Central field loss creates driving hazard

By Joanna Lyford, Senior medwireNews Reporter

People with central field loss (CFL) experience a range of difficulties when driving that could pose a danger to themselves and pedestrians, researchers believe.

Writing in JAMA Ophthalmology, P Matthew Bronstad (Schepens Eye Research Institute, Boston, Massachusetts, USA) and co-authors say that people with CFL show delayed detection of hazards, such as pedestrians, as well as loss of visual acuity and contrast sensitivity.

In an accompanying editorial, Gordon Legge (University of Minnesota, Minneapolis, USA) advises practitioners to raise with their patients this "compelling" evidence of a potentially detrimental impact of CFL on driving safety.

Using a case-control design, Bronstad et al sought to determine whether size of the scotoma and position of the preferred retinal locus (PRL) relative to the scotoma would influence hazard detection.

They recruited 11 adults with CFL and 11 people with normal vision. All participants completed two 90-minute sessions in a driving simulator, during which they responded to virtual pedestrians who appeared on either the left or right side and approached the participant's lane on a collision trajectory.

While overall detection rates were high, patients with CFL had more detection failures than controls (2.7 vs 0.3%) and 2.1 times more misses for pedestrians in blind versus seeing areas, Bronstad et al found.

Furthermore, people with CFL reacted more slowly to hazards than controls (3.35 vs 1.27 seconds) and had slower reactions in blind versus seeing areas (4.28 vs 2.43 seconds). These differences were more pronounced when driving on simulations of rural highways than urban areas, note the authors.

The CFL patients were also more likely to have untimely reactions than controls (29 vs 3%), most often involving pedestrians in blind rather than seeing areas (50 vs 19%). Again, untimely reactions were more frequent in rural highways than in city streets.

Finally, scotoma size and contrast sensitivity predicted driving ability in blind and seeing areas, respectively, whereas visual acuity and age were not correlated with any outcome measure.

"The effects of CFL have been anticipated but have not been previously documented because of the difficulties of studying visually impaired driving," the researchers write.

They conclude: "Knowledge about how specific aspects of vision loss (CFL, visual acuity, and contrast sensitivity) affect certain aspects of performance should help improve vision rehabilitation and the design of mobility aids. The results may help practitioners in advising patients with CFL about difficulties they may face when driving."

In his editorial, Legge says that the results are important in showing that the configurations of PRL and scotoma have more impact on driving performance than acuity.

"Findings such as those reported by Bronstad et al begin to shed light on individual vision-related factors that can guide ophthalmologists, optometrists, vision rehabilitation specialists, and their patients in making driving decisions," he writes. "The findings also offer opportunities for improved educational and intervention programs for driving safety and for the development of onboard technology to assist driving mobility."

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