Judging an individual’s likelihood of causing a collision based on a vision screening is challenging, to say the least. In the United States, when you first get your license, your vision is tested along with your knowledge of driving rules and safety. (In fact, vision is the only biological capacity that’s evaluated in all 50 states.) Motor vehicle administrations and licensing bureaus in the various states—and all over the world—are asking for information regarding how to conduct that vision screening so that it’s more likely to reveal those in the greatest danger of having a collision.
To help answer that question, our group has been conducting a multi-year study funded by the National Eye Institute, including an examination of how visual field loss may impact driving ability and collision rates.
Our study was done in two parts. The first part involved running screening tests on 2,000 Alabama drivers age 70 and older, and then comparing that data to each subject’s history of motor vehicle collision involvement over the previous five years. The second part of the study has consisted of following these individuals for three years after the baseline testing, to see how well the baseline data could be used to predict their future involvement in motor vehicle collisions. (This was necessary because comparing our base-line data to the preceding years could be biased; some poorer drivers might have eliminated themselves from the pool before we collected our sample.)
We conducted our study in Alabama, for a couple of reasons. In addition to our being located here, Alabama is one of about 10 states that do not perform vision screening when you renew your license. This means that people with some level of visual difficulty are not automatically regulated off the road, leaving them in the population of older drivers. We focused on drivers over the age of 70 because that’s when collision rates start to increase in later adulthood, so there’s a greater public-health interest in that age group.
We’ve just completed the post-baseline three-year period and are currently analyzing that data. Ultimately, we’ll be able to tell whether any of the screening data we accumulated could have predicted future motor vehicle collision involvement.
Of course, this is not the first time that glaucoma’s impact on driving has been studied. Over the past 20 years a number of studies have looked at whether glaucoma patients had a higher motor vehicle collision rate or impaired driving performance, evaluated on the road; also, questionnaire-based studies have found that glaucoma patients tend to report more driving difficulty than people in the same age range who don’t have glaucoma. However, most of the previous studies have been clinic-based, recruiting patients who were being seen by a physician. While that’s a tried-and-true research strategy, it doesn’t necessarily generalize in a strong way to the broader public. To address that issue, we conducted a population-based study.
Testing a Limited Field
One important factor that was not addressed by previous studies was whether particular regions of the visual field might relate more strongly to motor vehicle collision rates. We only use certain parts of our visual field when driving; the physical structure of the car limits using the entire visual field. So part of our intent was to look for associations between collision rates and loss occurring within the part of the visual field used while driving. We also wanted to determine whether particular areas within that limited visual field were more important for avoiding collisions.
One previous study had already demonstrated that the visual field used when we drive is limited.1 That paper was important in that for the first time it brought to people’s attention the fact that drivers don’t use their entire visual field. The roof of the car provides an upper cutoff; the pillars at either side of the windshield that hold up the roof also reduce the field. That earlier study found that when driving, the useful visual field is only 15 degrees above the horizontal meridian, and, if you include the dashboard, a little bit farther below the horizontal meridian. (The numbers we used were slightly different, but the idea was the same.) At the same time, the horizontal extent of the field is pretty large, especially if you account for the driver turning his head from side to side.
Testing a visual field like that described above is not something that would be of interest to most glaucoma specialists in the clinic; they’re mainly interested in managing glaucoma, and the standard assumptions about visual field testing are very useful in glaucoma detection and management. The existence of an abundance of this data from conventional clinical field testing made that data very convenient to use in previous studies of driving and glaucoma. However, it may not have been an ideal way to address questions relating to driving, since the visual field being used when driving is significantly different.
To construct a test that was specific to a driver’s working visual field, we used a Humphrey Field Analyzer (Carl Zeiss Meditec). That instrument has a custom test that allows you to tell the instrument what points to test out to 60 degrees, horizontally and vertically. So, we created a custom test based around the visual field areas relevant when driving. (We chose the number of points we tested to keep the test under five minutes per eye.)
