RACETTE, LYNE PhD; CASSON, EVANNE J. PhD
Driving is a highly demanding task in which vision plays a crucial role.1 Visual acuity, contrast sensitivity, color vision, and depth perception all contribute to the analysis of the visual environment as it relates to driving; however, the integrity of the visual field has been demonstrated to be more important to driving fitness than other visual functions.2 Vision standards for driving must promote the safety of the driver as well as that of others on the road without unduly restricting people with reduced visual fields from driving. Although such standards should ideally be derived from research (i.e., evidence-based), in many jurisdictions they are determined by other, less empirical factors. As a result, the visual field standards2,3 and medical review procedures2 are highly variable across jurisdictions. There is a pressing need for research aimed at characterizing the relationship between visual fields and driving performance such that more unified, evidence-based visual field standards can be established.
Although several studies have investigated the relationship between visual field and driving performance, the evidence remains controversial. The normal boundaries of the visual field in each eye, as measured in degrees from the point of fixation, are approximately 60° superiorly, 75° inferiorly, 90° to 100° temporally, and 60° nasally.4 The normal binocular visual field therefore extends to approximately 135° vertically and 200° horizontally. Although severe visual field loss was shown to have a negative impact on driving performance, milder visual field loss may not result in impaired driving. In a landmark study that took annual mileage into consideration, Johnson and Keltner5 reported that drivers with severe binocular visual field losses have accident and violation rates twice as high as drivers of the same age with intact visual fields. Severe simulated restrictions of the binocular visual field (≤40°) were also shown to significantly compromise some aspects of driving (e.g., identification of road signs, avoidance of obstacles, time to complete the course) as assessed on a closed-road driving circuit.6 Furthermore, patients with severely reduced visual fields (≤20°) resulting from retinitis pigmentosa showed impairment on a driving simulator and were shown to have more accidents than people without visual field restrictions.7
The results of patients with milder forms of binocular visual field deficits from retinitis pigmentosa, glaucoma, and cerebrovascular accidents are less consistent. Some of these patients performed as well as age-matched control subjects on a driving simulator and had similar accident rates.7–9 A recent study showed that patients with glaucoma with visual fields restricted to <100° may be at greater risk of real-world and simulator accidents.10 Other studies in which adjustments for driving exposure were made failed to report higher crash rates for people with binocular visual field loss.11,12 Monocular drivers also have restricted visual fields; however, several studies report driving performance similar to control subjects.5,6,13–16
The variability in the findings associated with visual field and driving may be partly the result of the assessment methods. For example, simulated restrictions of the visual field are sudden and may be different from visual field loss that occurs secondary to eye diseases or stroke.3 Driving assessments on closed-road circuits and driving simulators have the advantage of presenting controlled environments; however, these environments may also be less challenging than driving under natural conditions. It is of interest to note that a number of studies in which real-world driving assessments were used report that patients with visual field loss are not at increased risk of having driving impairments.17–19 The methods of assessing visual fields may also have an impact on the relationship between visual fields and driving.20 In recent years, the Esterman binocular test has gained increasing acceptance.21
Perhaps the most important source of variability in reports on visual field and driving performance is the large degree of individual differences in developing compensatory strategies. Driving is a complex task that requires visual, but also cognitive skills such as visual attention. The useful field-of-view test measures selective and divided attention, as well as visual processing speed.22,23 A reduction in the useful field of view of older drivers was shown to be associated with crash involvement,12,24–26 and to be a good predictor of the outcome of closed-road27,28 and on-road driving assessments.29 The useful field of view was also found adequate to determine divided attention levels of patients with acquired brain injury and correlated well with their rehabilitation outcome.30 Furthermore, in stroke patients, the scores obtained on the useful field-of-view test significantly improved with training.31 This suggests that, at least in some patients, compensatory mechanisms based on visual attention can develop either spontaneously or through a training program.
The present study investigates the relationship between driving performance and the extent and location of visual field deficits in patients with visual field loss who were enrolled in a driving rehabilitation program. The rehabilitation experts take both vision and driving ability into consideration when assessing each patient and as such offer a unique source of information. An investigation of patients enrolled in a rehabilitation program allows for a direct assessment of the driving performance of patients with known visual defects under natural driving conditions.
