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Dear Ruth:
I have only seen plasmaphoresis used in Lupus-induced pripheral neuropathies, not in cases of CNS vasculitis of the intracranial vessels.
The potential causes of strokes in your case include vasculitis (which appears to be the diagnosis your physicians are leaning towards), antiphospholipid antibody syndrome, TTP, and cardiac valvular disease. If some of these possibilities have not been looked into,they are important to rule out.
Your current treatment sounds appropriate, although prednisone has been combined with calcium channel blockers to treat vasculitis with some success (Calabrese et al , Jul 1997).
I hope this has been helpful--good luck.