Alzheimer’s disease is the sixth leading cause of death in the US, affecting more than 5 million Americans. But researchers from the Mayo Clinic in Florida say they have identified a subtype of the disease that is often misdiagnosed.

The research team, led by Dr. Melissa Murray, an assistant professor of neuroscience at the Mayo Clinic, says their study suggests that around 600,000 Americans may have this variant, which they call “hippocampal sparing” Alzheimer’s disease.

To reach their findings, recently presented at the American Academy of Neurology’s Annual Meeting in Philadelphia, PA, they analyzed 1,821 brains with confirmed Alzheimer’s disease.

All subtypes of Alzheimer’s have two specific hallmarks in the brain. Amyloid beta is responsible for the formation of brain plaques, while tau produces tangles in the brain.

In order to classify each subtype, the team used tangle counts to create a mathematical algorithm. They found that while all Alzheimer’s subtypes had the same amount of amyloid beta, the hippocampal sparing variant showed tau tangles in unequal areas of the hippocampus.

Dr. Murray further explains the findings in the video below:

They discovered that in patients with this sub-type, tau specifically damages neurons in areas of the brain associated with behavior, motor recognition and awareness, and use of speech and vision.

‘Alzheimer’s disease does not necessarily equate to loss of memory’

Hippocampal sparing Alzheimer’s was identified in 11% of patients. They define the variant as a form of the disease that appears to have minimal impact on memory.

Instead, the condition appears to cause behavioral problems, such as uncontrollable and frequent outbursts of anger. The disorder may also trigger the feeling that the patient’s limbs do not belong to them and that movements are controlled by an “alien” force. Furthermore, it can cause visual interruption and language problems.

From their research, the investigators found that hippocampal sparing Alzheimer’s appears to be more common in males, with onset occurring at a much younger age than traditional Alzheimer’s. In addition, patients with this subtype seem to deteriorate more rapidly.