Agnès Pérez Millan and her team at Universitat Oberta de Catalunya have cracked a puzzle that has eluded neuroscientists for decades: Alzheimer's disease doesn't attack the brain evenly. This discovery, published in Brain Communications, could transform how doctors detect and track one of the world's most devastating neurodegenerative diseases—potentially identifying it years before symptoms ever appear.
For a long time, researchers believed that Alzheimer's followed a symmetrical pattern of damage, affecting both sides of the brain equally, much like healthy aging does. But Pérez Millan's international team discovered something different. While healthy people experience balanced gray matter loss as they age, and patients with frontotemporal dementia show dramatic asymmetrical damage, Alzheimer's patients fall somewhere in between—with uneven deterioration spread across different brain regions. The insight came almost by accident. "We were studying another disease, frontotemporal dementia, which is very asymmetrical, and we wanted to compare patients' brains with those of healthy patients and patients with Alzheimer's disease," Pérez Millan explained. "We assumed that patients with Alzheimer's disease would have a symmetrical brain similar to healthy individuals, but we discovered that this was not the case."
That unexpected finding became the seed for the new brain asymmetry index. The team studied 624 individuals in total: 60 participants from Hospital Clínic in Barcelona and 564 from the DIAN project led by Washington University in St. Louis, which includes European patients. Using magnetic resonance imaging to measure cortical thickness, they applied an algorithm to detect differences in how gray matter thins unevenly across the brain. The resulting index quantifies this asymmetrical deterioration with remarkable precision.
What makes this work particularly significant is the timeline at stake. Alzheimer's disease begins its invisible remodeling of neural tissue 15 to 20 years before any symptoms emerge. From that early stage through diagnosis and into dementia, the disease progresses along a continuum—and this new index can track that continuum with accuracy. "We see that the higher the value of the index, the more asymmetry in the brain, which correlates with greater neurodegeneration and more symptoms," Pérez Millan said. The index successfully distinguished between people with Alzheimer's disease and healthy individuals, opening doors to earlier detection and intervention.
The hippocampus, the seahorse-shaped region responsible for memory and learning, is one of the brain's most vulnerable structures during aging. In Alzheimer's patients, this critical area shows the uneven thinning pattern the team identified. While the index doesn't pinpoint exactly where deterioration is most severe—it's a general measurement rather than region-specific—it does something perhaps more valuable: it measures disease progression across the stages that precede dementia.
The implications extend beyond diagnosis. Researchers envision using this asymmetry index as a biomarker to identify Alzheimer's disease earlier, track its progression more accurately, and assess how well new treatments actually work. For patients with genetic forms of Alzheimer's, which account for fewer than 1% of cases but follow a more predictable trajectory, this tool could be particularly transformative. The discovery that Alzheimer's rewrites the brain's symmetry offers a new window into a disease that has long kept its secrets hidden.