The characterisation of subjective cognitive decline

Abstract

A growing awareness about brain health and Alzheimer's disease in the general population is leading to an increasing number of cognitively unimpaired individuals, who are concerned that they have reduced cognitive function, to approach the medical system for help. The term subjective cognitive decline (SCD) was conceived in 2014 to describe this condition. Epidemiological data provide evidence that the risk for mild cognitive impairment and dementia is increased in individuals with SCD. However, the majority of individuals with SCD will not show progressive cognitive decline. An individually tailored diagnostic process might be reasonable to identify or exclude underlying medical conditions in an individual with SCD who actively seeks medical help. An increasing number of studies are investigating the link between SCD and the very early stages of Alzheimer's disease and other neurodegenerative diseases.

Figure 1:. Schematic representation of trajectories of SCD and objective cognitive function over time

(A) SCD occurs but remits fully, and objective cognitive functions remain stable. Conditions underlying SCD include depression, side-effects from medication, or intermittent sleep disturbances. (B) SCD occurs and continues without remitting (continuous green colour); objective cognitive functions remain largely stable. The normal ageing process can be an underlying cause of this type of SCD. (C) SCD occurs and objective cognitive functions subsequently deteriorates to dementia. This deterioration can be caused by neurodegenerative disease, including, but not limited to, Alzheimer’s disease. The proportion of individuals in each of the three SCD trajectories has not yet been fully delineated; however, the majority of individuals with SCD will not decline to dementia. The colour scheme indicates an absence of SCD (white), SCD (green), and objective impairment or dementia (red). SCD=subjective cognitive decline.

Figure 2:. Symptomatic stages of Alzheimer’s disease according the NIA-AA research framework

The schematic shows the symptomatic stages of disease in Alzheimer’s disease according to the NIA-AA research framework of Alzheimer’s disease. The stages apply only to individuals who are in the Alzheimer’s disease continuum, which is defined by biomarker evidence of amyloid pathology with or without tau pathology, and is irrespective of the status of neurodegeneration. The colour scheme indicates the continuous progression of cognitive impairment in an individual, from no objective cognitive impairment (light green) to severe objective cognitive impairment (dark red). According to the framework, there are six stages of symptomatic manifestation of Alzheimer’s disease. Stage 1 is the fully asymptomatic disease stage. Stage 2, also known as the transitional stage, includes individuals showing the first subtle signs of Alzheimer’s disease, manifesting as a combination of one or more of the following symptoms: subjective cognitive decline, subtle objective decline, or mild behavioural symptoms. Stage 1 and stage 2 are classified as preclinical Alzheimer’s disease, according to the 2011 NIA-AA criteria. Stage 3 reflects, in principle, mild cognitive impairment, whereas stages 4 reflects mild dementia, stage 5 reflects moderate dementia, and stage 6 reflects severe dementia. NIA-AA=National Institute of Aging-Alzheimer’s Association.