By Lisa Ravdin, PhD

Associate Professor of Neuropsychology in Neurology, Weill Cornell Medicine, New York

SYNOPSIS: Functional cognitive disorder (FCD) can describe cognitive difficulties that are present where there is no biologic cause, but a lack of consensus in diagnostic criteria limits its use in clinical practice and research. The authors proposed an operational definition for FCD as the cognitive phenotype of functional neurological disorder.

SOURCES: Ball HA, McWhirter L, Ballard C, et al. Functional cognitive disorder: Dementia’s blind spot. Brain 2020;143:2895-2903.

Kapur N, Kemp S, Baker G. Functional cognitive disorder: Dementia’s blind spot. Brain 2021;144:e37.

Subjective cognitive complaints that present without an identifiable cause or objective evidence of impairment are observed commonly and may present at greater rates in older adults. The term functional cognitive disorder (FCD) has been used to describe unexplainable persistent cognitive complaints and can be seen as a cognitive variant of the broader term, functional neurological disorder (FND).

The authors conveyed that internal inconsistency is at the core of the diagnosis of FCD, which is present when the patient’s subjective sense of cognitive dysfunction is discrepant with intact objective test scores, presentation, and independence in activities of daily living, as well as a collateral’s report reflecting reduced concern compared to that reported by the affected person. In FCD, there is variability in performance within a particular cognitive domain where an individual shows “the ability to perform a task well at certain times, but with significantly impaired ability at other times, particularly when the task is the focus of attention.”

This is not simply normal variability where performance fluctuates over time as in cognitive disorders that have waxing and waning symptoms. Internal inconsistency needs to be seen within a particular cognitive domain. This also is discrepant from individuals who intentionally perform poorly and fail effort testing (i.e., malingerers).

FCD is common in clinical practice but is rarely diagnosed as such. Patients with subjective cognitive complaints with no identifiable neurologic disorder often are diagnosed with mild cognitive impairment (MCI) or subjective cognitive decline. The authors contended FCD terminology also could be useful to de-emphasize the expectation that these subjective cognitive complaints necessarily progress to dementia. Importantly, definitions of FCD lack consensus, and the unclear trajectory of these symptoms, as well as the likelihood of comorbidity with underlying neurodegenerative processes, precludes its common use and understanding in clinical and research settings. The authors proposed an operational definition for FCD as the cognitive phenotype of FND.

In a letter to the editor, Kapur et al indicated some of the main points raised in the formulation of the FCD definition proposed by Ball et al are potential sources of confusion. Specifically, differentiating between internal and external inconsistency and its applicability to the diagnosis is questioned.

These authors noted using this definition does not account for naturally occurring neurologic presentations that have features of inconsistency. It is suggested that this definition of FCD has an overreliance on internal inconsistency. Further, these authors recognized the neuropsychological evaluation examines patterns on cognitive testing, and these exams do, in fact, include consideration of non-organic factors in the interpretation of neurocognitive data, including fluctuating attention, alertness, effort, and environmental factors, both within and between cognitive domains.

COMMENTARY

It is not uncommon for individuals to present with cognitive complaints, even when there is no biologic evidence that meets the threshold of a diagnosable disorder. This often is labeled as MCI. When used in this manner, MCI is a term that does not convey diagnostic specificity, essentially creating a blind spot in discriminating subjective vs. objective cognitive compromise. The neuropsychological evaluation can help identify patient-specific factors that contribute to cognitive complaints as well as interpret patterns of performance that can be useful in examining subjective vs. objective cognitive concerns. Patients with cognitive complaints may benefit from evidence-based interventions that target factors that create or amplify the experience of cognitive dysfunction, such as depression, anxiety, sleep problems, substance use, stress, and chronic pain.