In their frustration at being unable to treat victims of Alzheimer’s disease, some researchers now think that the condition has to be attacked at least 10 years before it changes people’s behavior. But nobody knows how to recognize the problem that far in advance.

“By the time an Alzheimer’s patient is diagnosed even with mild or moderate Alzheimer’s there is very, very extensive neuron death,” researcher John Morrison of Mount Sinai Medical School in New York tells NPR. “And the neurons that die are precisely those neurons that allow you to navigate the world and make sense of the world.”

Once the neurons die, the researchers say, no amount of rehabilitation can return them to life. The researchers believe that is the reason many recent trials of new Alzheimer’s drugs have failed to improve patient mental health.

“It’s possible we’ve tested things and discarded them prematurely,” says Sam Gandy, also a researcher at Mount Sinai. “Now that we can see the changes in the brain that underlie Alzheimer’s, we really should re-evaluate some of the things we’ve looked at.”

The researchers say the treatment could involve giving middle-aged Americans with a family history of Alzheimer’s disease drugs that might reduce their risk of developing the devastating mental illness.