Hyacinth I also have multiple and diffuse high signal changes in my periventricular and subcortical white matter. Had the lumbar puncture and evoked potentials, but they didn't think MS. They've been through various theories, including being told they were age related (at the ancient age of 36 
) then them being caused by migraines, but in the end have just said they are, as yet, unexplained and to go back if I develop any acute symptoms such as loss of bladder or bowel control or loss of one side of my body. So that's reassuring then - not.
According to the private consultant I went to see, they are very common among people with chronic conditions like ME/CFS/Fibro/Autonomic Dysfunction, but not thought to be dangerous in any way, iyswim. I worried about mine for years, but since I'm still here I figure he must be right.
The reason I think it relates to my pots is that my report said one suggestion for the lesions could be ischemia and I have read quite a bit recently about hypoperfusion in the white matter producing small vessel type damage/lesions. One BMJ article said:
"The affected vessels are presumed to induce lesions in deep white matter through chronic hypoperfusion of the white matter ...."
Given that I have had pots since childhood and therefore have been chronically hypoperfusive, I think that may explain the presence of wml's at a younger age.
I would love to find a study that looks into the prevalence of wml's in pots patients.
They can be caused by severe migraine and as I suffer from severe, complex migraine too, that is another potential cause, although I think with the number of lesions I have, there are likely to be more than one cause. My MRIs also showed both historic and recent encephalitis. I knew I'd had it as a child, as it was initially diagnoses as meningitis, but turned out not to be. I had also been extremely ill on holiday in Ireland a few weeks before the scan that showed up the recent signs. I was bedridden for almost a fortnight with what I thought was the worst migraine of my life. Turns out it was most likely encephalitis and my neuro was 
that I got through it without being admitted to hospital.
It is true that many people develop wml's as they age, but they are still only found in 10% of 60 year old patients, so I am not willing to accept being told they are 'normal' at 36 or like one particularly dreadful locum neuro told me, 'everyone has them'. I think if they don't know the answer some doctors will try and fluff their way through explaining your symptoms away, instead of admitting they don't know. Fortunately my usual neuro had already told me that they weren't normal and required monitoring. That said, a screw up by the crap locum got be dropped from the neuro list three years ago and I haven't gone back. I'd been under them since 2006 with no answers or help, so decided I wouldn't bother unless I absolutely had to.]
... aaand that turned out to be a much longer post than I intended - sorry. It's something I have read and thought a lot about, after spending years under neuro being prodded, poked, scanned and tested - only to be told they had no idea what the problem was or if my lesions were causing my symptoms. By contrast, three appointments privately, with a Consultant in General Internal Medicine and I had my diagnosis.