Basal forebrain atrophy along the Alzheimer’s disease spectrum and its relevance for subjective cognitive decline

Abstract Background There is growing evidence in the literature that the cholinergic basal forebrain might be one of the earliest affected structures in Alzheimer’s disease (AD). Recent data suggest that individuals with preclinical Alzheimer’s pathology already show atrophy of the posterior nucleus basalis of Meynert and that this even precedes the atrophy of the entorhinal cortex. Here we investigated whether basal forebrain volume reductions might not only be detectable in the mild cognitive impairment (MCI) and dementia stage of AD, but also in subjective cognitive decline (SCD) individuals who represent an at-risk population for preclinical AD, and examine the relationship with cognitive performance and amyloid-beta pathology. Methods Basal forebrain volumes of 341 participants from the multi-center German Center for Neurodegenerative Diseases Longitudinal Cognitive Impairment and Dementia Study, including 135 healthy controls, 110 SCD, 60 MCI, and 36 AD, were analyzed using high-resolution T1-weighted images. Healthy controls and SCD participants were further grouped into amyloid-positive and amyloid-negative cases according to their cerebrospinal fluid Aβ42/40 ratio. Associations between basal forebrain volume, neuropsychological performance, and amyloid load were evaluated. Results Apart from confirming progressive basal forebrain atrophy from MCI to AD, atrophy of the posterior of nucleus basalis of Meynert was also observed in subjective cognitive decline with confirmed evidence for preclinical Alzheimer’s pathology, based on the Aβ42/40 ratio. This atrophy was neither evident in subjects with SCD without amyloid pathology nor in healthy controls with amyloid pathology. Additionally, the volume of the posterior of nucleus basalis of Meynert was significantly correlated with amyloid Aβ42/40 ratio in SCD but not in healthy controls. Conclusion Our results confirm that basal forebrain atrophy occurs early along the Alzheimer’s disease trajectory. The observed volume reduction of the cholinergic basal forebrain in Aβ-positive participants with subjective cognitive complains and the absence of any volume reductions in the Aβ-positive healthy controls suggests that these ‘subjective cognitive decline’ symptoms reflect progression from stage 1 (asymptomatic) to stage 2 (transitional cognitive impairment) of the Alzheimer’s continuum (according to the recent National Institute on Aging-Alzheimer’s Association Research Framework), revealing the beginning neurodegeneration on a macroscopic level.