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ORIGINAL ARTICLES

Does Correcting Astigmatism with Toric Lenses Improve Driving Performance?

Cox, Daniel J.*; Banton, Thomas; Record, Steven; Grabman, Jesse H.§; Hawkins, Ronald J.§

Author Information
Optometry and Vision Science: April 2015 - Volume 92 - Issue 4 - p 404-411
doi: 10.1097/OPX.0000000000000554
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Abstract

Driving is a routine part of daily living in industrialized nations, with more than 211 million licensed drivers in the United States alone.1 It is a complex skill that comes with a significant risk to safety. For example, in 2012, there were about 10.8 million documented vehicular collisions in the United States, accounting for nearly 100 deaths daily.2 In addition, vehicular collisions are expensive, costing the US economy more than 276 billion dollars in 2012.3 Given the prevalence of driving and the risks associated with it, safety is imperative.

Driving performance depends on adequately performing complex skills such as braking, steering, and controlling speed. These multidimensional skills require drivers to have good visual, biomechanical, and cognitive abilities, and the relative importance of these abilities to various aspects of driving performance is beginning to emerge.4,5 The present study focused on vision-dependent abilities to evaluate a potentially overlooked effect of vision on driving performance: the consequences of driving with cylindrical blur from uncorrected astigmatism.

The effect of cylindrical blur on driving performance is of particular interest because there are a substantial number of people who drive with some degree of uncorrected astigmatism and potentially pose a risk to driving performance. This group comprises those who fail to update their prescriptions in a timely manner, those with residual astigmatism after lens or refractive surgeries, and contact lens patients with mild astigmatism (≤0.75 diopters [D]) who are intentionally not corrected for astigmatism. These latter patients are often fitted with spherical contact lenses because the acuity loss from uncorrected astigmatism is small and a toric lens alternative can sometimes be unstable as well as more expensive than spherical lenses.6,7 Although this latter group is shrinking with advances in toric contact lenses,8,9 the prescribing rates of toric contact lenses do not yet match the number of contact lens patients with astigmatism of 0.75 D or more.10 For people with mild astigmatism, it is usually assumed that there are few functional consequences of foregoing the cylindrical correction, but there are little data on which to base this assumption.

Evidence from studies of spherical blur suggests that cylindrical blur might negatively impact driving as well. Both spherical and cylindrical blur degrade some visual functions relevant to driving, such as high7,11–20 and low7,12,16,17 contrast acuity, reading speed,15,21,22 and subjective clarity.11,12,15,23 With respect to driving, spherical blur has been weakly associated with reduced driving performance.24,25,27,28 Increases in visual impairment with age, which include instances of uncorrected refractive error, are associated with increased crash rates.29–31 This association might actually be stronger than it looks, because a portion of the effect is presumed to be masked by self-restriction of driving as blur increases with age.31 If one can generalize from these similarities between cylindrical and spherical blur, then it might be expected that cylindrical blur would also reduce driving performance and perhaps safety.

To our knowledge, Wolffsohn et al.15 published the only test to date of driving-relevant performance with cylindrical blur. In their study to discern the everyday challenges posed by uncorrected astigmatism in presbyopic subjects, 21 participants with up to 4.00 D of induced cylindrical blur performed several tasks including a split attention task in which they viewed a 14-in computer monitor showing the perspective of a person sitting in a driver’s seat. During a 90-second simulated drive, two hazards (a lead vehicle braking and an intrusion by a pedestrian) were presented three times each. Reaction times to the hazards were recorded, and the test was repeated seven times. In this context, cylindrical blur had no effect on reaction time. The lack of an effect could be attributed to any number of factors, such as limitations imposed by using a small, graphically sparse desktop simulator.32 Therefore, the safety of astigmatic drivers using spherical-only corrections is still an open question.

The goal of the present study is to determine if contact lens wearers with mild to moderate astigmatism (0.75 to 1.75 D) are safer drivers when corrected with toric versus spherical lenses. This is the first study to objectively evaluate simulated driving performance among people with astigmatism wearing different types of contact lens corrections. It is hypothesized that contact lens wearers with low to moderate astigmatism will exhibit better simulated driving performance with toric lenses than with spherical lenses. To explore mechanisms for this expected improvement, the effect of these contact lens corrections on a set of newly developed driving-specific visual measures was initially tested.

