ANTERIOR INTEROSSEOUS SYNDROME
Pure Motor Branch of Median Nerve | No Sensory Loss | OK Sign Weakness
Clinical Severity
Critical Must-Knows
- Pure Motor Palsy: NO sensory loss distinguishes AIS from median nerve compression
- OK Sign Test: Cannot make circle with thumb IP and index DIP - both remain extended
- Muscles Affected: FPL, FDP to index (and middle), pronator quadratus only
- Natural History: 60-70% recover spontaneously within 6 months - observe first
- Compression Site: Usually between two heads of pronator teres or fibrous bands
Examiner's Pearls
- "No sensory loss = AIN syndrome not median nerve
- "OK sign weakness = FPL + FDP index involvement
- "Pronator quadratus weakness hard to detect clinically
- "Observe 3-6 months before surgery - high spontaneous recovery
Clinical Imaging
Imaging Gallery
Critical AIS Exam Points
Pure Motor Palsy
Zero sensory loss. If there is ANY sensory involvement, it is NOT isolated AIN syndrome - consider median nerve or pronator syndrome.
OK Sign Test
Cannot pinch with thumb tip and index tip. Both IP joints remain extended forming a pinch pulp-to-pulp instead of tip-to-tip.
Natural History
60-70% recover spontaneously. Observation for 3-6 months is first-line treatment. Do not rush to surgery.
Differential Diagnosis
FPL tendon rupture, FDP rupture, pronator syndrome. Check for sensory loss and palpate tendons to differentiate.
Quick Decision Guide
| Presentation | Likely Diagnosis | Treatment | Key Pearl |
|---|---|---|---|
| Weak OK sign, no sensory loss, recent trauma | Acute AIS (traumatic) | Observation 3-6 months, EMG at 6 weeks | 70% recover spontaneously |
| Weak OK sign, no sensory loss, gradual onset | Idiopathic AIS | Observation 3-6 months, consider imaging | Most common presentation |
| Weak OK sign with sensory loss palmar | Pronator syndrome | Different treatment - may need earlier surgery | Sensory loss rules out pure AIN |
FPL FDP PQAIN Motor Innervation
Memory Hook:FPL FDP PQ - The three muscles that make the OK sign work! All motor, zero sensory!
SENSORYAIS vs Pronator Syndrome Differentiation
Memory Hook:SENSORY tells you what's missing in AIS - no sensory loss!
FIBROMACauses of AIS
Memory Hook:FIBROMA - Think of anatomical causes from fractures to masses to anomalies!
Overview and Epidemiology
Why AIS Matters
AIS is the classic pure motor median nerve palsy. Understanding that it has ZERO sensory loss and high spontaneous recovery rate is critical for exam scenarios and preventing unnecessary surgery.
Anterior Interosseous Syndrome is isolated compression or injury of the anterior interosseous nerve, a pure motor branch of the median nerve.
Demographics
- Age: Any age, peak 30-50 years
- Sex: No clear gender predilection
- Bilateral: Rare (suggests systemic cause)
- Occupational: Can occur with repetitive pronation
Much less common than carpal tunnel syndrome.
Clinical Significance
- Diagnosis often delayed: Mistaken for tendon rupture
- High spontaneous recovery: 60-70% within 6 months
- Pure motor palsy: No sensory loss key to diagnosis
- Surgical outcomes: Good if selected appropriately
Observation is first-line treatment in most cases.
Pathophysiology and Mechanisms
AIN Anatomy
The anterior interosseous nerve branches from the median nerve 4-6cm distal to the lateral epicondyle, just AFTER the nerve passes between the two heads of pronator teres. It is a PURE MOTOR branch with zero cutaneous sensory distribution.
