Yours is a complex question and it is hard to do it justie in this forum. Despite the similarities in the name, PAN and microscopic PAN are likely 2 separate diseases. Although both can affect the kidney, other organ systems are different. For instance microsopic PAN an affect the lungs while PAN does not. Microscopic PAN is associated with an antibody blood test called p-ANCA. Since many of the drugs used to treat both of these problems are similar, you are right that it may be more important how severe the kidney problem is and whether it progresses in determining whether or not to move forward with more aggressive therapy such as cylophosphamide.
Microscopic Polyangiitis or Polyarteritis Nodosa
An indisputable biopsy ten years ago indicated PAN which seemed at the time to be somewhat limited to skin and joints with a relapsing and remitting course. I have had a history of microscopic hematuria. Upon examination there were a significantly large number of casts, but creatinine clearance was fine, and there was no evidence of proteinuria. The nephrologist believes this indicates microscopic polyangiitis. The decision was made to remain on 15 mgs of methotrexate a week along with 10 mgs of prednisone rather than more aggressive cyclophosamide therapy unless there is further indication of damage as the hematuria was present all along and has not affected kidney function. Is it possible to have two forms of polyarteritis nodosa at the same time? Does the disease mutate? Does it amke a difference to treatment to have a different name attached to the same symptoms? Would a course of cyclophosamide help beat this into remission?