Boston, Massachusetts – Drivers with an eye condition commonly associated with stroke or head injury have significantly more difficulty detecting pedestrians than normally-sighted people, according to a new study by the Schepens Eye Research Institute, an affiliate of the Harvard Medical School in Boston.
The condition, called hemianopia, causes blindness in one-half of the visual field in both eyes. When tested in a driving simulator, patients with the condition had trouble detecting pedestrians on their blind side. The results, published in the November 2009 issue of Investigative Ophthalmology and Visual Science, contradict some recent on-road studies that found most people with hemianopia were safe to drive.
“The results are important because they mean we need to continue to look carefully at people with this condition and evaluate them individually to determine their fitness to drive,” said Dr. Eli Peli, principal investigator of the study.
In at least 22 U.S. states and several countries, people with hemianopia are prohibited from driving because they do not meet the visual field requirements for licencing. The researchers said the study urges caution in allowing people with the condition to start or to continue driving again, especially since the simulator was used for a longer period of time than a typical on-road study where there is often little control over the appearance of potential “blind side” hazards.
More than a million people in the U.S. suffer from hemianopia, and often do not know what they can’t see and frequently bump into or trip over objects while walking. The researchers compared 12 subjects with hemianopia to 12 visually normal people, matched by age, sex and years of driving experience. All subjects drove for about 120 minutes along simulated city roads and rural highways, where pedestrians appeared at random intervals along the roadway and at intersections. The subjects pressed the horn button every time they saw a pedestrian, with detection rates and response times measured based on the responses.
The scientists found that drivers with hemianopia detected only about 45 per cent of pedestrians on their blind side, on average, compared with normally sighted subjects, or with their own seeing side. When pedestrians were seen on the blind side, response times were about twice as long as those of normally-sighted drivers. Drivers with the condition also had a larger variability in blind-side detection rates, ranging from six to 100 per cent, with the lower rates found among the older subjects.
The researchers’ next step is to study whether peripheral prism glasses, which expand the visual field, might be useful for drivers with hemianopia.