In addition to testing our subjects’ visual fields, we interviewed them about their driving exposure—how many miles they drive in a typical week. (The literature suggests that most people accurately report the amount of driving they do if you walk them through the interview and ask them about the typical places they drive in a week.) This allowed us to extrapolate how much exposure they were getting on the road and adjust their rate of motor vehicle collisions accordingly. (This is why your auto insurance company asks how far you drive to work; the likelihood of a crash increases the more you drive.)
When talking about older drivers, many things can elevate the risk of a motor vehicle collision, such as impaired mental status—which is not uncommon in older adults—and other medical conditions, including other vision problems. To account for these concerns, we measured many aspects of vision besides the visual field, including visual acuity, contrast sensitivity and visual processing speed, all of which have been implicated as being related to someone’s likelihood of having a motor vehicle collision. And, we looked into the cognitive status of our subjects.
What We’ve Found (So Far)
Data from the first part of our study showed a clear association between visual field problems and a history of motor vehicle collisions among the general population in this age group.2 Furthermore, it appears that the left portion and lower part of the visual field are most informative in terms of revealing increasing motor vehicle collision rates. (This was a novel finding.)
This finding makes sense to us, because a number of important concerns are happening on your left when driving in the United States. For example, you don’t want to unintentionally cross over the line on the road; and the lane to your left may have oncoming traffic or people passing you from behind. Of course, it is possible to hit something on the right side of the road, but there’s a whole lot of activity happening to your left. We don’t know whether the left visual field is equally important in other countries like Australia and the United Kingdom where people drive on the left side of the road. It’s certainly possible that in those countries the right visual field could turn out to matter more. (That’s a study waiting to be done.)
The greater relevance of the lower visual field also makes sense. In a vehicle, the lower part of the field contains the things that are closest to the vehicle. Things in the upper part of the visual field are farther down the road, so you have some time to adjust your vehicle before you get there. It’s the objects that are lower in the field that you’re most likely to hit or run over if you’re not careful. (This visual field data brings to mind findings from the literature relating to individuals at risk of falling; those studies found that individuals who have loss in the lower visual field are more likely to have postural instability and falling problems.)
Next, we focused in on the drivers in our study who had glaucoma; we verified their diagnosis via their medical records. Overall, we found that glaucomatous drivers were about 65-percent more likely to have a history of motor vehicle collision involvement than drivers who didn’t have glaucoma. The data also indicated that it’s probably an impaired visual field that’s mediating that association, rather than other issues such as contrast sensitivity or visual acuity. Of course, an impaired visual field is the signature visual impairment of glaucoma; visual acuity can remain quite good very far into the disease.
We also divided up the field as we had done in the earlier part of the study, this time only looking at glaucoma patients. Again, we found stronger associations for the left-hand portion of the field, but with the glaucoma patients we also found significant associations for both the lower and upper visual fields. Unfortunately, we only have so many glaucoma patients in our sample; whether we can further refine our findings without a larger data pool isn’t clear. For that reason it will require further research to nail down which areas of visual field loss in glaucoma make it particularly difficult to drive. I would predict that the left side of the visual field will be verified, as it was in the general population, but it’s possible that particular patterns of field loss may make glaucomatous drivers more vulnerable. For now, it seems that difficulty in any regions of the visual field may contribute to driving problems in a glaucoma patient, but there’s a tendency for the left side to be more important.
Acuity and Contrast Sensitivity
In terms of the other visual factors we checked, it’s well-established in the literature that visual acuity doesn’t tell you too much about motor vehicle collision rates. Of course, an individual with really bad acuity shouldn’t be driving, but there’s a lot of evidence that out to 20/100 a person is safe to drive. Where that individual will run into trouble is in reading road signs. That’s why there’s a visual acuity standard when you get a license; the states want to make sure you can read signs. In fact, the size and design of signs on interstates and expressways are chosen so that drivers can read them at an appropriate distance for braking and stopping—but you still need reasonably good acuity to be able to read them.
In terms of contrast sensitivity, our data revealed something that we can’t explain. We found that impaired contrast sensitivity among drivers with glaucoma had somewhat of a protective effect on a person’s collision rate. The effect was borderline and not statistically significant, but it looked like the worse an individual’s contrast sensitivity was, the lower his risk of motor vehicle collision.
