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Median Nerve Measurement on Ultrasound

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Median Nerve Measurement on Ultrasound

  • See Also
  1. Wrist Ultrasound
  2. Carpal Tunnel Syndrome
  • Indications
  1. Carpal Tunnel Syndrome Diagnosis
  • Preparation
  • Positioning
  1. Patient sits with Forearm supinated at rest
  2. Linear, high frequency probe in short axis for the Median Nerve (and the wrist)
  3. Consider towel roll beneath patient wrist
  • Images
  1. usWrist_VolarDistal.png
  1. Landmarks - start position at proximal wrist crease
    1. Median Nerve is superficial, but within the flexor Retinaculum
    2. Median Nerve is at radial aspect of palmaris longus
    3. Median Nerve is adjacent (radial) to flexor pollicis longus
      1. Wrist adduction and abduction will move flexor pollicis longus and will push on Median Nerve
  2. Landmarks - measurement position at distal wrist
    1. Slide probe in SAX distally along volar wrist towards palm
    2. Goal view
      1. Flexor Retinaculum most superficial
      2. Scaphoid shadowing posteriorly at radial or lateral aspect
      3. Mid-line wrist view contains three main components
        1. Median Nerve (most superficial)
        2. Flexor tendons (9)
        3. Lunate (deepest, shadows posteriorly)
      4. Pisiform shadowing posteriorly at ulnar or medial aspect
  3. Precautions
    1. Avoid compressing Median Nerve with excessive probe pressure
    2. Biphid Median Nerve or a split Median Nerve (present in >8%)
      1. Measure each nerve separately and add their areas
  4. Measurement (in mm2)
    1. Use the machine cross-sectional area measurement (draw oval around nerve inner boundary)
    2. Record measurement
  1. Slide probe proximally (in short axis) from proximal wrist crease, approaching mid-Forearm
    1. Measurement position is 12 cm from the View 1 measurement
  2. Landmarks
    1. Flexor digitorum superficialis
    2. Median Nerve (within facial layer)
      1. Improved visualization when patient makes a loose fist
    3. Flexor digitorum profundus
  3. Measurement (in mm2)
    1. As above (draw oval around nerve inner boundary)
  • Interpretation
  1. Wrist to Forearm ratio = (distal wrist median mm2) / (proximal wrist median mm2)
    1. Ratio >1.4 (some use ratio > 2.0)
  2. Increase in distal mm2 measurement >2 mm over proximal mm2 measurement
  3. Proximal Median Nerve width
    1. Normal if <9 mm2
    2. Abnormal >12 mm2 (severe if >14 mm2)
  4. Wrist Median Nerve cross sectional area at Carpal Tunnel inlet
    1. Cross sectional area >9 mm2 is sensitive and specific for Carpal Tunnel
    2. Tai (2012) Ultrasound Med Biol 38(7): 1121-8 [PubMed]
  • References
  1. Moore (2016) GCUS Musculoskeletal Ultrasound Course, St. Pete's Beach, FL
  2. Moore (2013) GCUS Upper Extremity Ultrasound