Many people are probably familiar with the idea that Alzheimer’s disease is linked to the dangerous accumulation of plaques in the brain (amyloid-beta plaques, to be exact). But just why the plaques accumulate in some people’s brains, leading to Alzheimer’s, has presented a bit of a mystery — till now. The authors of a new study say that it’s not the production of the plaques that’s the problem, it’s their clearance. In other words, Alzheimer’s brains seem to have trouble ridding themselves of the plaques as healthy brains can.

The brains of people with Alzheimer’s cleared out the plaques 30% slower than did the healthy controls. The plaques were produced at virtually the same rate in both groups of participants.

To explore the idea, researchers looked at how fast the amyloid-beta plaques were produced in the brains of people with Alzheimer’s disease and in healthy individuals, and how fast the plaques were cleared from the brain.

Over a period of 36 hours, they found that there were significant differences — not in the rate of production of the plaques, but in how fast they were removed from the brain. The brains of people with Alzheimer’s cleared out the plaques 30% slower than did the healthy controls. The plaques were produced at virtually the same rate in both groups of participants.

Though the results seem solid, the authors point out that cause and effect was not actually shown in the study. But given the findings described above and the fact that there also was less beta-amyloid in the cerebrospinal fluid of the Alzheimer’s patients (suggesting that less was being shunted away from the brain), the team says that it’s pretty likely that the clearance rate is the issue at play here. Future research will need to address just why some people’s brains develop problems clearing out the plaques over time.

The results seem extremely promising, and are very likely a big step on the road to understanding this devastating and mysterious disease.

The study was carried out by researchers at Washington University School of Medicine and published in the December 9, 2010 online issue of Science.