Relationship between plaques, tangles, and dementia in Down syndrome.

In patients with Down syndrome, senile plaques and neurofibrillary tangles accumulate in the cortex at an earlier age than in persons of normal karyotype. We studied 20 Down syndrome patients dying after age 30 (average age, 49); all had neocortical plaques and tangles, but only 3 of 20 had been demented. In 12 cases (average age, 53), tissue was available for quantitative study of plaque and tangle densities and estimation of cell loss in the hippocampus. Although at least 8 of these 12 cases had plaque and tangle densities comparable to those previously reported in demented old people, only 1 had dementia. The regional distribution of plaques and tangles in the hippocampus of these Down cases differed from the pattern in senile dementia. Although Alzheimer-like dementia occurs in Down disease, it is less prevalent than the plethoric plaques and tangles in the cortex.