July 2009




Driving with cataracts

by Matt Young EyeWorld Contributing Editor


Horswill’s Ph.D. student Megan Preece in a pair cataract goggles used in the study

Preece demonstrating the experimental setup used in Dr. Horswill’s study Source: Mark S. Horswill, Ph.D.

Many of the nation’s elderly may comply with minimum legal standards for driving and still put themselves and others at serious crash risk because of cataracts. Decreased contrast sensitivity already has been linked to increased crash rates, but now a group of researchers has implicated even mild cataract in slowing a driver’s hazard detection response time. “We found that both mild and moderate simulated cataracts significantly slowed hazard detection response times as assessed using a change detection task, while moderate though not mild simulated cataracts slowed participants’ ability to anticipate hazardous situations in a validated video-based hazard perception test,” according to a scientific report by Mark S. Horswill, Ph.D., School of Psychology, University of Queensland, Brisbane, Australia, published in the December 2008 issue of Optometry and Vision Science. Based on study results, even moderate cataract, which slowed hazard perception, may not cause enough visual impairment in an actual road context to cause a crash, Dr. Horswill noted. At the same time, the research supports the possibility that “drivers who may meet the current legal standards for driving (based on visual acuity) may be at increased crash risk due to impairments in their ability to detect traffic hazards.”

A cataract simulation

Dr. Horswill and colleagues analyzed the results of 186 drivers with normal vision who completed a video-based hazard perception driving test and change detection task based on traffic hazards. These participants wore goggles with simulated mild cataract, moderate cataract, or without lenses. Dr. Horswill described the hazard perception test as follows: “Drivers viewed unstaged Queensland road scenes filmed from the driver’s perspective and were required to use a computer mouse to click on any road users who represented a potential traffic conflict. A traffic conflict was defined as any situation in which the camera car had to brake or take evasion action to avoid a collision with another road user (for example, a pedestrian crossing the road).”

The hazard change detection task involved “participants viewing pairs of alternating images of traffic scenes, in which one image in each pair contained a hazard while in the other image the hazard had been removed,” Dr. Horswill noted. “Participants were instructed to detect the difference between each pair of images. Participants were asked to click on the single difference between the two images as soon as they detected it.”

Study participants with simulated moderate cataracts were slower than controls both in the hazard perception test and the hazard change detection task. Those with simulated mild cataract were slower than controls only in the hazard change detection task.

“To gain a sense of the importance of the effect sizes found in the present experiment, we referred to our finding that moderate simulated cataracts slowed drivers’ hazard perception response times by 351 [milliseconds],” Dr. Horswill reported. “This was calculated to be equivalent to 5.85 meters of travel for a driver traveling at 60 kilometers per hour, which plausibly maps onto the difference between having and not having a collision.”

Why were hazards in motion generally easier to detect with any simulated cataract than hazards in still images? “The hazard perception task involved moving stimuli and therefore the motion itself might have provided additional cues to aid hazard detection that were not available in the change detection task,” Dr. Horswill suggested. In conclusion, even mild cataracts have an impact on the way people react to hazards while driving. Performing cataract operations as early as possible is optimal for persons who wish to continue driving, Dr. Horswill suggested. “Our data suggests that the link between cataracts and elevated crash risk may be due in part to impairments in hazard perception ability,” Dr. Horswill noted. Mohan Rajan, M.D., medical director, Rajan Eye Care Hospital, Chennai, India, suggested that elderly patients around the world have something that this research did not note: common sense. In India, he said, upward of 65 years old, many people opt not to drive. “It’s a Herculean task,” Dr. Rajan said, especially in India where traffic is heavy, regulations are few, and the roads are narrow. “Driving is a major problem because of the local problems,” he said. As a result, people tend to stay off the road before they experience the deleterious effects of cataract. Dr. Rajan also noted that even in the West where more elderly people do drive, once they experience blurring of vision, most would stop and see a doctor. Cataract surgery, meanwhile, is a solution that is safe, efficacious, and widely embraced.

Editors’ note: Dr. Horswill has no financial interests related to this study. Dr. Rajan has no financial interests related to his comments.

Contact information

Horswill: m.horswill@psy.uq.edu.au
Rajan: +65 6254 6330, rajaneye@vsnl.com

Driving with cataracts Driving with cataracts
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