METHODS

Subjects

Recruitment

Young adult licensed drivers were recruited through flyers and social media from the local university and community college. Eleven participants were enrolled. The study was approved by the University of Virginia Institutional Review Board and followed all the tenets of the Declaration of Helsinki.

Screening

All subjects responding to the study advertisements were screened for inclusion over the telephone by DJC. The nature and possible consequences of the study were explained, and candidates satisfying the following inclusion criteria (subject to verification) were invited to participate:

  • Best-corrected binocular visual acuity of 0.30 logMAR (logarithm of the minimum angle of resolution) or better
  • 0.75 to 2.0 D of astigmatism in each eye
  • 0 to 6.0 D of myopia in each eye
  • No active eye infection
  • No visual field defects
  • No bifocal correction
  • Routine contact lens wearer (>4 d/wk)
  • Routine driver (drive a car >4 d/wk)
  • No history of motion, sea, or big screen (e.g., IMAX) sickness, to avoid possible simulation adaptation syndrome
  • Between 18 and 39 years

Informed Consent and Clinical Measures

Qualified subjects scheduled a visit with SR, who obtained informed consent and then completed a brief ocular evaluation to (1) measure visual acuity, (2) measure contrast sensitivity, (3) assess anterior segment health, (4) confirm contact lens prescriptions, and (5) assess the contact lens fit. Acuity was measured both uncorrected and with toric contact lens correction using a printed Snellen chart viewed from a distance of 20 ft under full room illumination. Likewise, contrast sensitivity was measured uncorrected and with toric contact lens correction using a printed Pelli-Robson chart at 10 ft under full room illumination. Contact lens fitting was done according to the manufacturer’s fitting guidelines, using each participant’s current (≤1 year old) contact lens prescription. Anterior segment health and contact lens fit were assessed with a slit lamp biomicroscope. Masked study lenses (spherical and toric designs) were dispensed at this visit, but participants did not open them until study testing began. Spherical ACUVUE MOIST and ACUVUE MOIST for Astigmatism contact lenses were used throughout the study.

Of the 11 study participants, 2 were male and 9 were female. Their ages ranged from 19 to 35 years (mean [±SD], 22.2 [±4.6] years). Their myopia ranged from −1.00 to −5.00 D (mean [±SD], −2.7 [±1.5] D), and their astigmatism ranged from −0.75 to −1.75 D (mean [±SD], −0.9 [±0.4] D). Uncorrected Snellen acuity ranged from 0.48 to 1.30 logMAR, and binocular Snellen acuity corrected with toric contact lenses was between 0 and 0.10 logMAR for all participants. As compensation for participation, subjects received $30/h.

Equipment

We used the Driver Guidance System simulator, produced by General Simulation (Milpitas, CA). As seen in Fig. 1, the simulator consisted of a driver’s seat; steering wheel; gas and brake pedals; turn signal; dashboard; right, center, and left rear-view mirrors; and air conditioning, situated within an 8-ft cylinder displaying a 210-degree image of a virtual world on a curved white screen. The display was created by aligning and synchronizing 70-degree images from three projectors (InFocus, model IN2106, with a resolution of 1280 × 800 and a refresh rate of 60 fps). Participants were seated 4 ft from the display screen.

F1-5
FIGURE 1:
Driver Guidance System simulator, produced by General Simulation. The driver is viewing a 210-degree-wide projected environment that contains virtual side- and rear-view mirrors. A color version of this figure is available online at www.optvissci.com.

Measures

Participants completed a simulated 15-mile drive under rural, urban, and highway conditions. A variety of traffic, signal, and road conditions had to be negotiated during the drive. Driving performance (tactical skills) was assessed by the simulator, and driving-relevant visual performance (operational abilities) was assessed by the experimenter during the drive. All 11 participants completed the measures of operational performance. Two dropped out during tactical testing, leaving nine with complete tactical data.

Tactical performance was scored using an established measure of driving performance, the tactical composite score. The tactical composite score is an overall measure of driving performance that has been demonstrated to predict future driving collisions of seniors,33 differentiate high- from low-risk drivers such as young seniors versus older seniors,34 differentiate drivers with and without attention-deficit/hyperactivity disorder,35 differentiate drivers with and without Alzheimer disease,36 and differentiate high-versus low-risk conditions, such as intoxication versus sobriety,37 diabetic hypoglycemia versus euglycemia,38 and drivers with attention-deficit/hyperactivity disorder on methylphenidate versus placebo.39 Therefore, the tactical composite score is the primary outcome variable for this study.

