Comorbid depressive symptomatology or diagnosed depression is not more prevalent in early, mild or severe Alzheimer's disease. This is an important conclusion from a recent systematic review. Before theories existed about the relationship between Alzheimer's and depression. These theories said that the prevalence of depression either decreases or increases with the increasing severity of Alzheimer's disease.
This is comparable to the relationship between depression and personality, does personality predispose to depression or is personality affected by going through depression.
Previous reviews stated that no conclusions could be drawn due to large differences between studies. This review tried to overcome this problem.
They made sure all studies consisted of patients with Alzheimer, valid assessments of depression and severity of Alzheimer were performed.
Instead of a linear correlation between Alzheimer's disease and depression this result could be explained by a theory that neurological and psychosocial factors can reinforce or diminish each other, depending on the specific situation of a patient.
The real answer can only come from a longitudinal study with Alzheimer patients.
Some news about the relation between depression and Alzheimer on NYT:
A history of depression is associated with an increased risk for Alzheimer’s disease, a new study has found, especially when the depression develops before age 60.
Verkaik, R., Nuyen, J., Schellevis, F., Francke, A. (2007). The relationship between severity of Alzheimer's disease and prevalence of comorbid depressive symptoms and depression: a systematic review. International Journal of Geriatric Psychiatry, 22(11), 1063-1086. DOI: 10.1002/gps.1809