My father has it too. After years of nondiagnosis, lots of scans, etc, finally the idea was postulated (and proven) that it was B. burgdorferi, aka Lyme's (transmitted by a tick many years ago). A great shame, because it would have been easy to treat. The progression has stopped now, but the damage has been done.
As there's a plethora of other potential diagnoses, including the most frequent 'no diagnosis', it's unlikely that the same is true for your case, but always do think about the possibility of Lyme's as it is just too great a shame to miss something like that and it could have severe implications in the longer term, when untreated.