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