“My grandfather had Alzheimer’s disease. I am having some trouble finding the words I want but my memory is ok. What are my chances of developing Alzheimer’s disease?”

First, the greatest risk for developing Alzheimer’s disease is age. The prevalence for developing Alzheimer’s disease before age 60 is less than 2%. Prevalence increases to less than 15% by age 80 and about 70% by age 100 by some estimates. In short, we are all at risk for developing Alzheimer’s disease if we live long enough.

Early-onset Alzheimer’s disease (before age 65) is rare and develops between 30 and 60. In some cases, early onset is familial Alzheimer’s disease with genetic mutations on chromosomes 21, 14, and 1. In these rare cases, the inheritance is referred to as “autosomal dominant.” The offspring in the same generation have a 50/50 chance of developing Alzheimer’s disease if one of their parents had it — the same pattern as for Huntington’s chorea. There are strong trends in the family of early-onset Alzheimer’s disease with three or more family members diagnosed with Alzheimer’s disease before age 60.

For the rest of us we are at risk of developing late-onset Alzheimer’s disease. The answer for “what’s my risk” is genetically complicated. The risk for those with no first degree relatives (parents or siblings) with a diagnosis of Alzheimer’s disease is 7% or less. The risk for those with one first degree family member is about 15-20%. For second degree relatives (e.g., grandparents) the risk may be about 10%. Even for identical twins the risk is not 100% (it is 84%).

In short, the risk for those with first degree relatives who develop Alzheimer’s disease before age 85 may be about three to four times the risk as compared to those who have no Alzheimer’s disease in first degree relatives. Genes are clearly not destiny for those with late-onset Alzheimer’s disease in their family.

Briefly, there is a “genetic marker” for Alzheimer’s disease called the Apolipoprotein effect (APO). Each of us has a variant of the APO gene as 2, 3, or 4. The APO 4 variant is associated with greater risk of late-onset Alzheimer’s disease. However, even those with APO 4 have a better than even chance of not developing Alzheimer’s disease. Therefore, the APO genotype is of little value in clinical diagnosis or prognosis.

The risk of getting Alzheimer’s disease is very complicated. Furthermore, having mild Alzheimer’s disease is not disabling. Many who have Alzheimer’s disease are either not demented or, if they have planned well, still lead a very good life. Early detection of changes in memory is still the best protection in combination with a proactive treatment program. If you are at risk or worried, seek out a good evaluation that focuses on challenging memory evaluation.