Operational performance was quantified using an exploratory measure of visual abilities, the operational composite score. The operational tests contributing to this score are new driving-specific metrics based on standard vision and reaction time tests but presumed to have the benefit of ecological validity. These tests are still under development. The operational composite score is the secondary outcome variable for this study.

Tactical Tests

The overall tactical composite score consists of nine different performance variables: two speed control, two braking, and five steering variables:

  1. Speed control
    • a. Driving too slow: The total time spent driving 20 mph below the posted speed limit.
    • b. Driving at inconsistent speeds: The SD of driving speed in the open road.
  2. Braking
    • a. Stop sign hesitation: The average time drivers remained stopped at a stop sign after gaining the right of way.
    • b. Inappropriate braking: The average amount of time the brakes were applied in the open road.
  3. Steering
    • a. Driving across the midline:
      • 1) The average time spent driving in the oncoming lane of traffic.
      • 2) The variability in the time spent driving in the oncoming lane of traffic.
    • b. Driving off-road:
      • 1) The number of times the passenger side wheels were on the shoulder of the road.
      • 2) The average time the passenger side wheels were on the shoulder of the road.
    • c. Lane position maintenance: The deviation in vehicle position relative to the center of the lane.

Operational Tests

The overall operational composite score was composed of the following six tests. All were presented as part of the simulated driving environment.

  1. Static visual acuity: Participants read six sets of three white letters presented on a black overhead road sign while the driver was stopped and standing outside of the vehicle. The three-letter sets were made progressively smaller by using letter sizes of 0.80, 0.65, 0.49, 0.38, 0.32, and 0.24 logMAR from the 8-ft viewing distance. The outcome variable was the smallest size at which the subject correctly read at least two of the three letters.
  2. Contrast sensitivity: Participants read 10 sets of three 1.84-logMAR letters presented on a white overhead road sign while the driver was stopped. The gray level of each set of letters was made progressively higher, reducing the letter contrast against the white background. Projector luminance was uncalibrated in this preliminary study; hence, letter contrast was approximated by using the computer-defined gray levels in the formula for Weber contrast. This yielded an approximation of contrast that did not account for potential display nonlinearities. Although the absolute contrast values reported are approximations, their ordinal ranking is correctly represented, which was useful in determining an appropriate stimulus range for this test. Bearing in mind these limitations, Weber letter contrast was changed from 50 to 2.4% in nine equal steps. The outcome variable was the lowest contrast at which the subject read at least two of the three letters.
  3. Dynamic vision: Participants read sets of three white 1.30-logMAR letters that scrolled across the black overhead road sign while the driver was stopped. The first set moved slowly, and subsequent sets scrolled at progressively faster rates. Nine different scrolling speeds were presented, ranging from 10.3 to 44.2 degrees/s. The outcome variable was the fastest speed at which the subject correctly read at least two of the three letters.
  4. Glare sensitivity: Participants repeated the dynamic vision test in the presence of a glare source. Glare was produced by positioning a 100-W incandescent light bulb 12 in from the subject’s eyes and 25 degrees below the dynamic acuity target, roughly simulating glare conditions that might occur for drivers near sunset.
  5. Foot detection time: While driving at a controlled speed and distance behind a lead car (cruise control), participants released the accelerator as soon as the brake lights of a lead vehicle were detected. The lead vehicle braked 10 different times. The outcome variable was the time between the brake lights coming on and when the subject’s foot was removed from the accelerator.
  6. Arm detection time: While driving at a controlled speed and distance behind a lead car (cruise control), participants turned the steering wheel as soon as they detected a pothole driven over and revealed by a lead vehicle. Ten different potholes were displayed. The outcome variable was the time between a pothole’s appearance and when the subject initiated a turn of the steering wheel.

Data Reduction

The means and SDs for each tactical and operational variable were calculated. Based on this, z scores were derived for each variable for each subject, with a mean of 0 and an SD of 1. A negative score reflected a below-average performance.

Tactical and operational z scores were then summed into composite tactical and operational z scores to serve as indices of overall driving skill and driving abilities.40,41 This approach has been used previously in closed-road driving, simulated driving, and neuropsychological studies.26,38,42–44 Composite z scores under three conditions (no lenses, spherical lenses, and toric lenses) were compared.