From 1976 to 2004, the Bloorview MacMillan Rehabilitation Centre, located in Toronto, Canada, offered a driving rehabilitation program designed to 1) assess the driving performance of patients after injury or illness and 2) provide driving instructions to those patients who showed potential for rehabilitation. The program counted on the services of an occupational therapist (OT) specialized in driving and on two driving instructors. The two driving instructors who evaluated the patients included in the present study were experienced and had been at the Bloorview MacMillan Center for numerous years (25 and 15 years). Information pertaining to the health status and driving ability of each patient was recorded in a computerized database and in patient charts. Working in collaboration with the center, we used this information to study the impact of different types of visual loss on driving performance in natural settings.
Procedure for the Driving Assessments
Each patient first met with an OT specialized in driving rehabilitation. The interview included the following: a review of health, medication and driving history, a physical and functional evaluation, an assessment of the patient's level of activity within the community, a vision assessment, a reaction time assessment, and an evaluation of the knowledge of road rules. The visual assessment focused on visual acuity, depth perception, peripheral vision, night vision, and glare. To assess peripheral vision, patients were asked to look straight ahead while a target was brought in from the periphery toward the center. Patients had to verbally report the detection of the target. Neglect was assessed using the bells test.
The OT then met with the driving instructors to inform them of the results of the interview. The OT informed the driving instructors of any special equipment needed by the patient. Furthermore, the driving instructors were informed of the results of the tests performed at the interview that could affect driving performance such as reduced peripheral vision. The on-road driving assessment was performed in a dual-controlled car owned by the rehabilitation center. Patients were accompanied by one of the two driving instructors. The on-road assessment consisted of an approximately 50-min drive in the area surrounding the rehabilitation center and was conducted during the regular opening hours of the center (between 10 am and 3 pm). The first 10 min of the drive were used to assist patients in getting familiar with the car, the neighborhood, and with the testing situation. This familiarization period was not used to evaluate patients. The patient drove in the area surrounding the rehabilitation center on streets of variable traffic flow and with variable speed limits to allow the evaluator to judge the driver in a wide range of driving situations. The assessment always began on quiet neighborhood streets. Patients were then asked to cross a major street at a lighted intersection. They then drove through an industrial area onto a four-lane road (two lanes in each direction) through school areas and then onto a high-speed freeway if the patient was used to this type of traffic conditions. The evaluation could be aborted at any point if the driving instructor felt that it would be unsafe to continue.
During the on-road evaluation, the driving instructors wrote comments on a form on which the following aspects of driving were listed: entry into the vehicle, prestart routine, starting up, putting the vehicle in motion, use of gas, use of brakes, signaling, steering, left and right turns, speed, lane observance, lane changes, intersections, following and meeting traffic, putting to use knowledge of road rules, parking, reversing, tracking, perception, attitude/behavior, use of adaptive equipment (if applicable), and ability to understand and follow instructions. Based on these elements, the driving instructors generated one of nine overall assessment outcomes for each patient. If the driving instructors had doubts, they consulted with the OT before generating an assessment outcome. Drivers could be deemed clearly safe or unsafe to drive or the outcome could indicate that, with proper help, the drivers would be fit to drive in the future. For the purposes of this study, we combined some of these outcomes and created three categories of drivers: 1) those who were deemed safe to drive, 2) those who were deemed unsafe, and 3) those for whom no clear safe/unsafe judgments could be reached at the time of the evaluation (unknown) either because they needed more practice or instructions or because it was too soon after the stroke to determine whether they would be safe or unsafe drivers. In some cases, the driving instructor felt that the evaluation needed to be performed again. Figure 1 illustrates the outcomes used by the rehabilitation center and how we combined them into three categories.
We reviewed the files of 1350 patients (out of over 6000 files) at the Bloorview MacMillan Rehabilitation Centre. We identified 131 patients with visual field loss who had undergone an on-road assessment. These patients had a primary diagnosis of visual impairment or a primary diagnosis of cerebral vascular accident (CVA) with a secondary diagnosis of visual impairment. Patients who had documentation of neglect based on the bells test were excluded from this study. Patients with substantial motor or cognitive deficits, as assessed by an experienced OT, were excluded from this study. No standardized tests were performed to assess these deficits.
