Cognitive process in preclinical phase of dementia.

Several recent prospective studies have demonstrated the existence of a preclinical stage of dementia, identifiable by neuropsychological assessment showing impairments with a great variety of cognitive tests. However, test scores are often colinear, largely because common cognitive components are involved in different tests; in spite of an apparent heterogeneity, it is still possible that a common cognitive component may be responsible for the deterioration shown in different tests in the preclinical phase. We studied the cognitive performances of 1159 elderly residents in the PAQUID (Personnes Agées quid) cohort, at a fixed lag time of 2 years before the clinical diagnosis of dementia. Seven neuropsychological tests were administered (Mini-Mental Status Examination, Benton Visual Retention Test, Wechsler Paired-Associates Test, Isaacs Set Test, Zazzo's Cancellation Task, Digit Symbol Substitution Test and Wechsler Similarities Test). Among the initially non-demented 1159 subjects, 25 developed a dementia 2 years later, of whom 16 were classified as cases of Alzheimer's disease. In order to dissect the multicolinearity of the tests we used a multivariate approach with principal component analysis (PCA). The patients' loading on each of the first four PCA factors were subsequently correlated with the risk of dementia and Alzheimer's disease 2 years later. The logistic regression with backward stepwise selected only the first factor as an independent predictor of dementia or Alzheimer's disease. Analysis shows that there are good reasons to suspect that the first PCA factor represents a general factor corresponding to aspects of control in the tasks used. Our results therefore seem to show that preclinical deficits in dementia and Alzheimer's disease reflect the deterioration of a general cognitive factor, which may be interpreted as the disturbance of central, control processes.