Design and Procedure

This was a randomized, crossover, single-masked design, with testing under three vision conditions (uncorrected, corrected with spherical contact lenses, and corrected with toric contact lenses). The vision conditions were counterbalanced, and both the participants and the research assistant were unaware of the directional hypotheses. Participants were told that this study was investigating differences in driving ability when wearing contact lenses for drivers with astigmatism.

On the day of testing, participants came to the Virginia Driving Safety Laboratory wearing their usual correction. They brought with them the spherical and toric contact lenses dispensed by SR. Subjects removed their personal lenses and were instructed by TB (via a random assignment table) as to which lenses to wear, if any, for each testing condition. All experimental testing was conducted by RH.

The procedure followed in each test condition was identical: A 30-minute lens adaptation period was provided, during which participants watched a video about Thomas Jefferson. This was followed by a 45-minute testing period in the simulator where they first performed operational tests followed by tactical tests. Subjects completed this sequence for each of the three lens conditions. This resulted in a 4-hour testing session. Condition order was counterbalanced across subjects.

RESULTS

Primary Outcome Variable, Tactical Measures

Composite z Score

The means (±SDs) of the composite scores for the no-lens, spherical lens, and toric lens conditions were −1.73 (±3.53), 0.19 (±2.62), and 1.54 (±1.75), respectively. A one-way repeated-measures analysis of variance (ANOVA) yielded a main effect of lens condition on tactical skills (F = 3.561, p = 0.05). Individual comparisons demonstrated that the toric correction was superior to uncorrected vision (p = 0.05), but spherical correction was not superior to uncorrected vision (p = 0.118). There was no significant difference in tactical driving performance when directly comparing toric to spherical lens correction (p = 0.276).

Individual z Scores

For each driving skill comprising the tactical composite, Table 1 details the z scores and significance testing outcomes for the three lens conditions. None of the individual driving skills could alone account for the main effect of lens condition.

T1-5
TABLE 1:
Tactical variables: z score means and SDs and p values for the three lens conditions

Secondary Outcome Variable, Operational Measures

Composite z Score

The means (±SDs) of the composite scores for the no-lens, spherical lens, and toric lens conditions were −2.77 (±4.30), 1.37 (±2.37), and 1.40 (±2.65), respectively. A one-way repeated-measures ANOVA yielded a main effect of lens condition on operational abilities (F = 9.501, p = 0.007). Individual comparisons demonstrated that spherical (p = 0.008) and toric (p = 0.011) lenses were superior to no lenses, but they were not different from each other (p = 0.941).

Individual z Scores

Table 2 details the z scores and the significance testing outcomes for each driving ability contributing to the operational composite score. Two individual operational tests substantially contributed to the effect of lens correction: static visual acuity and arm detection time.

T2-5
TABLE 2:
Operational and pretest variables: raw score means and SDs, z score means and SDs, and p values for the three lens conditions

DISCUSSION

In this preliminary study on the driving performance of people with mild to moderate astigmatism, the hypothesis that there is an advantage of toric over spherical contact lens wear with respect to driving performance could not be confirmed. Although toric (but not spherical) lens wear led to safer tactical driving than without a lens correction, the lack of a significant difference between toric and spherical lenses may indicate either equivalent driving performance with these lenses or a lack of power in our design. In support of the former speculation, spherical and toric lenses could not be differentiated by operational testing (although both were superior to the no-correction condition). Support for the latter speculation comes from comparing the study sample of 9 to the results of a power analysis suggesting that a sample of 43 would provide an 80% chance of finding a significant difference in driving performance between toric and spherical lenses. These competing speculations justify further research with larger samples and more sensitive measures. However, establishing the nature of the relationship between driving performance and toric lens correction is a significant step.