Patients were included in one of five categories of visual field loss based on the primary and secondary diagnoses, as well as on the extent of visual field entered in the database (obtained by confrontation testing by an OT). These categories are as follows: hemianopia, quadrantanopia, moderate visual loss, mild visual loss and monocular vision. Patients with hemianopia and quadrantanopia had a localized visual field defect that extended to an entire hemifield (hemianopia) or to an entire quadrant (quadrantanopia). In all cases, localized visual loss occurred secondary to a CVA. Patients with moderate and mild visual loss had diffuse visual loss secondary to any ocular condition. Patients with moderate visual loss had <135° of horizontal visual field measured at the midline, whereas patients with mild visual field loss had between 135° and 186° of horizontal visual field. These visual field cutoffs were selected arbitrarily as a reasonable way to categorize patients with diffuse visual loss into severity groups. Monocular patients had vision in only one eye secondary to any cause and no residual vision in the fellow eye. The causes of diffuse visual loss and monocular vision were not consistently documented in the database or in the charts and are therefore not reported in the present study.
We report the data obtained from 13 patients with hemianopia, 7 patients with quadrantanopia, 10 patients with moderate peripheral loss, 76 patients with mild peripheral loss, and 25 patients with monocular vision.
The results of this study were analyzed with the chi-square (χ2) test for noncontinuous variables using JMP software (SAS, Cary, NC). The data were analyzed to assess the impact of the extent and location of visual field loss on driving performance. The percentage of patients with each of the three driving outcomes (safe, unsafe, or unknown) is shown in Figure 2 for each type of visual field loss. Fifty-eight patients obtained an “unknown” outcome on the driving assessment (6 patients with hemianopia, 3 patients with quadrantanopia, 11 patients with monocular vision, 3 patients with moderate visual field loss, and 35 patients with mild visual field loss) and were excluded from all the statistical analyses reported here; they are, however, included in the tables and figures.
Extent of Visual Field Loss
No significant difference in the outcome of the driving assessment was observed when patients with all five types of visual loss were included in the analysis (χ2 = 4.37, p = 0.358). Similarly, no significant difference was observed between patients with localized visual field loss only (hemianopia vs. quadrantanopia) (χ2 = 3.33, p = 0.068). Although this effect did not reach statistical significance, it is interesting to note that a greater percentage of patients with hemianopia (42.86%; 3 of 7) were unsafe to drive compared with patients with quadrantanopia who obtained no “unsafe” driving outcomes (0%; 0 of 4). Approximately the same percentage of patients with hemianopia (46%; 6 of 13) and quadrantanopia (43%; 3 of 7) were excluded from this analysis as a result of an “unknown” outcome on the on-road driving assessment.
No significant difference in driving assessment outcome was observed between patients with diffuse visual loss (mild vs. moderate) (χ2 = 0.70, p = 0.402). The proportion of patients with diffuse visual loss excluded from the analysis as a result of an “unknown” outcome on the on-road driving assessment was different for patients with mild visual field loss (46%; 35 of 76) and moderate (30%; 3 of 10) visual field loss.
A large percentage of monocular drivers (79%; 11 of 14) obtained a “safe” outcome on the on-road driving assessment.
Location of the Visual Field Defect
When patients with all three types of visual field loss (localized, diffuse, and monocular) were included in the analysis, no impact of the location of visual field loss on the driving outcome was observed (χ2 = 1.05, p = 0.30).
The impact of the location of localized, diffuse, and monocular visual field loss on driving performance was then analyzed separately. The data obtained from patients with hemianopia and quadrantanopia were combined and used to look at the impact of the location of localized visual field loss on the outcome of the on-road driving assessment. The data were divided according to the location of the deficit (either left or right hemifield defect) and are presented in Table 1. None of the patients with localized visual loss in the left hemifield were deemed safe to drive. In contrast, none of the patients with localized visual loss in the right hemifield were deemed unsafe to drive. This effect was statistically significant (χ2 = 9.561, p = 0.002) and suggests that localized defects in the left hemifield may have a worse impact on driving performance than defects in the right hemifield. A slightly greater percentage of patients with an “unknown” outcome had localized visual field loss in the right (66%; 6 of 9) compared with the left (50%; 5 of 10) hemifield.
