That’s an interesting question! Please understand, I’m not a medical professional. Anything I say is strictly from my own personal experience.
That being said…
Anon, please forgive the assumption (and feel free to correct me!) I’m going to guess you’re female. You note that your blood tests are normal, but not what they tested you for. So I’ll give you my best general guesses.
I don’t know if there’s a disorder that evolves from bipolar, but I can describe a condition that makes it much worse. Very generally speaking, manic / depression (aka Bipolar) goes on around a 2 year cycle. Six months manic, six months leveling off, six months depressed, six moths leveling up, repeat. Your mileage may certainly vary – the point is that it’s a long cycle.
My own memory loss was triggered by a significant hormone imbalance which was never caught in a blood test. It tended to mimic Alzheimer’s, coming and going throughout the month. Initially my doctor described my diagnosis as “rapid cycle bipolar” (which is babbling nonsense – there’s no such thing) before I spoke to a female OBGYN, who suggested I had Premenstrual dysphoric disorder (PMDD). PMDD does what bipolar does, more or less. Only it does it in a month instead of 2 years. One week manic, one week level off, one week depression, one week level up, repeat. The intense mood swings aside, the rapid flux of hormones caused at least 4 migraines per month (4 days each) and left me with “hole head”. I could sometimes retrieve memories, but usually if they fell in a hole they were gone forever. It was almost as though I could watch them evaporate. Depending on the time of month even my speech slurred. If you happen to suffer from migraines you may want to look into the effect they have on the brain – in some cases they can actually cause damage and trigger memory loss in the same way a mini-stroke might.
I started taking .035 mg of progesterone daily to level out the mood swings, but it didn’t do much to improve my memory. My psychiatrist recommended fish oil to combat the worst of the symptoms. He said it was so beneficial he took it himself. It made for the healthiest brain possible with the fewest side effects. I’m not sure how much it helped, but I figured it couldn’t hurt.
What made me think of this was your comment that your mother is getting worse. When I started going through menopause, it was the worst period of my life. Every symptom I had suddenly pegged at 10 and stayed there. I thought I was literally losing my mind – I could barely finish a sentence! I feel like I was repeatedly walking into a wall for about 5 years.
The only relief I found was on the other side of menopause. I’ve been post-menopausal for about a year and the difference is amazing. I’m much calmer than I’ve been in ages, the migraines backed off and best of all I’m starting to regain my ability to think clearly. I still occasionally slur my speech but I’m hopeful that may retreat in time.
If your progressing condition is in any way connected to hormonal swings, you might want to have a talk with your OBGYN in addition to your psychiatrist. In the end, she was the one who figured out the most helpful plan of attack. To this day my psychiatrist doesn’t even believe PMDD exists. <smh>