--would be diagnosed through standard nerve conduction/EMG testing, as the small, unmyelinated fibres that control autonomic functioning cannot normally be reached with such testing (which only can contact larger, myelinated nerves).

Usually, tests for autonomic functioning are those mentioned by Aussie99, which are more indirect.

And, of course, the MS posssibility needs to be explored--any dysfunction of the central nervous system (brain/spinal cord) can produce similar symptoms of autonomic dysfunction (as well as many others).

Have you had a lumbar puncture, or evoked potential testing (indriect ways to check for central nervous system disorders in absence of obvious central nervous system lesioning)?

And, have you been checked for B12 and other nutrient malabsorption?