Dermatomes

 

  • A dermatome is an area of skin that is served by a single spinal nerve
  • Dermatome maps like the one above vary somewhat from one text to another, likely due to differences in testing methods and individual variations.
  • Familiarity with the dermatomes is clinically useful in localizing the lesion to a nerve root or spinal cord
  • You should have a good feel for the dermatomes in the arms and legs and know that for the chest/abdomen, the nipple line is ~T4 and the umbilicus is ~ T 10 (and you can “guess-timate” for the rest)
  • In spinal cord lesions, it is important to look for a sensory level (dermatome level)
    • Start from the abnormal and move up the chest or back on either side with a pin or cold tuning fork until you reach normal sensation. You can also check vibration along the vertebrae as well.
    • The sensory level is typically 2-3 levels below the lesion
  • It is important to know the dermatomes if you are trying to figure out if the problem is due to a radiculopathy (root compression)
    • Roots commonly compressed from herniated discs include L5 and S1 and less commonly C6 and C7.
    • The most common presentation of this is sharp, shooting pain along the course of the dermatome served by the affected spinal nerve
    • Damage to a single nerve root, even when severe, usually does not have any sensory loss because of the striking overlap of dermatomal sensory distribution
    • There may be slight loss, often accompanied with paresthesias (tingling or pins and needles) in small areas of the distal limbs where the sensory overlap is not great.

 

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