PIN SYNDROME
The Finger Drop
Key Definitions
Critical Must-Knows
- PIN Syndrome presents as 'Finger Drop' but sparing of wrist extension (ECRL is intact).
- Wrist extends in radial deviation (Radio-Carpal extension via ECRL).
- No sensory loss (Superficial Radial Nerve branches off proximally).
- The most common compression site is the Arcade of Frohse (proximal edge of Supinator).
- Lipomas are a common cause of 'spontaneous' PIN palsy.
- Rheumatoid synovitis at the elbow can also compress the PIN.
Examiner's Pearls
- "If the patient has WRIST drop, the lesion is proximal to the PIN (High Radial Nerve).
- "In PIN palsy, the patient can extend the wrist (ECRL) but it deviates radially (due to ECU paralysis).
- "Pain alone is Radial Tunnel Syndrome, not PIN Syndrome.
The Trap: Tendon Rupture vs Palsy
The Diagnosis
Vaughan-Jackson? RA patients can hav EDC tendon ruptures (Vaughan-Jackson) OR PIN palsy (Synovitis). Missing the difference leads to the wrong surgery. Tendon rupture: Sudden 'ping', dropped fingers one by one (Ulnar to Radial). PIN Palsy: Slow onset (usually), all fingers affected together (or specific pattern).
The Test
Tenodesis Test Flex the wrist. If the fingers extend passively (Tenodesis), the tendons are intact (nerve issue). If the fingers remain floppy/flexed then it is a Tendon rupture. Ultrasound confirms.
| Condition | Motor Loss | Sensory Loss | Site |
|---|---|---|---|
| PIN Syndrome | Finger/Thumb Ext + ECU | None | Arcade of Frohse |
| Radial Tunnel | None (Pain only) | None | Arcade of Frohse |
| Wartenberg's | None | Dorsal Webspace | Fascia (Forearm) |
| High Radial | Wrist + Fingers + Sensation | Dorsal Webspace | Spiral Groove |
ASEMuscles Innervated (PIN)
Memory Hook:The PIN supplies All Supracondylar Extensors? No, that's wrong. It supplies All Distal Extensors.
FREASSites of Compression
Memory Hook:Radial nerve FREAS up.
E-E-EOrder of Recovery
Memory Hook:Actually, Brachioradialis recovers first in High Radial.
Overview
Posterior Interosseous Nerve (PIN) Syndrome is a compressive neuropathy of the deep motor branch of the Radial nerve. It typically occurs at the proximal forearm within the Radial Tunnel, most commonly at the Arcade of Frohse.
Clinically, it presents as weakness of finger and thumb extension ("Finger Drop"). Crucially, wrist extension is preserved (ECRL is innervated proximal to the PIN), but the wrist deviates radially during extension because the Extensor Carpi Ulnaris (ECU) is paralyzed.
Pathophysiology and Mechanisms
Course
- Division: The Radial Nerve divides into Superficial (Sensory) and Deep (Motor/PIN) at the level of the radio-capitellar joint.
- Entry: The PIN enters the "Radial Tunnel" under the edge of the ECRB.
- Supinator: It pierces the Supinator muscle between its two heads. The proximal edge is the Arcade of Frohse.
- Exit: Exits the supinator distally to supply the deep extensor compartment.
The course within the supinator is 4-5cm long.
Classification Systems
Clinical Types
- Type I (Complete PIN): All PIN muscles paralyzed. Finger drop + Thumb drop + ECU paralysis.
- Type II (Partial PIN): Only some muscles (e.g., just Thumb/Index). Mimics tendon rupture.
- Type III (Radial Tunnel): Pain only. Dynamic compression.
Type II often confuses diagnosis with tendon rupture.
Clinical Assessment
Physical Exam
- Inspection: "Finger Drop". Wrist can extend but deviates Radially (ECRL acts, ECU fails).
- Power: Test EDC (MCP extension), EIP (Index extension), EPL (Thumb extension).
- Tenodesis: Check passive extension to rule out tendon rupture.
- Pain: Vague ache in proximal forearm (unlike Lateral Epicondylitis which is at the epicondyle).
Pain + Weakness = PIN Syndrome.
Imaging and Electrodiagnostics
MRI
- Mandatory: For any non-traumatic PIN palsy.
