Median nerve

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MEDIAN NERVE

Anatomy
Local lesions
Anterior interosseus syndrome
Carpal tunnel syndrome
Ligament of Struthers
Pronator syndrome

Median nerve: Anatomy

Formed by
- C5 to C7 roots from lateral cord of brachial plexus
- C8 and T1 roots from medial cord of brachial plexus
Branches
- Forearm: Muscular branches
  - Pronator teres
  - Flexor carpi radialis
  - Flexor carpi sublimis
- Anterior interosseus (motor)
  - Flexor pollicis longus
  - Flexor digitorum profundus to 2nd & 3rd fingers
  - Pronator quadratus
  - External link: Anatomy & Exam
- Palmar cutaneous
  - Sensory distribution: Skin over thenar eminence
  - Branch arises proximal to carpal tunnel
- Terminal motor
  - Abductor pollicis brevis
  - Opponens pollicis
  - Lumbricals: 1st & 2nd
  - ± Flexor pollicis brevis: Also innervated by ulnar nerve
- Terminal sensory
  - Sensory to palmar surface of thumb, 2nd, 3rd & lateral 1/2 of 4th finger
Anomalies
- Martin-Gruber anastomosis (10% to 44% of normals)
  - Branches from median to ulnar nerve in forearm: 2 patterns
    - Median nerve in proximal forearm: To ulnar nerve in middle to distal third of forearm
    - From anterior interosseus nerve: To ulnar nerve
  - Innervates
    - 1st dorsal interosseus: Most common
    - Other: Adductor pollicis, Abductor digiti minimi
  - Often bilateral: 60% to 70%
  - NCV features
    - Larger CMAP from stimulation at elbow than wrist
    - Initial positive CMAP from stimulation at elbow
    - Median mononeuropathy: Median forearm conduction is artificially fast
  - External link: e-Medicine
- Riche-Cannieu anastomosis
  - Connections between deep ulnar & median nerves in hand
  - Frequency: 77% of hands
  - May cause
    - Median innervation of normally ulnar innervated muscles
    - Ulnar innervation of normally median innervated muscles
    - Rarely: May produce all ulnar innervated hand
  - External link: e-Medicine

Median nerve: Lesions

Gowers
Normal hand Thumb is perpendicular to plane of palm	Median nerve lesions Thumb is externally rotated into plane of palm. Thenar eminence is wasted.

High Median nerve lesions
- General
  - Hand posture: Thumb, index & middle fingers remain extended with attempted grip
  - Pain: Present in region of entrapment
- Axilla
  - Crutch compression
  - Missle injury
  - Anterior shoulder dislocation
  - Lipoma or other neoplasm
- Upper arm
  - Stab wounds: ± Brachial artery injury
  - Sleep palsy: Near pectoralis major tendon
  - Tourniquets
  - Fracture: Humerus shaft
- Elbow
  - Supracondylar spurs & ligaments (Ligament of Struthers)
  - Fracture
    - Humerus supracondylar: Children; Anterior interosseus distribution
    - Medial epicondylar
  - Elbow dislocation
  - Injection injury
  - Pronator syndrome
- Supracondylar spurs & ligaments (Ligament of Struthers)
  - Median nerve anatomy
    - Entrapped by band from medial epicondyle to supracondylar spur on distal, medial humerus
    - Spur occurs in 1% of population
      - May be associated with high origin of: Pronator teres & Anterior interosseus nerve
      - Under ligament: Median nerve; Brachial artery
      - Over ligament: Ulnar artery
  - Pain: Volar forearm
  - Paresthesias: Median-innervated digits
  - Weakness: Pronator teres; Other median muscles
  - Sensory loss: Mild; Median distribution, including thenar eminence
  - Exacerbated by: Supination of arm + Extension of elbow
    - Radial pulse may also be reduced
- Pronator teres syndrome (Pronator syndrome)
  - Pain
    - Locations: Volar forearm; Proximal arm
    - Exacerbated by
      - Repeated or resisted pronation
      - Resistance to isolated flexion of PIP joint of long and ring fingers
    - Associated with: Paresthesias in median innervated fingers
  - Pronator teres muscle
    - May be enlarged or firm
    - Tinel's sign over site of entrapment
  - Weakness: Mild or None
    - May be present in APB or Flexor pollicis longus
  - Sensory loss
    - Objective: Mild or none
    - Subjective: Sense of numbness
  - Laboratory
    - EMG: Denervation in median-innervated muscles (70%)
    - NCV: Abnormal median sensory responses (65%)
  - Treatment: Surgical release produces improvement in 50%
Anterior interosseus syndrome
- Anatomy: Arises from median nerve, 5 cm above medial epicondyle
- Causes
  - Fracture: Radius midshaft
  - Excessive exercise
  - Idiopathic
  - Direct trauma: Stab wound; Venipuncture
  - Anatomic anomalies
    - Vessel: Radial artery in mid forearm; Ulnar artery thrombosis
    - Muscle: Deep palmar compartment; From aberrant accessory muscles
      - Gantzer's muscle: Accessory head to Flexor pollicis longis
      - Origin of: Palmaris profundis; Flexor digitorum sublimis; Flexor carpi radialis
    - Fibrous bands: Origin of Flexor Digitorum Sublimis
    - Course of nerve deep to pronator teres
- Muscles innervated
  - Flexor pollicis longus
  - Flexor digitorum profundus to 2nd & 3rd fingers: FDP to other fingers innervated by ulnar nerve
  - Pronator quadratus
  - Martin-Gruber anastomosis: Intrinsic hand muscles weak
  - External link: Anatomy & Exam
- Weakness
  - Flexion of distal joint of thumb & 1st finger
  - Pronation of forearm wiht elbow flexed
  - Martin-Gruber anastomosis: Intrinsic hand muscles weak
- Sensory loss: No cutaneous innervation
- Muscle MRI (T2W fat-saturated images)
  - High signal in AI innervated muscles: Especially in pronator quadratus
- Weakness also seen with
  - Brachial neuritis: Lateral cord lesion
  - Supracondylar fractures
  - Proximal median nerve lesions: Neuroma or other
  - Tendon rupture: Flexor pollicis longus & Flexor digitorum profundus in rheumatoid arthritis
- Course: Spontaneous recovery in > 50%
- External link: Wheeless
Median neuropathy in forearm
- Bleeding into flexor compartment
  - Hemophiliacs; Anticoagulants; Brachial artery puncture
- A-V fistula for dialysis: Pain common; Onset days to weeks after surgery
Carpal tunnel syndrome

