April 2016

 

RETINA

 

In the drivers seat with AMD


by Maxine Lipner EyeWorld Senior Contributing Writer

 
   
Patients with AMD

Patients with AMD who have blind spots tend to have slower response times to hazards in the road.

Driving simulator

Driving simulator used in the study Source: Peter Mallen, Schepens Eye Research Institute

Focusing on reaction times behind the wheel and scotomas

Driving for age-related macular degeneration (AMD) patients with scotomas may be problematic, according to Alex Bowers, PhD, assistant professor of ophthalmology, Harvard Medical School, and assistant scientist, Schepens Eye Research Institute, Massachusetts Eye and Ear, Boston. Those with central visual field loss from the condition may experience delayed reaction times when it comes to avoiding pedestrian hazards, she reported in a study published in the September 2015 issue of PLOS One.

Dr. Bowers and colleagues, Eli Peli, OD, professor of ophthalmology, and Matthew Bronstad, PhD, instructor in ophthalmology, were spurred to examine the issue by concerns about what driving with scotomas could mean for many. AMD is an eye condition where the older you get, the more likely you are to have the condition, Dr. Bowers pointed out, adding that with patients living longer than ever, there is a worry about what this could mean for those who continue to drive. In the U.S., regulations regarding visual requirements for driving vary by state and usually only take into account the need for visual acuity to be around 20/40. None of the states that Im aware of in the U.S. say anything about the integrity of the central visual field, Dr. Bowers said. But other countries like the U.K. and Australia dothey have a clause that says there should be no significant scotomas or field loss within the central 20 degrees or so. This speaks to the fact that, along with reduced visual acuity, patients with AMD usually also have a central scotoma. But thats not often taken into account in evaluating visual fitness to drive here in the U.S., Dr. Bowers said.

Studying the effect of scotomas

The idea was to consider how patients ability to detect and respond to potential road hazards while driving might be impaired if they have a blind area in the center of their vision. The reason that this is relevant here in the U.S. is that even if people dont meet the 20/40 (acuity) requirement, they may still be able to drive with a restricted license, Dr. Bowers said, adding that acuity could be as low as 20/200 with a restricted license in some states. Those with AMD who have 20/200 vision are going to have a fairly sizeable central scotoma. To safely examine how patients might react to such situations, investigators employed a driving simulator and compared reactions of those with AMD to those of controls. Use of the driving simulator enabled investigators to give exactly the same test to everyone, as well as to program exactly where pedestrians would appear, Dr. Bowers explained. In this current study, the sizes of the patients scotomas ranged from 7 to 25 degrees in diameter and were located either above or below the patients gaze points. In a study published in the March 2013 issue of JAMA Ophthalmology in which Dr. Bowers also took part, she reported results for patients with scotomas that were either to the left or to the right of the gaze point. Participants were asked to spend 2 sessions in the simulator, each involving about 1 hour of test drives, Dr. Bowers noted, adding that every participant was first given time to get used to the simulated driving conditions. Across both driving simulator sessions, there would be a total of more than 100 pedestrians, she said. They were programmed so that if it had been in the real world, there would have been a collision if the person kept driving at the same speed without taking any evasive action. However, to avoid subjecting participants to what would have been a lot of collisions, investigators programmed the pedestrians to stop before they entered the travel lanes. Whenever drivers saw one of these pedestrians, they had to respond by pressing the horn. From that we were able to measure detection rates and their reaction time, Dr. Bowers said.

Results from the first study indicated that when the patients scotomas were to the left or to the right of their gaze point and a pedestrian stepped off the curb in that area, the response time to pressing the horn was slow. They had very long response times, Dr. Bowers said. In most cases, they would have seen the pedestrian so late that they wouldnt have been able to stop. Response times to pedestrians not in the area of visual field loss were delayed relative to normally sighted controls but were not as delayed as those involving the area of the scotoma, she reported.

Key findings

With this in mind, in the latest study involving central scotomas, investigators expected that subjects who had a blind area above their gaze point might be a little slower to react than controls but that this would not affect their ability to detect pedestrians. We didnt think that we would find any effect of the scotoma, she said, but investigators found that response times were elevated in some instances. This, they determined, only occurred when the scotoma obscured the drivers view of the pedestrian, which was an issue at times when some glanced down toward the speedometer or something else. Across these 2 studies, the key finding is that response times of those with central scotomas are longer than for normally sighted people, she said. If people have a scotoma that is to the left or to the right, there is a marked increase in response time to pedestrians on the corresponding side. Even those who have a central blind area above or below the position of gaze can run into trouble if their gaze is even momentarily averted in a direction that obscures a potential hazard from view. To our knowledge, this is the first time that this has been systematically investigated, Dr. Bowers said. While it would be possible to potentially compensate for the scotoma by scanning from side to side, elevated response times here suggest that drivers are in fact looking straight ahead most of the time, she said.

From a clinical perspective, the study indicates that licensing authorities should take into account large scotomas in the central visual field as some countries already do. The people who we tested here would not have been permitted to drive in the U.K., Dr. Bowers said. Yet, 6 of the drivers retained their U.S. licenses and 1 had relinquished the privilege within the last 2 years.

Dr. Bowers hopes that practitioners seeing patients with AMD will take into account the size and the density of the scotoma when advising them to continue driving or not. Physicians should make patients aware that they have this blind area in their vision and that it potentially means that they might not see something, she said. Going forward, Dr. Bowers would like to see a larger follow-up study done on this.

References

1. Bronstad PM, et al. Driving with central visual field loss II: how scotomas above or below the preferred retinal locus (PRL) affect hazard detection in a driving simulator. PLoS One. 2015 Sep 2;10(9):e0136517.

2. Bronstad PM, et al. Driving with central field loss I: effect of central scotomas on responses to hazards. JAMA Ophthalmol. 2013;131(3):303309.

Editors note: Dr. Bowers has no financial interests related to her comments.

Contact information

Bowers: alex_bowers@meei.harvard.edu

In the drivers seat with AMD In the drivers seat with AMD
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