- Purpose: To rule out a mass (Lipoma, Ganglion).
- Finding: Denervation edema in supinator/extensors. Mass lesion compressing nerve.
A negative MRI does not exclude dynamic compression.
Management Algorithm
Non-Operative
- Indication: Neuritis (Parsonage-Turner), Transient compression, No mass.
- Splinting: Cock-up dynamic splint (outrigger) to hold fingers in extension (prevent overstretching).
- Time: Observe for 3-6 months.
- Steroids: Limited role unless inflammatory.
Activity modification (avoid pronation/supination) is key.
Surgical Technique
Anterior Approach (Henry)
- Incision: Volar curvilinear.
- Interval: PT / BR.
- Deep: Identify Radial Nerve. Trace it distally.
- Release: Ligate Leash of Henry. Retract ECRB. Identify Arcade of Frohse.
- Action: Divide the arcade and the superficial head of supinator.
Stay on the nerve at all times visually.
Complications
Intraoperative Complications
- Iatrogenic Nerve Injury: Direct injury to PIN during release, especially at the Arcade of Frohse.
- Vascular Injury: Damage to Leash of Henry (radial recurrent vessels) causing significant bleeding.
- Incomplete Release: Failure to release all five compression sites (most commonly missing distal supinator edge).
- ECRB Denervation: Variable innervation can lead to unexpected weakness if motor branch damaged.
- Superficial Radial Nerve: Injury during anterior approach causes sensory loss on dorsal hand.
Meticulous technique with loupe magnification is essential.
Rehabilitation
- Dynamic Splint: Low profile radial nerve palsy splint with metacarpophalangeal extension assist.
- Purpose: Allows active flexion, passive extension. Prevents extensor overstretching.
- Wound Care: Keep incision clean and dry; suture removal at 10-14 days.
- Edema Control: Elevation and gentle active finger movements encouraged.
- Range of Motion: Active-assisted exercises to maintain joint mobility.
- Scar Management: Silicone gel or massage once wound healed.
- Continue Splinting: Night splinting particularly important to prevent contractures.
- Recovery Monitoring: Monitor for "flicker" of EDC - first sign of reinnervation.
- Order of Recovery: Brachioradialis → ECRL → ECRB → Supinator → EDC → EPL.
- Strengthening: Gentle isometrics progressing to resistance as power returns.
Recovery progresses at 1mm per day from site of compression. Sensory re-education is NOT required (pure motor nerve). Full recovery may take 6-12 months depending on severity.
Prognosis
Expected Outcomes by Etiology
- Compression (Arcade): Excellent recovery expected if decompression performed within 6 months; greater than 85% return to full function.
- Mass Lesion (Lipoma): Excellent with excision and neurolysis; function returns in 3-6 months post-surgery.
- Traumatic (Monteggia): Variable; depends on mechanism and timing. Neurapraxia recovers well, neurotmesis requires grafting.
- Inflammatory (RA): Good if synovectomy performed early; ongoing disease may cause recurrence.
- Radial Tunnel Syndrome: Unpredictable; 60-70% success rate for pain relief even with surgery.
Early intervention correlates strongly with better outcomes.
Evidence Base
Arcade of Frohse Anatomy
- Anatomical study of the Arcade of Frohse
- Found fibrous arch in 30% of adults
- Absent in fetuses (suggests acquired fibrosis)
- Identified as main compression point
Radial Tunnel vs PIN
- Defined Radial Tunnel Syndrome as pain without motor loss
- Proposed dynamic compression theory
- Results of decompression were mixed for pain
- Excellent for motor PIN palsy
Lipomas and PIN
- Lipomas are the most common solid tumor causing PIN palsy
- Often undetected on X-ray
- MRI is diagnostic
- Surgical removal restores function in most cases
Tendon Transfers
- Described standard transfers for radial nerve palsy
- PT to ECRB, FCU to EDC, PL to EPL
- Emphasized timing (wait 1 year?)
- Excellent outcomes reported
Ultrasound Diagnosis
- US vs Surgical findings in Radial Nerve compression
- High sensitivity for visualizing the nerve and arcade
- Can see nerve swelling proximal to compression
- Useful for dynamic assesssment
Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Spontaneous Drop
"A 45-year-old woman presents with inability to extend her fingers. It started gradually over 3 weeks. No trauma. Wrist extension is present but deviates. What is the diagnosis?"
