Objective The term “functional cognitive disorder” (FCD) has been proposed to describe a subjective experience of cognitive dysfunction in the absence of underlying brain pathology. Features of FCD include discrepancies between cognitive symptoms and objective performance, memory perfectionism, over-interpretation of attentional lapses and heightened self-monitoring for cognitive errors. Fibromyalgia and non-cognitive functional neurological disorders (FND) such as non-epileptic attacks (NEA) can be associated with subjective cognitive symptoms, sometimes referred to as “brain fog”. The relationship between archetypal FCD and cognitive symptoms in the context of fibromyalgia and FND is unclear. We conducted a systematic review of cognitive findings in fibromyalgia and FND in order to define their key neuropsychological hallmarks. Our hypothesis was that the cognitive profile associated with functional motor and pain syndromes (i.e. brain fogs) will be similar to that seen in isolated FCD.

Method We performed a systematic review of studies evaluating cognition in fibromyalgia and FND. We identified 56 studies on fibromyalgia and 40 studies on FND. Study characteristics were highly heterogeneous.

Results A high frequency of cognitive symptoms was described in fibromyalgia but no consistent neuropsychological deficits were seen. However, some authors found memory symptoms to correlate with attentional abnormalities (in particular tests of divided attention), and these to pain. In NEA problems with memory and attention were also reported. However, most studies comparing NEA with epilepsy did not report generalised neuropsychological differences. Discrepancies between cognitive symptoms and objective performance were rarely investigated, and evidence was inconsistent. Effort testing was rarely included in neuropsychological assessment, particularly in fibromyalgia. However, only a minor proportion of patients failed on effort testing. Notably, effort definition and assessment tools were highly variable. “Memory perfectionism” was not evaluated.

Conclusion Both fibromyalgia and FND appear to show prominent cognitive symptoms. Regarding objective performance, reduced attentional reserve has been postulated to explain symptoms such as distractibility and attentional lapses. Evidence for discrepancies between cognitive symptoms and objective performance, and for poor effort on testing, remains contradictory. Overall, although limited, the data suggest that the neuropsychological mechanisms underlying ”brain fog” are similar to those in isolated FCD.