Behavioral changes such as depression and irritability may be the first symptoms of the onset of dementia or Alzheimer's disease, researchers say.

These could be early, non-cognitive kinds of symptoms, a study by the Washington University School of Medicine in St. Louis suggests.

Non-cognitive symptoms are those involving self-control and emotions, as opposed to cognitive symptoms centered on memory and thinking, they explain.

For the study, the researchers looked at 7 years' worth of data on 2,416 people age 50 and above who had no memory or thinking problems at the time of their first visit to one of 34 Alzheimer's disease centers located in the United States

Of that number, slightly more than half eventually developed dementia in the study period, and they experienced behavioral changes, such as depression, apathy, sleep problems and irritability, earlier than those in the study who did not go on to develop dementia, the researchers report in the journal Neurology.

"We are trying to get a picture of some of the symptoms that occur along with memory and thinking problems when people get early dementia," said study author and neurology professor Catherine Roe.

Previous research found 90 percent of patients with Alzheimer's disease also displayed non-cognitive symptoms.

However, Roe cautioned about putting too much emphasis on what can be very common behaviors.

"I wouldn't worry at this point if you're feeling anxious, depressed or tired that you have underlying Alzheimer's, because in most cases it has nothing to do with an underlying Alzheimer's process," explains.

Whether depression and other mood or behavioral changes might result from the same physical changes in the brain underlying Alzheimer's disease or they're a psychological response to dealing with the condition is yet to be determined, Roe points out.

Showing an association between changes in behavior and an increased risk of developing Alzheimer's is not the same as proving a cause-and-effect link, the researchers emphasize.

Still, anyone noticing significant mood or behavioral changes might want to consider speaking to a physician, says Keith Fargo, director of scientific programs and outreach for the Alzheimer's Association.

"Don't try to tough it out and don't try to wait for it to go away," he says. "Those things are probably manageable through either lifestyle measures or medication, or they may be indicative of something larger going on such as dementia or Alzheimer's."

Alzheimer's disease, which causes not just memory loss but changes in personality, reasoning and judgment, currently afflicts more than 5 million Americans.