We normally think of bipolar disorder as a treatable for most patients. My experience is that if the patient is compliant with treatment (and that is a big "if"), then symptoms can be mostly controlled.
Unfortunately, there is some evidence that some persons with bipolar disorder may eventually develop more enduring memory problems and other cognitive deficits. I frequently have bipolar patients referred to me for assessment of memory problems or evaluation for possible dementia, and usually I do find mild to moderate problems.
Most of the patients referred to me in this situation are in their fifties, not in their sixties, seventies, and eighties, as usually occurs when I am testing patients for dementia. This suggests to me that there is something very different about this process than Alzheimers. It seems to develop earlier in life, causing problems in one's job. In addition, the patient seems to have more insight that they are having problems, whereas many of the patients with Alzheimers are brought by their family members and do not have insight that they are having cognitive problems. However, if there is such a thing as a bipolar dementia, we do not as yet have a clear picture of what t would look like in terms of specific symptoms like to differentiate it from Alzheimers, ischemic dementia, and so on.
In a situation of testing a person with bipolar disorder, it is always possible that their memory problems are actually due to their depression. Depression causes a generalized decrease in brain function and interferes with memory. And so, part of the evaluation always has to be looking at the current severity of depression. But the cognitive deficits associated with bipolar disorder do not appear to be linked to whether the person is currently depressed or not. There seems to be something else going on. It is possible for example that the excessive cortisol release caused by depression and mania over the lifespan have damaged the hippocampus, so that even after depression improves, there is a memory problem.
Three factors tend to make the problem worse: older age, having an onset at a younger age, and having a more severe illness overall. There is some evidence that the more episodes of depression and mania, the greater the cognitive deficits may be. Thus, the cognitive deficits in later life may be preventable if persons take management of their illness seriously and treat it conscientiously.
Problems occur in other aspects of brain functioning besides just memory. These include problems with executive functioning, concentration, and visuospatial skills.
The cognitive problems appear to be persistent and sometimes severe, so that sometimes a diagnosis of dementia may be appropriate. Usually, I do not diagnose dementia with these patients because the symptoms are not quite severe enough. I use the milder DSM-IV diagnosis of Cognitive Disorder Not Otherwise Specified. Until we have more information about the reversible or irreversible nature of this illnes and how it responds to treatment, I prefer to use the less serious diagnosis.