Alzheimer's disease and depression: preclinical comorbidity effects on cognitive functioning.

Both Alzheimer's disease (AD) and depression (D) are prevalent disorders in old age and may co-occur in the same individual. The present study examined whether a diagnosis of D in AD has negative effects on cognitive functioning in the preclinical stage of the diseases, as well as at the time when the diagnoses were rendered. Population-based samples of 13 individuals with incident AD and D, 109 incident AD cases without D, and 179 normal older adults were followed over a three-year period. The groups were compared preclinically and at the time of diagnosis on global cognitive functioning using the MMSE total and the specific item scores, as well as the occurrence of depressive symptoms. As expected, there were clear AD-related deficits preclinically, which were exacerbated at follow-up. In addition, there were D-related deficits on three MMSE items (i.e., following commands, reading, and writing). The poorer performance on the three MMSE items was linked to an elevation of depressive symptoms. However, D was not associated with greater decline in cognitive functioning over the three-year follow-up period. Thus, although depressive symptoms may result in slight cognitive deficits in preclinical AD, at the time of the dementia diagnosis these effects may be absorbed by the neurodegenerative process.