In fact not only does long-term fibromyalgia not lead to dementia, it appears that the brain fog in the disorder may not get worse over time. Whatever happens to cognition early in the disease happens and that's about it. This study suggests it is not progressive.
Compared with people who had been ill for a short time, long-term fibromyalgia patients showed no evidence of cognitive decline in 14 of the 15 different measures of neurocognition, despite that fact that their cognitive problems had lasted 12.6 years longer. In fact, measures of episodic memory and processing speed (which are good markers of preclinical Alzheimer’s disease) were in the normal range in both groups.
The authors conclude that the brain fog of fibromyalgia (fibrofog) was not associated with progressive cognitive decline. As they explain, “People affected by memory problems for an additional 12.6 years might be expected to display well-advanced cognitive problems, but in reality, they were not significantly more cognitively disabled than those with a history of cognitive problems for 12 months or less.”