Anatomical Course:
- Origin: Median nerve, 4-6cm distal to lateral epicondyle
- Course: Descends on anterior surface of interosseous membrane
- Termination: Pronator quadratus and wrist joint capsule (sensory only to joint)
Motor Innervation (3 muscles only):
- Flexor pollicis longus (FPL): Thumb IP flexion
- Flexor digitorum profundus to index (and middle): Index and middle DIP flexion
- Pronator quadratus (PQ): Forearm pronation (weak clinically)
Common Compression Sites:
- Between two heads of pronator teres
- Fibrous bands from deep head of pronator teres
- Accessory muscles (Gantzer's muscle)
- Lacertus fibrosus (bicipital aponeurosis)
- Thrombosed vessels or aberrant radial arteries
Classification Systems
Clinical Severity Classification
| Grade | Clinical Findings | EMG Findings | Treatment |
|---|---|---|---|
| Mild | Weakness only, no atrophy | Denervation in affected muscles | Observation 3-6 months |
| Moderate | Progressive weakness, no recovery at 3 months | Ongoing denervation | Continue observation to 6 months |
| Severe | Muscle atrophy or no recovery at 6 months | No reinnervation potentials | Surgical exploration |
Severity guides timing of intervention.
Clinical Assessment
History
- Onset: Acute (post-trauma) or gradual (idiopathic)
- Pain: May have initial forearm pain (Parsonage-Turner)
- Weakness: Difficulty pinching, writing, turning keys
- No numbness: Critical negative finding
- Red flags: Sensory loss (not pure AIN), progressive atrophy
Ask specifically about numbness to rule out pronator syndrome.
Examination
- OK sign test: Cannot make tip-to-tip circle
- Pinch pattern: Pulp-to-pulp instead of tip-to-tip
- FPL testing: Isolate thumb IP flexion
- FDP index testing: Isolate index DIP flexion
- Sensory exam: MUST be completely normal
- Tinel's: May be positive over AIN course
Always test sensation to confirm pure motor palsy.
Key Clinical Tests
| Test | Technique | Positive Finding | Indicates |
|---|---|---|---|
| OK Sign | Ask patient to make circle with thumb and index | Extended IP joints, pulp-to-pulp pinch | FPL and FDP index weakness |
| Isolated FPL | Stabilize thumb MCP, ask to flex IP | Weak or absent IP flexion | FPL denervation |
| Isolated FDP Index | Stabilize PIP, ask to flex DIP | Weak or absent DIP flexion | FDP denervation |
| Pronator Quadratus | Resist pronation with elbow flexed 90° | Subtle weakness (hard to detect) | PQ denervation |
Differential Diagnosis
Key differentials: FPL tendon rupture, FDP rupture, pronator syndrome (has sensory loss), high median nerve palsy, neuralgic amyotrophy. Palpate tendons and check sensation carefully.
Investigations
Investigation Protocol
Clinical diagnosis based on pure motor palsy affecting FPL, FDP index, and PQ with ZERO sensory loss. OK sign weakness is diagnostic.
EMG/NCS at 6 weeks post-onset. Denervation potentials in FPL, FDP (radial), and PQ. Normal sensory studies. Helps confirm diagnosis and establish baseline.
MRI forearm if mass lesion suspected, trauma history, or no recovery at 3 months. May show space-occupying lesion, hematoma, or nerve changes.
Repeat EMG to assess for reinnervation potentials. If no recovery, consider surgical exploration.
Electrodiagnostic Findings:
- Motor NCS: May be normal (nerve supplies only 3 muscles deep in forearm)
- Sensory NCS: NORMAL (critical finding)
- EMG: Denervation potentials (fibrillations, positive sharp waves) in FPL, FDP index/middle, and PQ
- Follow-up EMG: Reinnervation potentials indicate recovery
Management Algorithm
Conservative Management (First-Line)
Observation Protocol
Observation and activity modification. Avoid heavy gripping and pinching. Baseline EMG at 6 weeks to confirm diagnosis. Hand therapy for ROM.
Continue observation. Clinical reassessment monthly. Most spontaneous recoveries begin in this period. Maintain ROM exercises.
Repeat EMG at 3-4 months. If no clinical or electrical recovery, consider imaging and surgical exploration. If improving, continue observation.
Surgery if no recovery by 6 months. Some advocate waiting up to 12 months if showing gradual improvement. Balance against muscle atrophy risk.
Observation for 3-6 months is standard as most cases recover spontaneously.
