When people say its neuropathy is idiopathic, does this means, people don't know what cause it, or every exam is normal (including ski biopsies) so people should not have it...

By the way, how long is a leason in a large nerve detectable in a EMG? Is always detectable dispite its healness?

And also, are peripherical nerves composed by large and small fibers? Or do small fibers only exists in the skin?

Thank you;