A comparison of multiplex and simplex families with Alzheimer's disease/senile dementia of Alzheimer type within a well defined population.

A study was made on 150 clinically demented patients presenting at autopsy at Umeå University Hospital in Sweden. In 90 of the cases dementia was considered to be primary in nature and of these forty six per cent (41 cases), fulfilled both the clinical and histopathological criteria for the diagnosis of Alzheimer's disease/Senile dementia of Alzheimer type (AD/SDAT). The families of these 41 AD/SDAT cases were then studied, and a family history obtained through interviews with multiple family informants and from civil and medical records. Additional diseased family members suffering from progressive dementia (multiplex families) were observed in 12 probands out of 41 (29%). Multiplex families exhibited similar clinical and histopathological characteristics as simplex families containing a single affected individual. The secondary cases in the multiplex families exhibited similar demographic and clinical characteristics as the probands. 39% of the multiplex and 14% of the simplex cases had an early age of onset of the disease, that was under 65 years. The overall prevalence of progressive dementia disorders in the 41 families was 5.9%. The prevalence of a progressive dementia disorder was 11% in the multiplex families (14% for the early onset cases) and 3.5% in the simplex families (2% for the early onset cases). The prevalence of progressive dementia disorder for family members who had passed the mean age of the onset of the disease for their family, was 45% for multiplex and 18% for simplex families. Furthermore the incidence rate for dementia was significantly higher (p < 0.005) in multiplex families (5.5 per 1,000 person years) when compared to simplex families (2.5 per 1,000 person years). No differences could be seen in parental age at birth of the diseased when comparing the two sets of families. However in multiplex families the duration of the disease was significantly (p < 0.025) shorter, in subjects with parental age at birth over 35 years compared to those with a parental age under 35 years. The multiplex families contained significantly (p < 0.025) larger sibships; and showed a significantly lower age of onset for the disease (p < 0.001), and a significantly longer duration of disease (p < 0.05) compared to the simplex families. A significant intra familial correlation of age at disease onset was observed in both sets of the families.