iirc people with BIID have something gone wrong with their peripheral nervous system and it's not sending all the correct signals it should, which results in the brain thinking "hey this body part shouldn't be here" driving the person mad. So far there isn't a medication to alleviate it.

I really don't agree with doctors agreeing to surgically amputate on people like this, especially when most cases result in the person becoming legally disabled (not this case, which is why the doctor agreed to do it) but often someone with BIID ends up doing some DIY amputation or mutilation. Maybe therapy can help a person cope with the sensations and urges, but it doesn't sound pleasant either way.