"As you know I am a lumper, and not a splitter, and have rehearsed the arguments for making SP & PPMS the same disease many times on this blog.
What are the arguments for SP & PPMS being the same disease. MRI studiesdemonstrate that lesions on MRI in MSers with relapse-onset or PPMS are identical. The pathology and genetics of the two subtypes are the same. Similarly, when MS occurs in families and the first sibling develops RRMS the second and third siblings may develop either RRMS, or PPMS, in proportion to the incidence of these subtypes in the general population. The pathology of the two subtypes is similar. Once someone with relapse-onset disease develops SPMS it progresses, on average, at the same rate as PPMS. Importantly, a proportion of pwPPMS (~10-15%) will go onto to have relapses; although we classify these people as having progressive-relapsing disease they still have PPMS. I am not aware of any consistent biological differences between relapse-onset MS (RRMS & SPMS) and PPMS. In summary, relapse-onset MS (RRM & SPMS) and PPMS are the same disease." says Dr Giovannoni on the Barts MS Blogspot.
Did everyone else who reads the Blogspot know this is his view? I had managed to miss it, although I read the Blogspot with great regularity.
BTW, the rest of the Blogspot post I have quoted from, Multiple Sclerosis Research: ResearchSpeak: a New Year prediction, is a really interesting meditation on the way forward for all types of progressive MS.