Researchers studying the changes Alzheimer's triggers in human brains say the disease affects men and women differently.

The same disease causes more shrinkage in the brains of men, and less in women -- results that surprised the Canadian scientists doing the research.

"We thought women would show more atrophy than men because women are more vulnerable to Alzheimer's disease than men," said Dr. Sandra Black, a neurologist with Sunnybrook & Women's College Health Sciences Centre. "The surprising thing was that the opposite was found. Actually the men showed more atrophy."

The study is the first to examine specific sex differences in the portion of the brain believed to handle emotions and memory. It examined the brains of 40 men and women with probable Alzheimer's, as well as 40 control subjects.

Researchers have already shown that the brains of both men and women with Alzheimer's tend to have similar differences in comparison to brains of people without the disease. So, this latest study was designed to identify whether there were differences in the size of, or blood flow in, male and female patients' brains.

"The men displayed more atrophy (shrinkage) in the orbitofrontal cortex, middle and posterior cingulate cortices, hypothalamus and mamillary bodies, and more relative hypoperfusion (decreased blood flow) in the anterior and middle cingulate cortices," lead investigator Dr. David Callen said.

"The women, however, showed only anterior thalamic atrophy."

Despite the differences in brain changes, the report notes that women with less hypoperfusion and atrophy exhibit the same level of cognitive impairment as men whose brains appear worse off.

By controlling for other factors, such as differing levels of education and head size, the researchers say the biological mechanism behind the sex difference remains the subject of speculation.

But it could lead to new diagnostic tests and perhaps even different forms of drug treatments for men and women.

"What we suspect is that it will be important to have separate analysis for men and women," Black said. "So that you are treating them not together, but as separate groups when you are looking at how they respond to treatment."

The cause of the progressive, degenerative neurological illness that gradually robs victims of their independence as brain cells shrink or disappear is also a mystery.

It's estimated that 4.5 million Americans suffer from the disease named by Dr. Alois Alzheimer nearly a century ago. In Canada, approximately 280,000 people over the age of 65 suffer the toll Alzheimer's exacts on their ability to think, remember, understand and make decisions.

Although there is no known cure for the disease, some of its symptoms can be treated with drugs.

Direct annual costs of treating Alzheimer's Disease and related dementias in Canada has been pegged at roughly $5.5 billion.