When do you consider plasmapheresis instead of (or in addition to) IV steroids for patients with acute MS exacerbation?  

I would utilize plasmapheresis in a patient with severe, debilitating symptoms from an MS relapse and incomplete (or non-reassuring trajectory of) improvement after intravenous steroids. This is supported by AAN guidelines (Cortese et al., PMID 21242498). There is less robust data to support IVIG.