Interesting that you posted that question as that's what led me to this forum today (1st time). I'm 40 and have most of the upper right third and arm covered in a PWS. I've been experiencing a lot of changes to it and most recently have been experiencing pain. I'd always been told before (when I saw doc when I was much younger about this) that it couldn't/wouldn't cause pain, but I'm for sure feeling it now.
In the past ten years, I've developed numbness (comes and goes), a sort of numb uncomfortable ache in my right hand (esp fingers) when cold and my birthmark goes dark purple/blue. In the last year, I've had to take medication for unspecified neuralgia for tingling, stinging sensations in my right hand where my PWS is. Now I'm experiencing pain and a burning sensation in my right shoulder and chest area as well.
I don't really know the answer as to whether a PWS can cause pain, but I know I'm sure experiencing it and only in the area of my PWS, so I can only assume it's related somehow. Perhaps someone out there with more knowledge/experience can advise further?