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A dermatome is an area of skin that is mainly supplied by a single spinal nerve.There are 8 cervical nerves (note C1 has with no dermatome), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves. Each of these spinal nerves relay sensation from a particular region of skin to the brain.
The nerves from the upper cervical spine supply the skin of the neck. C5 to T1 nerves supply the arms. T2 to L2 nerves supply the chest and abdomen. L3 to S1 nerves supply the skin of the legs. S1-C1 nerves go to the groin.
Testing of dermatomes is part of the neurological examination looking for radiculopathy as sensation changes within a specific dermatome may help in determining the pathological disc level.
Dermatome Testing is done ideally with a pin and cotton wool. Ask the patient to close their eyes and give the therapist feedback regarding the various stimuli. Testing should be done on a specific dermatomes and should be compared bilaterally.
Pinprick test (pain sensation) - Gently touches the skin with the pin ask the patient whether it feels sharp or blunt
Light touch test (light touch sensation) -Dab a piece of cotton wool on an area of skin 
The comprehensive videos below on dermatomes also report on the clinical significance of these tests. From studies and a systematic review (2017) they report the finding attribute a weak clinical significance to these tests but, they point out, they are the best tools we have. 
- Wikipedia Dermatome. Available from: https://en.wikipedia.org/wiki/Dermatome_(anatomy) (last accessed 23.4.2019)
- M Roehrs. Dermatomes. Available from: https://www.youtube.com/watch?v=CYZBH6NX8wg&feature=youtu.be (last accessed 23.4.2019)
- Slide share. Dermatomes and myotomes. Available from: https://www.slideshare.net/TafzzSailo/special-test-for-dermatomes-and-myotomes (last accessed 23.4.2019)
- Physiotutors. Upper limb dermatomes. Available from: https://www.youtube.com/watch?v=VlPpdRTGH-o (last accessed 23.4.2019)
- Physiotutors. Lower limb dermatomes Available from: https://www.youtube.com/watch?v=SzAyUsA25MQ&t=8s (last accessed 23.4.2019)