Updated: 10/5/2016

Anomalous Extensor Tendon

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Introduction
  • Definition
    • variations of extensor tendons of the hand
    • usually discovered incidentally during surgery for other reasons (e.g. ganglion excision)
  • Epidemiology
    • incidence 
      • not uncommon
  • Mechanism
    • symptoms arise because of increased muscle volume within small muscle compartment
      • pain from synovitis or ischemia
Anatomy
  • Normal EIP 
    • occupies 4th dorsal extensor compartment (8-10mm wide)
    • ratio of 1:1 for muscle:tendon length
    • origin - posterior surface of distal third of ulna and adjacent interosseous membrane
    • insertion - dorsal expansion of index finger on ulnar side of EDC
Classification
 
Anomalous Extensor Muscle Forms
Anomalous extensor indicis proprius (aEIP)  • Most common cause of symptoms
  
Extensor digitorum brevis manus (EDBM)  • Less common cause of symptoms because muscle belly is proximal to extensor retinaculum
Extensor medii proprius (EMP)  
Extensor indicis et medii communis (EIMC)  • EIP and EIMC unlikely to be symptomatic because of narrow width
 
Presentation
  • Symptoms
    • usually asymptomatic 
      • discovered incidentally during surgery (e.g. ganglion removal)
    • mass on the dorsum of the hand  
    • intermittent dorsal wrist pain if muscle bellies impinge on and occupy the narrow dorsal compartments of the wrist
  • Physical Exam
    • inspection
      • mass does not transilluminate
      • moves with movement of local muscles (flexion and extension of hand and wrist(
      • becomes firmer with grasp
    • provocative tests
      • resisted extension triggers pain
Imaging
  •  MRI
    • indications
      • exclude other more common conditions e.g. ganglion
    • findings
      • mass is isointense with muscle tissue
      • anomalous extensor indicis proprius (aEIP) 
      • extensor digitorum brevis manus (EDBM)  
      • extensor medii proprius (EMP) 
      • extensor indicis et medii communis (EIMC) 
Differential
  • Ganglion
  • Synovitis
    • both produce dorsal wrist pain
Treatment
  •  Nonoperative
    • observation
      • indications
        • first line treatment
  • Operative
    • surgical decompression of 4th dorsal compartment (aEIP) or reduction of muscle belly (EDBM)  
      • indications
        • failed conservative treatment, and symptoms, signs and imaging point to anomalous muscle, with no associated conditions (e.g. ganglion)
 

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