Our rescued Jack Russell, Trigger had them and still can get them, or something similar. In his case its an auto immune issue and we treated him with steroids ( what a combination). We had to make the decision if they got worse that would be it, we did and do not have the money for it, having fluffed up insurance, going for a cheap option
The vet said, you can do all the tests you want, spend thousands and never really get to the end of it, in Triggers case he had only seen it twice in his career.
Anyhow they have receded over the years, hes 11 now and still as fit as a fiddle.
Here he is on a riding walk after 4 miles
https://vimeo.com/user12784146