One possible explanation is suggested by previous studies that found that individuals with contrast sensitivity impairment are more likely to report difficulty in visual tasks; in other words, they are aware they have a visual deficit. We also know from the literature that individuals who are aware that they have visual difficulty do a lot of self-regulation on the road, meaning they drive at the safest times of the day, don’t drive in inclement weather, drive slower and prefer that someone else drive. (Of course, we don’t know for certain whether or not that explains this result in our study.)
Although our findings about the importance of the left and lower areas of the visual field are worth being aware of, it may be early for clinicians treating glaucoma patients to act on them. After all, this is the first time this finding has been reported, and we don’t yet know what the second, prospective portion of our study will reveal. However, the existing evidence suggests certain sensible precautions worth taking with your patients.
First of all, the segment of the population that became our study group included those who were in the lowest quartile of visual field health; in other words, they were the 25 percent with the worst vision. These were individuals with severe visual field loss. I think if you have a patient in the chair who has a lot of visual field loss—particularly in both eyes—you should have a dialogue about driving. I wouldn’t be too concerned about an individual with early field loss, or a lot of field loss in one eye but not in the other eye. Since the visual fields in the two eyes largely overlap, the good eye can fill in the visual field where the bad eye is lacking or has scotomas.
The flip side of the coin is that you don’t want to unnecessarily alarm patients who are in the early stages of the disease or don’t yet have significant field loss in both eyes. Driving is a very important part of daily living for most people, even when they get older, so you have to be cautious about too aggressively suggesting that a patient not drive. There’s no reason to take away someone’s license just because he has glaucoma and minor field loss in one eye.
|When there’s some reasonable doubt ... giving the patient the opportunity to prove his or her skill may be your best option.
The Tough Cases
When you do encounter a patient with severe visual field loss that leaves you concerned about the individual’s driving ability—someone who insists he or she is a fine driver or simply isn’t inclined to stop driving—one good option is to refer the patient to a driving assessment clinic. We have one here at the University of Alabama at Birmingham, and they are popping up all over the country. Ours is run by an occupational therapist who is also a certified driving rehabilitation specialist, or CDRS. The clinic does a battery of in-clinic tests that include not only vision screening but also tests of mental status, visual processing speed and other motor skills relevant for driving. The therapist can also take the individual out on the road for a driving evaluation.
The reason we developed this clinic is that it’s beyond most physicians’ expertise to make a decision about somebody’s driving. That subject is not typically covered in medical school or an ophthalmology residency. So the clinic is run by a health-care provider you can refer out to, just as you might refer someone out for labs or imaging or any other type of medical consultation. Then you can sit down and talk to the patient about the outcome of the evaluation. Even in this scenario, you have to remember that no one can ever say, “This person is going to have a collision,” (or never going to have one). They can only say that this person has the potential for safe or unsafe driving, based upon all of the benchmarks.
The other option, if an unsafe patient won’t stop driving, is that most states allow physicians to report patients to the motor vehicle administration if you believe they are a threat to public safety on the road. (In some states the physician can’t be taken to court for a HIPAA violation because it’s in the public’s interest.) However, we always recommend that before you do that, at least let the patient demonstrate whether he can drive or not. That’s what the driving assessment clinic can do.
Of course, you always have to use your own medical impressions. If you have someone in front of you who has completely dark visual fields, you don’t need to send him to a driving clinic. You know he shouldn’t be driving. But when there’s some reasonable doubt—for example, if the visual field loss is in different areas in the two eyes—giving the patient the opportunity to prove his or her skill may be your best option. REVIEW
Dr. Owsley is the Nathan E. Miles Chair of Ophthalmology at the School of Medicine at University of Alabama at Birmingham.
1. Vargas-Martin F, Garcia-Perez MA. Visual fields at the wheel. Optom Vis Sci 2005;82:675–681.
2. Huisingh C, McGwin G Jr, Wood J, Owsley C. The driving visual field and a history of motor vehicle collision involvement in older drivers: A population-based examination. Invest Ophthalmol Vis Sci 2015;56:132–138. DOI:10.1167/iovs. 14-15194.