Surgical Technique
Anterior Interosseous Nerve Exploration
Surgical Steps
Supine, arm on table, tourniquet. Volar approach, curvilinear incision from 2cm distal to elbow flexion crease extending 10-12cm distally along ulnar border of mobile wad.
Identify and protect lateral antebrachial cutaneous nerve. Divide lacertus fibrosus. Retract brachioradialis laterally.
Identify median nerve between FDS and FCR. Trace distally to identify pronator teres. Release nerve from between two heads of pronator teres.
Identify AIN branching from median nerve 4-6cm distal to epicondyle. Follow nerve distally along interosseous membrane. Release all fibrous bands, anomalous muscles (Gantzer's), or compressing structures.
Release nerve throughout course. Excise any mass lesions. Ensure nerve glides freely. Inspect FPL, FDP, and PQ muscle bellies for atrophy.
Layered closure. No drain typically needed. Bulky dressing and forearm splint in neutral position.
Complete decompression from origin to pronator quadratus is essential.
Complications
Complications of AIS Surgery
| Complication | Incidence | Management |
|---|---|---|
| Incomplete recovery | 20-40% | Counsel pre-operatively, therapy |
| Cutaneous nerve injury | 5-10% | Careful dissection, neurolysis if symptomatic |
| Median nerve injury | Rare (under 2%) | Prevent with gentle handling |
| Recurrent compression | Rare (under 5%) | Ensure complete release |
| CRPS | Rare | Early therapy, multidisciplinary care |
Incomplete recovery is the most common outcome issue. Counsel patients that surgery aims to prevent further deterioration and allow recovery, but full strength may not return, especially if atrophy is present.
Postoperative Care
Postoperative Protocol
Forearm splint in neutral position. Wound care. Finger and elbow ROM out of splint. Suture removal at 10-14 days.
Discontinue splint. Begin gentle active ROM all joints. Scar massage. No strengthening yet.
Progressive strengthening. Pinch and grip strengthening exercises. Functional activities.
Monitor for nerve recovery. Repeat EMG at 6 months post-op to assess reinnervation. Recovery may take up to 12-18 months.
Return to work: Light duty at 2-4 weeks. Full duty at 8-12 weeks depending on demands and recovery.
Outcomes and Prognosis
Non-Operative Outcomes:
- Spontaneous recovery: 60-70% within 6 months
- Time to recovery: Most within 3-6 months, can take up to 12 months
- Complete recovery: Common in idiopathic and post-traumatic cases
- Neuralgic amyotrophy: Recovery in 80-90% but may take 18-24 months
Surgical Outcomes:
- Overall improvement: 70-80% good to excellent results
- Complete recovery: 50-60% (lower than spontaneous)
- Partial recovery: 20-30%
- No improvement: 10-20%
Prognostic Factors:
| Factor | Better Outcome | Worse Outcome |
|---|---|---|
| Cause | Traumatic, mass lesion | Idiopathic, neuralgic amyotrophy |
| Duration | Short (under 6 months) | Long (over 12 months) |
| Atrophy | Absent | Present pre-operatively |
| Age | Younger | Elderly |
Early surgical decompression for compressive masses has excellent outcomes. Prolonged observation (over 12 months) may lead to irreversible muscle atrophy.
Evidence Base
- 23 patients with AIN syndrome
- 74% spontaneous recovery
- Average recovery time 9.3 months
- Observation first-line for idiopathic cases
- 17 patients with AIN palsy
- EMG findings correlate with recovery
- Reinnervation potentials predict good outcome
- Serial EMG useful for monitoring
- First comprehensive description of AIS
- Emphasized pure motor nature
- Described anatomical compression sites
- Established OK sign as diagnostic test
- 10 patients with surgical decompression
- 70% good to excellent results
- Best results with identifiable compressive lesion
- Idiopathic cases had variable outcomes
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Classic Presentation
"A 35-year-old man presents with 2 months of difficulty pinching objects. He cannot make an 'OK' sign properly. His thumb and index finger IPs remain extended when he tries. Sensation in the hand is completely normal. What is your diagnosis and management?"
