Imagine your surprise when a colleague, who's normally a sharp dresser, shows up in a checked sports coat with striped suit slacks.

Six months later, he's having trouble pronouncing and finding words. Then at 52, he can no longer write. His mind is sharp yet the hard part for his family is he doesn't care about anyone or anything.

This disorder is called Frontotemporal dementia or FTD. It used to be considered rare but now FTD is the most common cause of early onset dementia after Alzheimer's.

The typical age when symptoms begin is between 45 and 65 years old. Patients die within five to fifteen years.

What happens in people with FTD is the outer layers of the brain – just above the eyes and near the ears ' begin to shrink. Patients can have a hard time making or understanding speech.

But most often, they have behavioral changes.

Some behavior becomes impulsive and can be socially inappropriate. Patients can lack empathy, express more interest in sex, and even change their food preferences. Others become apathetic, which can include blunted emotions, bad personal hygiene, and low energy.

In the majority of FTD cases, small pockets of brain cells die off, leaving the front part of the brain pockmarked with holes. A smaller percentage of cases are called Pick's disease, where there's a buildup of an insoluble protein called tau in the brain.

In both these diseases, researchers linked the cause to mutations in genes on chromosome 17. But more recently, scientists are identifying chromosomal abnormalities passed on in families. In fact, in forty percent of FTD cases, patients have a family history of the disease.

Now scientists are studying the brains of 271 patients who had FTD before they died. They hope to better understand this disease, which for now has no cure or way of slowing its progression.