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"When I am an old woman I shall wear purple … and spend my pension on brandy and summer gloves … sit down on the pavement … gobble up samples in shops and press alarm bells … go out in my slippers in the rain … learn to spit … eat three pounds of sausages."When I Am Old by Jenny Joseph

The idea that aging is associated with dramatic changes in personality, reasoning or thinking has been perpetuated for years, but this is not aging.

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Do the following scenarios sound familiar?

"My mother is fighting with her grandchildren."

"My father is no longer the life of the party."

"We don't go to church any more, because Harold hugs everyone, even people he doesn't know."

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"We can't go to restaurants because my wife, who used to be so picky, will now eat out of other people's plates."

"My mother never left the house without being 'dressed to a T' and now she doesn't shower for days."

"My husband lost all our money by making some bad investments."

"Mom, who used to be so frugal, now buys from every salesperson who calls."

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"My husband thinks I am having an affair and that his children are stealing his money."

It seems everyone knows that a change in memory, language or the ability to navigate may be a sign of dementia. But where do changes in personality or behaviour come from? Some of the same brain structures and networks that are responsible for attention and memory also have important roles in your personality, your ability to relate to others and your ability to function socially.

Not surprising then that many people developing a neurodegenerative disease such as Alzheimer's disease, Lewy body disease, frontotemporal dementia or vascular disease dementia will first exhibit changes in their personality. Their families and friends may find them withdrawn, moody and irritable. In some illnesses, their social skills deteriorate first and they may act inappropriately or utter comments they would have never said before: "You're fat," "That dress looks funny on you." They use profanity although they used to threaten to wash their children's mouths with soap. They may push their way into line. A normally gentle person may grab something out of your hand. These changes occur because a disease has taken hold of their brain.

These same illnesses that sometimes lead to difficulty with memory, concentration or language in some can also lead to a change in personality, behaviour, insight or in judgment in others. In frontotemporal dementia, the personality changes are so central to the disease that they are used as criteria for diagnosis.

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But how do you recognize whether these changes are the result of a type of dementia and not something such as depression, an anxiety disorder or just having a bad day? This can be tricky at first, which is why many people get misdiagnosed as "depressed" or worse as just "aging." Without an accurate diagnosis, people don't receive the medical attention they deserve.

But does making this differentiation really matter? Why make a diagnosis when there is no cure? Why not let people just believe it's aging?

The problem is that the changes in the brain that lead to changes in behaviour and judgment can affect more than just the patient. The person will alienate their family members and friends. They may accuse them of stealing or cheating. These delusions can be detrimental because a person can become paranoid that others, including family members and friends, want to harm them. They may lose their savings by making bad decisions or become too trusting and give money away to strangers. They stop participating in activities they once loved and become withdrawn from their families.

Going undiagnosed has consequences. There is a brain disease and named or unnamed, it will take its toll.

Although we have no cures, giving family members knowledge about what is happening can help them better prepare to deal with the changes. Understanding that a behaviour is due to an illness and not because the person doesn't care can have bearing on a relationship. The guilt that families go through when they figure out that the person's actions were the result of illness can have long-lasting consequences, sometimes for years after the person is gone. Understanding that judgment and insight are impaired because of Alzheimer's disease or another such illness can prevent misunderstandings and financial loss, keep people safe and preserve relationships.

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Your brain defines you, it's where your personality resides. It's your brain that loves, hopes and dreams as well as remembers, thinks and speaks. Diseases of the brain such as neurodegenerative diseases not only change your ability to remember and plan, they can change you.

Dr. Carmela Tartaglia is a neurologist at the Krembil Neuroscience Centre's Memory Clinic at Toronto Western Hospital, researcher at the Tanz Centre for Research in Neurodegenerative Disease and assistant professor at the University of Toronto.