sports: grip under fatigue, protective equipment assessment
• High-rep, low-resistance endurance training for FPL, FDP, PQ
• Functional testing: strong "OK" sign, sustained pinch against resistance, sport-specific grip tasks
• Restore full proximal kinetic chain strength (shoulder, elbow, wrist)

Rehab Exercise Program PDF

• General Finger and Hand Home Exercises PDF

Return-to-Play Criteria

• FPL and FDP strength ≥ MMT 4/5
• Tip pinch strength ≥80–90% of contralateral side
• Grip strength ≥80–90% of contralateral side
• Complete "OK" sign without compensatory thumb IP hyperextension
• Pain-free sport-specific drills at full intensity
• Normal forearm pronation strength (PQ recovery)
• EMG evidence of reinnervation (supportive, not mandatory if clinical criteria met)
• Psychological readiness and confidence in hand function

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Complications

Prognosis

• Overall
  • Favorable in the majority of cases
  • 30% of patients may be left with permanent weakness or palsy if untreated.
• 75% of patients improve following surgery if clear space occupying lesion (need citation)
• Conservative treatment
  • 80% of limbs had a good recovery in a prospective study
  • Biggest predictor of recovery in this study was ≥1 MMT improvement within the first 6 months
  • A seperate study showed recovery signs within 6 months in 12 of 14 patients
• Surgical treatment
  • Interfascicular neurolysis performed within 8 months of onset yielded good recovery (MMT ≥4) in 10/13
  • Surgery delayed beyond 12 months resulted in poorer outcomes (MMT ≤3).

Complications

• Persistent Motor weakness
  • Up to 30% of AINS patients may have permanent weakness or palsy without intervention
  • Recovery among 12 patients followed for 20 months: complete (3), partial (6) and none (3)
• Compensatory Movement Patterns and Overuse
  • Patients develop abnormal pinch substitution patterns (hyperextension at thumb IP joint)
  • Can become habitual and difficult to correct
• Persistent Pain and Fatigue
  • Up to two-thirds of patients followed ≥3 years have persisting pain and/or paresis among all neuralgic amyotrophy
• Joint Contractures
  • Denervated muscles and prolonged immobility can lead to DIP and IP joint contractures
• Recurrence
  • The recurrence rate for idiopathic NA is ~25–26%, for hereditary NA (SEPT9 mutation) up to 75%
• Salvage Procedures for Refractory Cases

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See Also

Internal

• Hand and Wrist Anatomy (Main)
• Wrist Pain (Main)
• Forearm Pain (Main)
• Elbow Pain (Main)
• Physical Exam Wrist
• Neuropathies (Main)

External

• Sports Medicine Review Wrist Pain: https://www.sportsmedreview.com/by-joint/wrist/

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