It is worth emphasizing that none of the individual tactical tests alone (Table 1) accounted for the superior simulated driving performance with toric lenses compared with no lenses. Rather, it was only the composite tactical score that explained the effect of toric lenses. Therefore, it may not be surprising that in the study by Wolffsohn et al.,15 induced cylindrical blur did not significantly alter performance on a single driving test (reaction time to hazards) because single tests are, to date, insufficient measures of overall driving performance. Successful indicators of driving performance ought to reflect the gamut of performance variables used in driving, which may be why the tactical composite score has been useful in discriminating driving performance under a variety of conditions. Other factors, such as the use of Wolffsohn et al.15 of a small desktop display, may have also contributed to their lack of an effect, because display size can be important to performance on a variety of spatial tasks including navigation.45

To address the secondary aim of investigating mechanisms for improved driving performance, an exploratory set of driving-relevant vision and reaction time tests was developed and included in this study to see if it might differentiate toric and spherical lens wearers with respect to driving performance. It was reasoned that driving-specific vision tests might improve the predictability of driving performance because in tasks involving cognition, ecologically valid tests are better predictors of function than those in artificial testing situations.46 The operational test battery did differentiate the performance of spherical and toric lens wearers from those with uncorrected vision, but it was not able to differentiate between spherical and toric lens wearers. Although it was encouraging that the operational battery differentiated large performance differences, its inability to differentiate spherical from toric lens performance was not surprising given the sample size limitations discussed above, as well as the spatial and temporal limits imposed by the computers and projectors used in this study. Using projectors and computers with greater resolution and faster refresh rates will allow the creation of more challenging vision tests that might be able to differentiate more subtle differences.

One strength of this study was the use of an immersive simulator with a 210-degree horizontal visual display to measure driving performance. This allowed for precise control of the driving environment across subjects and conditions and objective measurement of driving performance during both unremarkable and high-risk driving conditions. This degree of control, objectivity, and exposure to road hazards could not be achieved in on-road testing. Although the simulator can only approximate the experience of on-road driving, training in a highly immersive simulator such as used in this study can produce results similar to those found with on-road training.32,47 Performance suffers when smaller, less immersive desktop displays are used,32 which may have been the case in the study by Wolffsohn et al.15 Other strengths of the present study are that a within-subject design was used to control for differences in driving experience and ability, visual abilities, motor abilities, and cognitive abilities across test conditions. Furthermore, composite scores such as these have been previously used to successfully predict collisions and to identify high-risk drivers and driving conditions. Finally, bias was reduced by keeping the research assistant naive to the lens condition and keeping both the research assistant and subject naive to the hypotheses.

Both tactical and operational tests may have been impacted by some limitations. Because this was a preliminary study, its scope was fairly narrow. The sample was small (tactical, n = 9; operational, n = 11) and homogeneous (college students with relatively mild astigmatism). Therefore, the findings should be replicated with a larger sample reflecting the general population of drivers with astigmatism. The study was also restricted to a line of toric lenses that are notably stable during eye movements used in driving.48,49 It remains to be seen if other toric lens designs also provide the stability needed to support an improvement in driving performance. Lastly, this was a single-masked study in which only the simulator examiner was naive to the corrective condition, which could introduce bias. The uncorrected vision condition was obvious, and subjects might have differentiated spherical from toric lenses based on the axis markings on the toric lenses or from the quality of vision after lens insertion. Subjects did not report on which lens they thought they were wearing.

Despite these limitations, correcting vision with toric lenses resulted in significant benefits to simulated driving skills and safety compared with no lens correction. Future studies are necessary to address the limitations and to further investigate the current findings. If the original hypothesis (that contact lens wearers with low to moderate astigmatism exhibit better simulated driving performance with toric lenses than with spherical lenses) can be confirmed, the scope of these results would extend beyond contact lens wearers to include drivers who have uncorrected astigmatism from other sources, such as wearing outdated spectacles that do not correct emerging astigmatism or residual astigmatism after surgical lens extraction. The results could have significant personal, professional/clinical, and public policy implications for drivers with astigmatism. Subjects with astigmatism may reduce their risk of being involved in a driving collision by wearing a correction for astigmatism while driving. Clinicians may be more likely to encourage the use of toric lenses for their patients with astigmatism. Insurance companies and departments of motor vehicles may even require this type of correction to extend insurance coverage or the privilege to drive. Further, this finding may imply potential benefits in other activities of daily living, such as playing sports for example.

Daniel J. Cox

Departments of Ophthalmology and Psychiatry

Box 800-223, University of Virginia Health Systems

Charlottesville, VA 22908

e-mail: [email protected]

ACKNOWLEDGMENTS

This study was made possible by an investigator-initiated grant funded by Johnson and Johnson. These data were presented at the 2014 annual scientific meeting of the Association for Research in Vision and Ophthalmology.

Received April 17, 2014; accepted February 5, 2015.

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Keywords:

astigmatism; driving; safety; toric contact lenses; simulator

© 2015 American Academy of Optometry