How long has it been since the skin biopsy was done?

Based upon the answer:

Before you submit to the Cellcept or Imuran, why don't you ask for a repeat skin biopsy to see if the nerve fiber density has increased? That would indicate that the IVIG is, in fact working.

This is one of the benefits of the skin biopsy...that it can be repeated many times to gauge improvement (or progression) of disease process.