Visual clarity: people with MS maintain eyesight with drug.

A medication prescribed to limit nerve damage caused by multiple sclerosis also seems to prevent subtle vision loss in many patients.

People with multiple sclerosis (MS) experience pain, weakness, loss of muscle control, and slurred speech as a result of inflammatory damage to the fatty sheaths that insulate nerves. More than half of people with the illness also have episodes of blurry vision caused by optic neuritis, inflammation of the nerves that carry signals from the eyes to the brain.

In 2004, the Food and Drug Administration approved the drug natalizumab (Tysabri) for MS after two clinical trials found that monthly injections alleviated some MS symptoms, which flare up during relapses. In one trial, participants received either natalizumab or a placebo. In the other trial, all the participants were taking interferon beta, a standard MS drug, and also received natalizumab or a placebo.

Natalizumab stops nerve-sheath damage by neutralizing a protein that facilitates entry of inflammatory immune cells into the central nervous system (SN: 3/4/06, p. 131).

The new report analyzes the data on visual acuity that were collected during the 2-year studies. In those trials, doctors monitored 2,113 patients' sight by using a standard eye chart with black letters and another chart, with pale-gray letters, that measures low-contrast vision. In both trials, natalizumab didn't affect the scores of the participants on the standard eye chart. But on the low-contrast chart, patients getting no natalizumab were more likely to show decreased acuity than were those getting the drug.

Laura J. Balcer, a neuro-ophthalmologist at the University of Pennsylvania in Philadelphia, and her colleagues report these results in the April 17 Neurology.

It's noteworthy that patients getting interferon beta plus natalizumab fared better than did those getting interferon beta alone, says Edmond J. FitzGibbon, a neuro-ophthalmologist at the National Eye Institute in Bethesda, Md.

Some doctors may be reluctant to prescribe natalizumab. Its label warns that the immune suppression triggered by the drug can unleash a latent virus that causes a rare nervous disorder.

The new work bolsters the value of testing for subtle vision impairments. Balcer says that many people with MS complain about their eyesight yet score 20/20 on the standard chart. However, they fare poorly on a low-contrast test.

Because low-contrast-vision impairments don't necessarily show up on a standard eye exam, "they have been underappreciated," says FitzGibbon. "You need contrast sensitivity when driving at night, in fog, or in low-light situations," he says.

For people with MS, low-contrast eye charts might offer an inexpensive yet effective tool for assessing vision, says Nicholas LaRocca of the National Multiple Sclerosis Society in New York.