You do not have to just go along with whatever they say, if you are unhappy about it, or have questions.
As you rightly say, Lemtrada is usually only indicated for active or very active RRMS.
Are you quite sure it’s Lemtrada that’s on offer, and not a simple steroid infusion? As it appears from your post that Lemtrada was not actually named, and you have simply gone home and Googled infusions for MS, and that was the name you came up with. It seems possible there is some confusion here, and your husband is only really being offered intravenous steroids, which are a common form of symptomatic relief.
Also, response to steroids can be a useful indicator of whether someone has RRMS or PPMS, as RRMS will generally respond - PPMS won’t. As there seems to be uncertainty about which your husband has, that might be another very good reason why the consultant would like to try steroids. Steroids do also have risks and side-effects, but not (as I understand it) on a par with Lemtrada - nevertheless, your husband’s consultant would be duty-bound to point them out.
Double-check with the nurse, but I really have a feeling it’s a false alarm, and your husband is NOT being offered Lemtrada.
If I’m wrong, and he is, I would certainly want greater clarity about the rationale for the decision - why a DMD at all, if it’s not yet clear which type of MS your husband has (DMDs only work for RRMS), and why straight to one of the “big guns”, without trying a milder drug first? It does all seem a bit drastic, if they can’t even confirm he has RRMS.
My bet is it’s steroids.
Many here have had them, and NICE guidelines consider it a routine treatment for an MS episode. Sometimes they are given intravenously, and sometimes as pills.
I can’t advise you further, as I haven’t personally had them. As with any drug, there are some risks and side-effects, and it could be possible to have a rare reaction, but I think most people are perfectly OK with a course of steroids, and side-effects, whilst they can be unpleasant, are usually temporary.
Completely no idea about the thromboembolism treatment! Clerical error? Has your husband actually had a thromoembolism (blood clot) or considered at risk of one? If not, I’d guess it’s a hospital c*ck-up.