Barbara Boughton

November 03, 2009

November 3, 2009 (San Francisco, California) — Some patients with hemianopia and quadrantanopia are safe drivers and should be offered the chance to prove it with on-road driving tests, according to researchers here at the American Academy of Ophthalmology Joint Annual Meeting With the Pan-American Association of Ophthalmology. In a study in which patients with visual field defects were compared with age-matched controls, 73% of those with hemianopia and 88% of those with quadrantanopia were rated as safe drivers, lead investigator Michael Vaphiades, DO, from the University of Alabama at Birmingham told meeting attendees.

The small study, which compared the driving skills of 22 people with hemianopia and 8 with quadrantanopia with 30 age-matched control subjects with normal visual fields, is the largest to date on driving safety in patients with homonymous visual field defects, according to Dr. Vaphiades. The study calls into question governmental policies that deny licensure to these patients without a road test, he asserted. "These policies should be reassessed," he said.

Dr. Vaphiades noted that denying licenses to those with visual field defects might not only be unfair, it might reduce these patients' independence, employment options, and access to healthcare, and increase their risk for depression. "One of my patients told me: 'If you take my license away, it's a death sentence. I won't be able to do anything'," Dr. Vaphiades said.

Dr. Vaphiades and colleagues assessed baseline visual acuity, visual field, and contrast sensitivity in all subjects. Patients also took cognitive screening tests, including the Mini-Mental Status Examination and the Digit Symbol Substitution Test (DSST), which measures mental processing speed, short-term memory, and attention switching. Each driver in the experimental and control group then took a 14.1-mile road test, 7.8 miles of which were on an interstate roadway, and each was rated independently by 2 blinded researchers who sat in the back seat of the car. For safety reasons, each car was outfitted with dual brakes, which could be used by a third researcher who sat in the front passenger seat of the car.

Each driver was rated on parameters that included scanning, lane position, steering steadiness, speed, gap judgment, braking, using directional indicators properly, and obeying traffic signals. Individuals in the experimental and control groups were rated on a scale of 1 to 5 for their driving abilities, with 1 being "so unsafe that they shouldn't drive" and 5 being "good driving," Dr. Vaphiades said.

All subjects in the control group were rated as safe drivers. Those with hemianopia and quadrantanopia had more problems with lane position, steering steadiness, and gap judgment than those in the control group, although they did not differ significantly on other safe driving behaviors. The researchers acknowledge that the sample size limited their conclusions.

Dr. Vaphiades and team analyzed what characteristics were associated with unsafe driving among those with hemianopia and quadrantanopia. They found that unsafe driving was associated with slower visual processing speed, poorer executive function as measured by the DSST, and reduced contrast sensitivity.

Although the study is an interesting one and raises some important questions, the conclusion that some patients with hemianopia and quadrantanopia are safe drivers might be premature, cautioned Kenneth Shindler, MD, PhD, a neuro-ophthalmologist and assistant professor of ophthalmology at the University of Pennsylvania in Philadelphia.

"The [test] route was fairly controlled and the parameters they looked at might not be the right parameters," he said. The biggest problem that those with hemianopia and quadrantanopia, who are missing part of their visual field, face when driving are obstacles on the road (or approaching cars or pedestrians), Dr. Shindler explained. "Even if they're scanning and bringing their visual field over, you have to believe they have an expanded blind spot," he said.

However, Dr. Shindler concurred that many requirements for drivers' licenses are not based on rigorous scientific testing, but on hunches of what might or might not be safe. "Designing good studies to look at safety is a great idea, but a lot more testing has to be done to really draw conclusions about how much visual field is needed to safely drive," he said.

Dr. Vaphiades and Dr. Shindler have disclosed no relevant financial relationships.

American Academy of Ophthalmology Joint Annual Meeting With the Pan-American Association of Ophthalmology (AAO-PAAO): Abstract SYM23. Presented October 26, 2009.