The results for moderate and mild visual field defects were combined and used to look at the impact of the location of diffuse visual field loss on driving performance when the loss was not concentric (Table 2). The results show that 90% (19 of 21) of patients with diffuse visual field loss in the left hemifield were safe drivers, whereas only 36% (4 of 11) of patients with diffuse visual field loss in the right hemifield were safe drivers. This effect was statistically significant (χ2 = 10.395, p = 0.001) and suggests that diffuse visual field loss in the right hemifield may have a worse impact on driving performance than defects in the left hemifield. This effect is opposite to what was observed for localized visual field loss. Thirty-three percent of patients (28 of 86) had concentric (equal loss in each hemifield) diffuse visual loss and were excluded from this analysis. The location of the diffuse visual field loss of patients with an “unknown” driving outcome was almost evenly distributed between the left (43%; 16 of 37) and right (48%; 10 of 21) hemifield.
The data obtained from the monocular drivers are illustrated in Figure 3. No statistical difference was observed (χ2 = 0.263, p = 0.608) suggesting similar driving performance for patients with vision in the right eye only compared with those with vision in the left eye only. The location of the monocular visual field loss of patients with an “unknown” driving outcome was slightly unevenly distributed between the left (38%; 5 of 13) and right (50%; 6 of 12) hemifield.
We conducted this retrospective study to further understand the impact of visual field loss on driving performance. Our results show that some patients with visual field loss can obtain “safe” driving outcomes on an on-road driving assessment. The results suggest that the extent of visual field loss may be related to driving performance. For example, patients with larger visual field loss (hemianopia and moderate visual field loss) were more likely to be unsafe drivers than patients with smaller visual field loss (quadrantanopia and mild visual field loss). Overall, the location of the visual field defect did not influence driving performance. However, the impact of the location of the visual field defect was different for patients with localized, diffuse, and monocular visual loss. Localized visual loss in the left hemifield and diffuse visual loss in the right hemifield were found to have a negative impact on driving performance. A large proportion of monocular drivers were fit to drive, and the location of their deficit was not related to driving performance.
The results obtained in the present study are consistent with previous findings showing that some patients with visual field loss can drive safely under natural driving conditions.17–19 Some of the patients included in this study who obtained a “safe” driving outcome on the on-road driving assessment did not meet the vision standards required for driving currently in place in many jurisdictions. Although visual field standards for driving are warranted based on previous research, our results suggest that their strict application to all drivers may prevent some competent individuals from driving. This can have a serious negative impact, particularly in a society in which driving is associated with a sense of independence and autonomy. Driving provides a means of performing daily activities, of getting to work, and maintaining social interactions.3 Furthermore, the ability to drive contributes strongly to health-related quality of life,32 and driving cessation is associated with social isolation and depression.33
Consistent with previous findings, our results suggest that the extent of visual field loss may be related to driving fitness. Overall, a greater percentage of “unsafe” driving outcomes were observed for patients with more severe visual losses. Similarly, a greater percentage of “safe” outcome was observed for patients with less severe visual losses. However, we observed large individual differences, suggesting that the absolute extent of the visual field should not be taken as the sole indicator of driving fitness.
Although it has been suggested that the location of visual field loss can have an impact on driving fitness, the overall results obtained in the present study argue against this claim. One explanation is that the small number of patients in each group did not allow us to obtain reliable results (many patients were excluded because they had an “unknown” outcome). The situation may be more complex, however, because different results were obtained for localized, diffuse, and monocular visual field loss. For example, localized visual field loss in the left hemifield had a negative impact on driving fitness. Visual loss in the left hemifield is associated with damage to the right side of the brain, which is also an important site for attentional control.34,35 Patients with visual loss in the left hemifield may therefore have reduced attentional skills affecting driving performance. This account is consistent with reports indicating a strong correlation between the useful field-of-view test (a test of visual attention) and driving performance. Although we excluded patients with neglect, it is possible that more subtle attentional deficits were present in some of our patients with localized visual field loss. Mazer et al.36 showed that patients with visual loss in the left hemifield are more likely to respond to an attentional training program to improve driving fitness.
