@glennaskinfolk, I'm not sure if you were aware of these two sites for PPMS. They might provide some help.

National MS Society - Treating PPMS
-- https://www.nationalmssociety.org/What-is-MS/Types-of-MS/Primary-progressive-MS/Treating-Primary-Progressive-MS

Medical News Today - What to know about primary progressive MS
-- https://www.medicalnewstoday.com/articles/315475

Another good search tool is Google Scholar - https://scholar.google.com/