Could a buildup of toxins in cerebrospinal fluid be causing Alzheimer's disease? Stanford University physician Edward Rubenstein hypothesized just that in 1998, noting how in young people, the fluid (which serves as a watery cushion for the brain) is refreshed about four times a day-twice as often as in the elderly.

Doctors at Stanford are now trying to find out whether draining stagnant fluid can slow, or even reverse, the fatal condition. The approach: a 1-meter tube that drains fluid from the brain into the abdomen. CS Fluids, a Redwood City, Calif., startup, has raised $4 million to develop the device, called COGNIShunt. In the nine elderly patients who've received the treatment so far, doctors have seen big drops in the levels of proteins known to damage brain cells.