There is actually a significant amount of evidence for plasmapheresis for a steroid-unresponsive multiple sclerosis relapse:
In fact, there was a randomized trial for this exact scenario with sham plasmapheresis which yielded significant results (" 8 of 19 (42.1%) [improved with] active treatment compared with 1 of 17 (5.9%) [with] sham treatment"):http://onlinelibrary.wiley.com/doi/10.1 ... O;2-Q/full
In other words, it works a little less than half the time
I also found a case series of 16 patients: http://europepmc.org/abstract/MED/16341 ... VH6T8fq.30
If I were you, I wouldn't let Marc's experience disuade you. For one, extreme exhaustion is not a common side effect of plasmapheresis. The most unpleasant part is generally having a central line placed (a large bore catheter usually inserted into the internal jugular or subclavian vein). The plasma exchange itself is generally not unpleasant and is well tolerated.
The highest risk portion of the procedure is the placing of the line (collapsed lung a small risk with subclavian lines), but there is also a risk of volume shifts/electrolyte changes or infection of the catheter.
Marc states that he has primary progressive MS for which this treatment is unproven. Unfortunately, it did not help him, but you are probably a better candidate for the treatment than he was.
If you are having a true relapse with significant enough symptoms that you would be willing to accept the inconvenience/risk, you may want to consider it.
best of luck. let us know what happens.