For the ASIA exam, one muscle group was selected to represent each spinal segment. However, each muscle group is innervated by at least two spinal nerves – the more rostral spinal nerve is assigned to the muscle group. So, while the elbow flexors are innervated by C5 and C6, they represent C5 for purposes of the ASIA.
If someone has the C5 nerve root intact but not C6, they will have some strength (but not 5/5) in their elbow flexors. If C5 and C6 are intact, they will likely have 5/5 strength in their elbow flexors. This makes sense when determining the motor levels of injury: someone with C5 only may score 3/5 at their elbow flexors and would have a motor level of C5 whereas someone with C5 and C6 would score 5/5 on their elbow flexors and likely at least 3/5 on their wrist extensors and would have a motor level of at least C6. Does that make sense?
The ASIA exam is a bit different than traditional myotome testing for upper and lower quarter screens in the absence of spinal cord injury, for which it is still reasonable to use elbow flexion/wrist extension for C6 and elbow extension/wrist flexion for C7.
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