Apathy and depression are the most frequent neuropsychiatric symptoms symptoms in Alzheimer's disease (AD). In a cross-sectional observational study of 734 subjects with probable mild AD, we evaluated the prevalence of apathy and depression. After the use of specific diagnostic criteria, we tested the interaction between the two syndromes and their relation with specific comorbidities, and different functional outcomes. Depression was diagnosed using the diagnostic criteria for depression in AD, and apathy with the diagnostic criteria for apathy in neuropsychiatric disorders.

According to the specific diagnostic criteria, depression has a 47.9% prevalence, as apathy prevalence is 41.6%. Apathy and depression are associated in 32.4% of patients (n=225). 9.4% (n=65) have only apathy, 15.4% (n=107) have only depression, and 42.9% have no apathy and no depression (n=298).

The three most frequent depressive symptoms are fatigue or loss of energy (59.4%), decreased positive affect or pleasure in response to social contacts and activities (46.2%) and psychomotor agitation or retardation (36.9%). Concerning apathy, loss of goal-directed cognition is the most frequently altered (63.6%), followed by loss of goal-directed action (60.6%) and loss of goal-directed emotion (43.8%). Patients with both apathy and depression more frequently require a resource allowance for dependency. Neurological comorbidities were more frequent in the “apathy and depression” and “depression alone” groups (p<0.001).

Apathy and depression overlap considerably, and this might be explained by the presence of some non-specific symptoms in both diagnostic criteria. The need for social support is higher when a patient fulfills the two diagnostic criteria.