Evidence of Subtypes of Alzheimer's Disease and Implications for Etiology

OBJECTIVE: Because age of onset does not reliably define two subtypes of Alzheimer's disease, classification based on the severity of neuronal degeneration was tested. DESIGN: Numbers of extracellular tangles and pyramidal neurons in the hippocampus were used to group patients. PATIENTS: The study population consisted of 46 elderly patients satisfying DSM-III criteria for dementia and NINCDS-ADRDA criteria for definite Alzheimer's disease after death. RESULTS: Univariate logistic regression analysis showed the numbers of neurofibrillary tangles and pyramidal neurons and the duration of dementia were significantly associated with grouping based on the presence of abundant extracellular tangles. Ninety-one percent of patients were correctly classified as compared with 85% correctly classified by age of onset data. Odds ratios showed that increasing numbers of neurofibrillary tangles predicted greater severity of neuronal loss. CONCLUSION: The results of the study indicate the importance of neurofibrillary degeneration, not the deposition of amyloid, in the pathogenesis of Alzheimer's disease. They support a classification of Alzheimer's disease related more closely to the severity of neurofibrillary degeneration than to age at onset.