Functional cognitive disorder: dementia's blind spot

Abstract

An increasing proportion of cognitive difficulties are recognized to have a functional cause, the chief clinical indicator of which is internal inconsistency. When these symptoms are impairing or distressing, and not better explained by other disorders, this can be conceptualized as a cognitive variant of functional neurological disorder, termed functional cognitive disorder (FCD). FCD is likely very common in clinical practice but may be under-diagnosed. Clinicians in many settings make liberal use of the descriptive term mild cognitive impairment (MCI) for those with cognitive difficulties not impairing enough to qualify as dementia. However, MCI is an aetiology-neutral description, which therefore includes patients with a wide range of underlying causes. Consequently, a proportion of MCI cases are due to non-neurodegenerative processes, including FCD. Indeed, significant numbers of patients diagnosed with MCI do not 'convert' to dementia. The lack of diagnostic specificity for MCI 'non-progressors' is a weakness inherent in framing MCI primarily within a deterministic neurodegenerative pathway. It is recognized that depression, anxiety and behavioural changes can represent a prodrome to neurodegeneration; empirical data are required to explore whether the same might hold for subsets of individuals with FCD. Clinicians and researchers can improve study efficacy and patient outcomes by viewing MCI as a descriptive term with a wide differential diagnosis, including potentially reversible components such as FCD. We present a preliminary definition of functional neurological disorder-cognitive subtype, explain its position in relation to other cognitive diagnoses and emerging biomarkers, highlight clinical features that can lead to positive diagnosis (as opposed to a diagnosis of exclusion), and red flags that should prompt consideration of alternative diagnoses. In the research setting, positive identifiers of FCD will enhance our recognition of individuals who are not in a neurodegenerative prodrome, while greater use of this diagnosis in clinical practice will facilitate personalized interventions.

Keywords: cognition; dementia; functional cognitive disorder; functional neurological disorder; mild cognitive impairment.

Figure 1

How FCD relates to other cognitive concepts. (A) Where FCD fits in relation to other key terminology used in the cognitive clinic. ‘Objective cognitive impairment’ denotes low scores on standardized testing. ‘Subjective cognitive concern’ denotes an individual’s perception of their cognitive difficulties (note some patients with MCI and dementia lack insight). Patients with FCD account for a proportion of those with MCI, and a proportion of those with SCD; rarely, those with FCD can meet criteria for dementia (i.e. severe enough to interfere with daily function and independence). Crosses represent biomarkers for neurodegenerative conditions. Biomarkers are clustered most densely among patients with dementia; a small number of true positive biomarkers also exist in the healthy population with neither subjective concerns nor objective impairment (indicating neurodegenerative tendency that has not yet manifested), and some will be false positives because a biomarker with 100% specificity seems unlikely (see McWhirter et al., 2020 for further discussion). (B) Trajectories in FCD (adapted from McWhirter et al., 2020). This illustrates the wide spectrum of potential trajectories within FCD, highlighting that some patients have considerable persisting symptoms and impairment even after serial testing, whereas others return to baseline functioning. The causes of these divergent trajectories may be explicable via co-morbidities or external factors, but often no such factors are identified. Disentangling this heterogeneity is an important area for future research. The x-axis represents each lifetime; those who remain above the x-axis to the end of their lifetime have died from other causes.