Scenario 2: The Monteggia
"A child has a Monteggia fracture (Ulnar fracture, Radial head dislocation) reduced 6 weeks ago. Still cannot extend thumb."
Scenario 3: Pain Only
"A tennis player complains of lateral elbow pain. Treated as 'Tennis Elbow' for 6 months with no relief. Injections failed. Exam shows pain on resisted middle finger extension."
Scenario 4: Rheumatoid Patient
"A 55-year-old woman with known rheumatoid arthritis develops progressive weakness of finger extension over 4 weeks. She has swelling around the elbow. How do you differentiate the cause?"
MCQ Practice Points
Anatomy
Q: What is the most common site of PIN compression? A: The Arcade of Frohse (proximal edge of Supinator).
Clinical Signs
Q: Why does the wrist deviate radially in PIN palsy? A: ECRL (Radial N) is intact, but ECU (PIN) is paralyzed.
Diagnosis
Q: How do you differentiate PIN palsy from multiple tendon ruptures (Vaughan-Jackson)? A: Tenodesis test. Passive wrist flexion should extend the fingers if tendons are intact.
Syndromes
Q: What is Wartenberg's Syndrome? A: Compression of the Superficial Radial Nerve (Sensory only) causing dorsal hand paresthesia.
Mass Lesions
Q: What is the most common soft tissue mass causing spontaneous PIN palsy? A: Lipoma. Always order MRI for spontaneous PIN palsy to rule out a mass lesion.
Australian Context
Referral Patterns:
- "Resistant Tennis Elbow" failing conservative management is a common trigger for referral.
- Spontaneous onset of finger drop requires urgent MRI and specialist referral.
- Hand surgery units at major centres (Royal North Shore, St Vincent's, Alfred) see PIN pathology regularly.
- HealthPathways provides streamlined referral for peripheral nerve disorders in most metropolitan areas.
Occupational Considerations:
- Workers with repetitive pronation/supination activities (electricians, mechanics, assembly line workers) are at increased risk.
- WorkCover claims for Radial Tunnel Syndrome can be challenging due to diagnostic uncertainty.
- Occupational therapy assessment is essential before return to work.
- Workplace modifications may be required to prevent recurrence.
Space-Occupying Lesions:
- Lipomas are the most common soft tissue tumour causing spontaneous PIN palsy.
- MRI is mandatory for any non-traumatic PIN palsy to exclude mass lesions.
- Ganglion cysts from the proximal radioulnar joint are another common cause.
- Rheumatoid synovitis should be considered, particularly in patients with known RA.
Antibiotic Prophylaxis (eTG):
- For planned surgical decompression: Single dose first-generation cephalosporin at induction.
- Clean surgery with low infection risk - prophylaxis is optional per surgeon preference.
- Lipoma excision may warrant prophylaxis if extensive dissection required.
Australian Rehabilitation:
- Dynamic radial nerve palsy splints are available through major hand therapy departments.
- Custom splinting is Medicare rebatable when prescribed by an orthopaedic surgeon.
- DVA and WorkCover typically fund comprehensive hand therapy programs.
- Recovery monitoring should include serial EMG at 3-month intervals.
High-Yield Exam Summary
Anatomy
- •FREAS (Fibrous, Recurrent, ECRB, Arcade, Supinator)
- •Arcade of Frohse = #1 Site (70% fibrous)
- •ECRL Spared (innervated by Radial Nerve proximally)
- •PIN = pure motor branch of Radial nerve
- •Supinator course: 4-5cm within muscle
Clinical
- •Finger Drop (Not Wrist Drop = PIN vs High Radial)
- •Radial Deviation on wrist extension (ECRL intact, ECU paralyzed)
- •Tenodesis Test (rule out Vaughan-Jackson tendon rupture)
- •Normal sensation (Superficial Radial branches off proximally)
- •Middle Finger Test: pain/weakness 4cm distal to epicondyle
Treatment
- •MRI mandatory (rule out lipoma/mass)
- •Observe 3-6 months if no mass
- •Release (Henry or Thompson approach)
- •Transfers if no recovery at 1 year (PT-ECRB, FCU-EDC, PL-EPL)
- •Splint (dynamic outrigger) prevents overstretching