The risk of developing Alzheimer's disease skyrockets when both parents are struck by the degenerative brain disorder, a study published Monday has found.
The dementia also seems to strike at an earlier age among patients with a wide family history of the disease, the study published in the Journal of the American Medical Association's Archives of Neurology found.
"Because Alzheimer's disease is so common in the general population, it is not uncommon for both spouses to develop the disease," wrote lead author Suman Jayadev, of the University of Washington, Seattle.
"Offspring of two such affected individuals would presumably carry a higher burden of these Alzheimer's disease-associated genes."
Jayadev and her colleagues studied the frequency of Alzheimer's disease in adult children of 111 families in which both parents had been clinically diagnosed with the disease.
They found that of the 297 offspring who reached adulthood, 22.6 percent developed Alzheimer's disease compared with an estimated six to 13 percent of the general population.
But since 79 percent of those still untouched by the disease had not yet reached the age of 70, it is likely that the actual risk is higher, the authors concluded.
That's because they found that the risk of developing the disease increased with age: 31 percent of those older than age 60 and 41.8 percent of those older than age 70 had been diagnosed with Alzheimer's.
At age 80, the cumulative risk was determined to "beyond 60 per cent." The presence of Alzheimer's disease elsewhere in the family did not seem to increase the likelihood of developing the disease.
But it was linked to an earlier onset: those whose family history of Alzheimer's was limited to their parents had an average onset of 72 years; those who had one parent with a family history of Alzheimer's had an average onset of 60 years; and those with a history of Alzheimer's on both sides of their family had an average onset of 57 years.