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Neuroscientists have defined a subtype of Alzheimer's disease (AD) that they say is neither well recognized nor treated appropriately. 

Researchers examined around 1,820 AD-confirmed brains and found that around 11 percent of them had a variant called hippocampal sparing AD. The figure suggests that this subtype is relatively widespread in the general population. 

According to the Alzheimer's Association, around 5.2 million Americans are living with AD.  And with nearly half of hippocampal sparing AD patients being misdiagnosed, this could mean that well over 600,000 Americans make up this AD variant, researchers said. 

Researchers added hippocampal sparing AD often produced symptoms that were substantially different from the most commonly known form of AD. 

The patients, mostly male, are afflicted at a much younger age, and their symptoms can be bizarre - behavioral problems such as frequent and sometimes profane angry outbursts, feelings that their limbs do not belong to them and are controlled by an "alien" unidentifiable force, or visual disturbances in the absence of eye problems, researchers said in the press release.

Patients with this subtype of AD also decline at a much faster rate than patients with most common form of AD.

"Many of these patients, however, have memories that are near normal, so clinicians often misdiagnose them with a variety of conditions that do not match the underlying neuropathology," said the study's lead author, Melissa Murray, Ph.D., an assistant professor of neuroscience at Mayo Clinic in Florida, in the press release.

"What is tragic is that these patients are commonly misdiagnosed and we have new evidence that suggests drugs now on the market for AD could work best in these hippocampal sparing patients - possibly better than they work in the common form of the disease."