Vision Damage--a New Clue in Alzheimer’s

Some patients with Alzheimer’s disease have severely damaged visual nerves, a new finding that may lead to a better understanding of the nation’s most frequent cause of dementia, researchers at the USC Medical Center reported today.

The discovery also may help explain why some patients who suffer from the baffling ailment complain of impaired vision even though they test normal on standard eye exams.

The USC researchers found “distinctive” patterns of nerve-cell death in the eyes of eight of 10 Alzheimer’s patients on whom they performed autopsies. The abnormalities were not found in the eyes of 10 age-matched people who died from other causes and who served as a comparison group.

The USC study was published in today’s issue of the New England Journal of Medicine.

“This research gives us firm anatomical evidence that something is going on,” said Dr. Alfredo A. Sadun, a neuro-ophthalmologist at USC and one of the authors of the study.

Vision Complaints

“Many patients with Alzheimer’s complain that they can’t read or that they bump into furniture, even though when we test them they have 20-20 vision,” Sadun said in an interview. “The cause may be subtle impairments of vision, in addition to difficulty thinking.”

Other neuro-ophthalmologists, such as Dr. Joseph F. Rizzo of Harvard Medical School and Dr. Michael Repka of Johns Hopkins Medical Center, said the USC study is important. But they cautioned that it may not necessarily lead to advances in either diagnosing or treating the disease.

“This is an interesting additional finding and it is likely to be true,” Repka said in a telephone interview. But for now, he pointed out, “it is of no value to a particular affected patient.”

Alzheimer’s, which causes loss of memory and deterioration of other mental processes, afflicts an estimated 2.5 million people. There is no definitive diagnostic test, no specific treatment and no known cause for the disease.

All people lose some nerve cell function as they age, but Alzheimer’s patients lose nerve cells in greater numbers and at a greater speed.

So far, these patterns of brain deterioration have not appeared to be specific for Alzheimer’s because similar changes have also been found during autopsies of elderly patients who were not demented.

Used a New Method

The USC researchers used a new method of staining nerve cells to detect degeneration of the optic nerve. The nerve is a cable usually containing more than a million nerve fibers that connect the visual receptors in the eye to the brain.

In the Alzheimer’s patients, the nerve fibers were sparsely packed and a form of scar tissue had accumulated, while in the control patients the fibers were tightly packed in bundles.

“It is extremely unlikely that eight of the 10 patients had severe bilateral optic neuropathies unrelated to Alzheimer’s disease,” the researchers concluded.

Physicians can examine the optic nerve directly by looking into the interior of the patient’s eye with a special instrument called an ophthalmoscope.

Changes in the optic nerve may be particularly significant in diagnosing diseases that may alter the appearance of the nerve, such as glaucoma. But the usefulness of the test is limited because patients sometimes must lose up to 50% of the fibers of their optic nerve before it begins to appear abnormal.

Nerve May Look Normal

In Alzheimer’s, USC’s Sadun estimates that patients lose at most 30% to 40% of their optic nerve fibers. “The optic nerve may look normal,” however, he said.

Ophthalmologists can use a variety of other visual tests to detect more subtle abnormalities of the optic nerve, such as evaluation of color vision. But these tests require that the patient have good mental function and often cannot be used in patients with dementia.

Sadun said his current research is focused on developing new techniques to diagnose visual impairment in Alzheimer’s patients.