Alzheimer's disease take a curious turn.

Physicians have described several cases of an extremely rare form of Alzheimer's disease that primarily assaults not memory but visual perception.

Although sufferers possess no eye defects, they often find themselves unable to drive a car in a straight line or to determine their distance from other cars while driving. Their ability to perform many other tasks that require hand-eye coordination or sustained visual attention also declines sharply.

Cerebral defects that trigger this type of Alzheimer's disease, which includes only moderate memory difficulties, may differ greatly from those that contribute to the condition's more common incarnation, a new study finds.

"We've found that Alzheimer's disease can begin in a separate neuronal system from the one it's usually thought to be linked to," contends study director Pietro Pietrini, a neuroscientist at the National Institute on Aging in Bethesda, Md. "In this subgroup of patients, Alzheimer's disease may occur well before any memory complaints."

Pietrini's group examined 10 individuals diagnosed with Alzheimer's disease who displayed primarily visual and spatial perception problems but some memory loss as well. Three of the patients have since died, and analyses of their brains revealed tissue changes consistent with Alzheimer's disease, Pietrini says.

The study also included 22 people whose Alzheimer's disease featured severe memory impairment without visual complaints and 25 adults without the disease.

Volunteers in all three groups averaged 65 years old.

As each participant rested with his or her eyes covered and ears plugged, researchers took positron emission tomography (PET) scans of cerebral glucose use, an indicator of the vigor with which groups of neurons function.

Relative to healthy adults, those with the visual variant of Alzheimer's disease displayed markedly lower glucose use in areas at the back of the brain, including the primary visual cortex, Pietrini and his coworkers report in the October American Journal of Psychiatry. The same individuals showed relatively normal activity in other regions, including some of those associated with memory.

In contrast, Alzheimer's disease patients who had no visual problems exhibited declines in glucose use throughout much of the brain, but the visual cortex was largely spared.

The visual form of Alzheimer's disease may stem from disconnections in a brain pathway that mediates the perception of motion and spatial relations among objects, Pietrini theorizes.

This unusual condition may fall at one end of a continuum of Alzheimer's disease symptoms, or it may represent a distinctive disorder, notes neuroscientist James V. Haxby of the National Institute of Mental Health in Bethesda, Md.