In preparation for my Physical Medicine practical exam next week, I've been practicing locating myotomes and dermatomes, and testing deep tendon reflexes (using Tony as a guinea pig; you will be happy to know that all of his myotomes, dermatomes, and reflexes are intact and functional).
Myotomes are muscles that are innervated by one nerve root of the spinal cord. We test myotomes to look for any signs of weakness in a group of muscles. If a particular group of muscles is weak, then it is likely that the spinal nerve responsible for innervating these muscles may be damaged. Spinal nerves may be damaged by trauma, spinal
cord lesions/tumors, or by disc herniation. For example, if you have trouble flexing your elbow, but you can extend your wrist with no problems, then it is not a cranial nerve 6 problem, but a more local problem involving the elbow flexors. If it had been a spinal/cranial nerve problem, then when the cranial nerve 6 myotome was tested (by resisting elbow flexion and wrist extension (with the actions being resisted at the same time)), then both actions would have been weak. Still confusing? Yeah, it's hard to explain myotomes in a blog post... am I am realizing this now when I've almost completed writing this post. Doh! Anyway, you can watch how mytomes are tested by watching this video posted here. It's a bit lengthy, but it's great for someone who needs to know how to test myotomes like me ;)
Dermatomes are areas of skin that are innervated by one nerve root of the spinal cord. While myotomes are test the motor functioning of a nerve, dermatomes test the sensory functioning. For instance, if you can't feel any sensation on your thumb but you can feel sensation on your index finger, than your cranial nerve 6 dermatome is still intact and healthy. Both areas would be affected if it were a spinal/cranial nerve problem. The lack of feeling in the thumb is likely due to a local cutaneous nerve injury and not due to a spinal cord/disc/nerve root problem. You can view the dermatomes in this pretty, visual video posted here. I love learning about dermatomes because all of the teaching material for them are so colorful!
Lastly, while searching for the videos linked above to help me better understand myotomes and dermatomes, I came across this creative dance video. Through the medium of dance, the girls in the video demonstrate the testing of both myotomes and dermatomes. You can view their dance here. Now, if only I could dance my way through my upcoming Physical Medicine exam...