My heart is with you and you are not alone. My husband with FTD becomes fixated on things too. It could be anything. In your case if the docs say that there is nothing physically wrong then its time to look at other issues. For me, I called and strongly advocated that he is not living the best of the rest of his days with the anxiety, stress, and obsessing he now does. It was paralyzing for him. Admittedly, it is often exaggerated or irrational but to him real. They finally made some med changes. I also tried to hug him more and reassure him that I am there and will call the doctor as needed. I got a pulse ox cheap online and he can see his heart rate and oxygen level because paranoia also comes in and this is an objective measure. The struggle affects me and is causing me stress-related issues so advocate for yourself and him. Your in my prayers.