The results obtained for patients with diffuse visual loss suggest that defects in the right hemifield may impair driving fitness. This finding is inconsistent with what we observed for patients with localized visual field loss. Therefore, predicting whether a patient with a visual field defect will be able to drive safely based on the extent and location of the deficit is difficult. Although the extent and location of the visual field loss may be related to driving performance, large individual differences exist. Based on our results, knowing where the deficit is located would not improve the prediction, and this information should not be used alone when trying to determine whether an individual may be fit to drive.
Our results for monocular drivers are consistent with many reports showing that patients with monocular vision can drive safely.5,6,13–16 No effect of the location of visual loss was observed for these patients. The main concern with monocular drivers has been their ability to perceive depth. Although stereopsis provides important cues for the perception of depth under certain conditions (i.e., within grasping space), there are a number of monocular depth cues (i.e., relative size, interposition, and texture) that are primary for depth judgments at greater distances. Therefore, depth can be estimated through different cues and its perception does not pose a significant problem when driving.
Limitations of the Present Study
The results of the present study must be interpreted in light of its inherent limitations. First, the purpose of the rehabilitation center was to assess the driving performance of patients with disabilities and to provide them with assistance to drive again whenever possible. As a result, no healthy control group was available to compare the driving outcomes of patients with visual field loss.
Second, the focus of the center was not on having standardized tests, but rather on making the most of whatever tests were available for each patient referred to the center. This was true for the assessment of cognitive and motor status, and for visual field loss assessment. Although all patients underwent confrontation visual field testing at the center, some were referred with results from automated perimetry. The assessment of cognitive and motor status was based on the experience and judgment of the occupational therapist.
Similarly, the on-road driving assessments were not conducted in a fully standardized fashion. For example, different driving instructors assessed the patients included in this study and no intergrader agreement analysis was performed. In addition, the driving instructors were not blind to the condition of the patients. Although the staff at the rehabilitation center made a substantial effort to be consistent in their assessment methods by meeting regularly to discuss these issues, we have no direct evidence to support their evaluation consistency.
This study was retrospective and in many cases, little documentation could be obtained with regard to the integrity of the visual system of the patients. For example, we did not have access to potential coexisting ocular conditions of the patients included in our study and in many cases, we did not have access to their visual acuity. Although all patients with localized visual field loss had experienced a CVA, we did not have access to the etiologies of the diffuse visual loss and we do not know how the monocular patients lost their vision. Furthermore, we could not determine with certainty whether each patient had an absolute or a relative visual field defect.
A relatively small number of patients were included in each group. This is in part because many patients obtained an “unknown” outcome on the driving assessment and could not be categorized as “safe” or “unsafe” drivers. Within the context of this study, we could not make any assumptions as to how patients with an “unknown” outcome would fare when reassessed at a later time.
Our results show that at least some patients with visual field loss can drive safely. Although the extent of visual field loss appears to be related to driving fitness, no clear conclusion could be drawn with regard to the location of the deficit. The presence of large individual differences within our sample highlights the need for individualized on-road assessments for patients with visual field defects. It also raises the possibility that, given appropriate training, some patients with visual loss may be able to drive safely. Prospective studies in this area are critical to determine the safety and sensitivity of this method. Whenever possible, patients with visual field defects should undergo a driving fitness evaluation, which should take the following factors into account: reports from an ophthalmologist or optometrist, good driving record, stability of the condition, motor skills, cognitive skills, and the results of an on-road driving assessment.37
The authors thank the Bloorview MacMillan Rehabilitation Centre and Margaret Young for their collaboration in this study.
University of California at San Diego
Hamilton Glaucoma Center
Department of Ophthalmology
9500 Gilman Drive
La Jolla, CA 92093-0946
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