From: C Zaidman
Ultrasound images of Median Nerves
- Causes
  - Idiopathic: ? Related to
    - Congenital smallness of carpal tunnel
    - Hand activity: Repetitive; Amount
    - More common in dominant hand
    - Occupational
  - Reduced space in carpal tunnel
    - Tenosynovitis: Rheumatoid arthritis or suppurative
    - Bone: Fracture; Osteophyte; Dislocation; Exostosis
    - Mass: Ganglion; Gouty tophi; Hematoma
    - Anomalous muscles
      - Flexor digitorum sublimis
      - Palmaris longus
    - Median artery, Persistent
    - Congenital smallness of carpal tunnel
    - Neoplasms: Ganglia; Hemangioma; Lipoma;
      Neurofibroma; Schwannoma
  - Susceptibility of nerve to pressure
    - Hereditary Liability to Pressure Palsies
    - Diabetic Neuropathy
    - Renal failure
    - Other polyneuropathies
  - Systemic disorders
    - Pregnancy: Often resolves after delivery
    - Endocrine: Hypothyroidism; Acromegaly
    - Multiple myeloma; Amyloidosis
    - Osteochondritis dissecans
    - Spasticity
    - Edema
    - Immune disorders: Rheumatoid arthritis; Sarcoid
    - Infection: Histoplasmosis; Lyme; Septic arthritis; Tubercuolsis
  - Hereditary
    - Carpal tunnel syndrome ¹
      l Dominant
      - X-linked: 1 family
      - Onset
        
        Age: < 2nd decade
        
        Pain in lateral fingers
      - Bilateral
      - Anatomy
        
        Wrist X-rays: Narrow carpal tunnel
        
        Thick transverse carpal ligament
    - Amyloidosis
    - Mucopolysaccharidosis
    - Mucolipidosis
    - HNPP
  - Acute
    - Wrist fractures (Colles), dislocations (lunate bone, hyperextension), hematomas
    - Pyogenic infections: tenosynovitis
    - Rhematoid arthritis: Acute exacerbation
    - Excessive unaccustomed manual work
  - Palmar cutaneous branch
    - Transverse incisions at wrist for carpal tunnel surgery
    - Anomalous palmaris longus muscle
    - Ganglion from blunt trauma to wrist
- Clinical features
  - Epidemiology
    - Female: Male:: 3:1
    - Age peak: 40 to 60 years
    - Dominant hand
      - Often affected 1st & more severely
      - More common in idiopathic cases
    - Commonly becomes bilateral
    - Rare in absence of pain
  - Clinical
    - Pain
      - Episodic
      - Temporal: Worse at night initially; Later during day also
      - Location: Arm, forearm, wrist, hand & fingers
      - Repetitive or sustained activity: Exacerbates pain
    - Paresthesias & Sensory loss
      - Palmar thumb, 2nd, 3rd & medial 4th fingers
      - Finger tips
      - Nocturnal: Common
      - Normal: Thenar eminence
    - Weakness: After sensory loss
      - Abductor pollicis brevis (APB): Most common
      - Opponens pollicis
    - Provocative tests
      - Phalen maneuver
        
        Wrist held passively flexed
        
        Symptoms in 30 to 120 sec
        
        Most common: Paresthesias in index or middle finger
      - Tinel sign
        
        Tapping over median nerve at wrist
        Produces paresthesias
        
        May be false positive
  - Electrodiagnostic
    - Slow sensory conduction across transverse carpal ligament: Most sensitive
    - Later changes: Long motor latency; Low CMAP; Denervation in APB
  - Systemic work-up: Thyroid; Blood sugar; Sed rate; CBC
  - Treatment
    - Splinting
    - Corticosteroid injections
    - Surgical decompression: Complete section of transverse carpal ligament
      - Primary treatment in acute carpal tunnel syndrome
      - Avoid in pregnancy
- External link: Ortho-u

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References
1. Acta Neurol Scand 2001;104:377-379

7/29/2008