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Hey friend! Common symptom in OCD, particularly if your theme revolves around health OCD. I have the same theme currently, and I’ve dealt with your particular issue in the past—although now it’s morphed into something else. The best part is I was having hyponopomic hallucinations upon waking up during that time as well. Like with all other obsessions, the answer is ERP therapy. For me, that meant exposures where I’d stare at images that cause visual hallucinations, or I’d set up screens to flash things in my periphery at random. Eventually, I stopped caring and focusing on it at all.

I am terribly sorry you feel this way, I also have this issue with my OCD. To be honest, I am glad I found someone else with it as that provides assurance nothing is wrong. I talked to my therapist about it, the visual field is quite tricky. Our visual mistakes things for other things all the time, especially out of the periphery. You must first ACCEPT that you’re anxious about this. Rather than being afraid of the things you may see, know that everytime it occurs it’s an OCD thing/typical occurance with your visual field.

I totally agree with both replies above. You said "how do I diagnose this". To me, that question appears misguided. Perhaps you can change it to "how can I respond to this effectively?"