Scenario 2: Post-Traumatic AIS
"A 28-year-old woman sustained a proximal radius fracture treated non-operatively 8 weeks ago. She now presents with difficulty writing and pinching. Examination shows weak OK sign but normal sensation. What is your approach?"
Scenario 3: Persistent AIS at 6 Months
"A patient with idiopathic AIS has been observed for 6 months with no clinical improvement. Repeat EMG shows persistent denervation with no reinnervation potentials. MRI shows no mass lesion. The patient has visible atrophy of the thenar eminence. What is your approach?"
MCQ Practice Points
Pure Motor Nature
Q: What distinguishes anterior interosseous syndrome from pronator syndrome? A: Zero sensory loss. AIS is a pure motor palsy (AIN has no cutaneous sensory distribution). Pronator syndrome has sensory loss in median nerve distribution because the nerve is compressed proximal to the AIN branching.
OK Sign Test
Q: What does the OK sign test? A: FPL and FDP to index. Inability to make tip-to-tip pinch (thumb IP and index DIP extended) indicates weakness of FPL and FDP index, pathognomonic for AIN palsy.
Spontaneous Recovery Rate
Q: What percentage of AIS cases recover spontaneously? A: 60-70% within 6 months. This high spontaneous recovery rate justifies observation as first-line treatment before considering surgical exploration.
Three Muscles Only
Q: What muscles are innervated by the anterior interosseous nerve? A: FPL, FDP to index (and middle), and pronator quadratus. Only these three muscles - no cutaneous sensation.
Branching Point
Q: Where does the AIN branch from the median nerve? A: 4-6cm distal to the lateral epicondyle. This is just distal to the pronator teres, which is why pronator teres compression can affect AIN.
Gantzer's Muscle
Q: What is Gantzer's muscle? A: Accessory head of FPL. Anatomical variant present in 45-65% of people. Arises from medial epicondyle and can compress the AIN.
Australian Context
Australian Guidelines:
- EMG/NCS recommended for confirmation before surgical intervention
- Medicare rebates available for electrodiagnostic studies and surgical decompression
- Most AIS exploration performed as inpatient procedure
Medicolegal Considerations:
- Document pure motor nature (zero sensory loss) to confirm diagnosis
- Obtain informed consent including risk of incomplete recovery
- Document observation period and rationale for surgery if performed
- Warn of nerve injury, wound complications, persistent weakness
Workcover/Insurance:
- May be work-related if associated with forearm trauma or repetitive activities
- Document occupational history and mechanism
- Prolonged time off work may be required for recovery (6-12 months)
Australian surgeons should follow RACS guidelines and document consent thoroughly, especially regarding the high spontaneous recovery rate and observation period.
ANTERIOR INTEROSSEOUS SYNDROME
High-Yield Exam Summary
Key Anatomy
- •AIN = pure motor branch of median nerve
- •Branches 4-6cm distal to lateral epicondyle
- •Innervates only 3 muscles: FPL, FDP (index/middle), PQ
- •Zero cutaneous sensory distribution
Clinical Diagnosis
- •OK sign weakness = cannot pinch tip-to-tip
- •Normal sensation = key to diagnosis
- •FPL and FDP index weakness, PQ weak (hard to test)
- •EMG: denervation in 3 muscles, normal sensory studies
Differential Diagnosis
- •Pronator syndrome = has sensory loss (palmar)
- •FPL tendon rupture = palpable tendon gap
- •High median nerve palsy = thenar weakness + sensory
- •Neuralgic amyotrophy = painful onset, shoulder involvement
Natural History
- •60-70% recover spontaneously within 6 months
- •Observation first-line treatment
- •EMG at 6 weeks to confirm diagnosis
- •Surgery if no recovery by 6 months
Surgical Technique
- •Volar approach, expose median nerve proximally
- •Trace AIN from origin distally
- •Release all fibrous bands and anomalous muscles
- •Gantzer's muscle common compression site
Outcomes
- •Non-op: 60-70% full recovery
- •Post-op: 70-80% good-excellent
- •Recovery takes 6-12 months
- •Atrophy portends incomplete recovery