MRI and nerve conduction (velocity) studies will not show small fiber damage, usually. The MRI shows large myleinated nerve damage, but the small unmyleinated nerves are too small to measure.
Also, in myleinated large fiber damage, it is sometimes difficult to ascertain whether the damage is in the peripheral system or in the central nerve system.
your facial symptoms, and not hands or legs - would lend a clue to look into the central nervous system, as well.