Thank you for your message. Welcome to this community.
It sounds as if you are having a difficult time. This is a supportive community so our members may have more information and support for you.
You may be interested to look at the lupus UK community on the health unlocked forums. You can also find more information at lupusuk.org.uk telephone 01708 731251 (office hours).
There is also a memory health forum on health unlocked.
I'm not familiar with 'brain fog' as a part of a lupus diagnosis, but I am as a part of fibromyalgia, having had a good friend suffer with this as a side effect of her diagnosis. Like 'brain fog' itself, fibromyalgia seems to be either poorly understood by many of the medical profession, or downright dismissed out of hand.
I think, as a non-professional, that 'brain fog' most certainly happens and that it can happen in several medical conditions.
I'm not completely sure why you are asking, but if it is that you aren't certain that your husband's symptoms are down to dementia, then, although I'd follow my doctor's advice and make sure he attends all outpatient appointments, I'd certainly still keep an open mind about it and make sure he's investigated for any other illness that could be the cause.
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