On-road driving impairments in Huntington disease

Hannes Devos, PhD Alice Nieuwboer, PhD Wim Vandenberghe, MD, PhD Mark Tant, PhD Willy De Weerdt, PhD Ergun Y. Uc, MD

Correspondence to Dr. Devos: [email protected]

ABSTRACT

Objective: To determine the driving skill impairments and underlying visual, motor, and cognitive deficits that lead to failure on road testing in manifest Huntington disease (HD).

Methods: Certified driving assessment experts scored performance on 13 specific on-road driving skills in 30 persons with HD and 30 controls and issued a pass/fail decision based on their overall impression. These on-road skill items were mapped onto an existing theoretical framework that categorized driving skills into operational, tactical, visuo-integrative, and mixed clusters. The HD group additionally completed a detailed off-road battery of motor, visual, and neuropsychological tests. Results: The HD group performed worse on all on-road items. Fourteen drivers with HD (47%) failed the road test compared with none of the controls. Scores on the Total Functional Capacity scale discriminated significantly between pass and fail groups. Total on-road score and performance in operational, tactical, and visuo-integrative clusters correlated strongly (Spearman r .0.50) with the pass/fail decision. The off-road tests showed variable strengths of association depending on the level of driving skill. Selective attention was strongly associated (Spearman r .0.50) with the total on-road score and all driving clusters. Conclusions: HD affects driving at many levels due to motor and cognitive deficits and leads to unsafe road performance even in mild stages. The high failure rate on the road test and difficulties in all aspects of on-road driving suggest that monitoring of fitness to drive should be initiated in the early course of HD. Neurology® 2014;82:956–962 GLOSSARY HD 5 Huntington disease; IQR 5 interquartile range; SDMT 5 Symbol Digit Modalities Test; TFC 5 Total Functional Capacity; TMT 5 Trail Making Test; TRIP 5 Test Ride for Investigating Practical fitness to drive; UFOV 5 Useful Field of View; UHDRS 5 Unified Huntington’s Disease Rating Scale.

Huntington disease (HD) is an autosomal dominant neurodegenerative condition characterized by a triad of motor symptoms, cognitive impairments, and psychiatric problems.1 Although HD can develop throughout the lifespan, the symptoms occur most often around the age of 40 years.1 Individuals in that particular time of life rely heavily on a car for work, domestic needs, and leisure. Driving a car is a mainly automatic yet highly complicated activity requiring a timely interplay of visual, motor, and cognitive processes to adequately react to a dynamic and constantly changing environment.2 A substantial portion of persons in the early stage of HD give up driving.3–7 Those who continue driving report changes in their driving behavior such as avoiding distracting conversations with passengers and driving smaller distances, less frequently, or more slowly.5–7 However, some drivers with HD may not be aware of their driving impairments and continue to drive unsafely.7 These persons are at an increased risk of car crashes.4,7 Few studies have sought to determine the best clinical predictors of fitness to drive in HD.3,7 In a review of medical charts and cognitive performance of 74 persons with HD, a comprehensive cognitive evaluation (total score of the Repeatable Battery for the Assessment of Neuropsychological Status) emerged as the sole predictor of driving status.3 A best-fit model additionally From the Department of Physical Therapy (H.D.), College of Allied Health Sciences, Georgia Regents University, Augusta, GA; Department of Rehabilitation Sciences (H.D., A.N., W.D.W.), KU Leuven, Heverlee; Department of Neurology (W.V.), University Hospitals Leuven; Department of Neurosciences (W.V.), KU Leuven, Leuven; CARA Department (M.T.), Belgian Road Safety Institute, Brussels, Belgium; Department of Neurology (E.Y.U.), University of Iowa, Iowa City; and Neurology Service (E.Y.U.), Veterans Affairs Medical Center, Iowa City, IA. Go to Neurology.org for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the article. 956

© 2014 American Academy of Neurology

retained the Total Functional Capacity (TFC) score of the Unified Huntington’s Disease Rating Scale (UHDRS).3 In a study of 30 active drivers with HD, we identified poor performance on the Symbol Digit Modalities Test (SDMT), Stroop Word Reading, and Trail Making Test (TMT) B as the most parsimonious set of screening measures to accurately predict failure on a comprehensive fitness to drive evaluation.7 The outcome on this fitness to drive evaluation was based on detailed visual and cognitive testing as well as on a standardized on-road test. Road testing is the gold standard in driving assessment.2 We are not aware of any study in the English medical literature that has investigated actual road performance in HD. Identifying specific driving impairments and their underlying mechanisms will increase our understanding of unsafe driving in HD and may help to develop mitigation strategies toward driving restrictions and eventually cessation of driving. Therefore, in this study, we concentrated on detailed analysis of the road testing component of the official driving evaluation. We used all clinical screening tools gathered from a previous study7 as well as additional specific visual and cognitive tests performed at the fitness to drive center to model road driving behavior in HD. The aims of this study were to (1) compare performance on a road test between drivers with HD and age-matched drivers without HD; (2) determine the critical driving skills that lead to failure on road testing in HD; and (3) investigate the underlying functional, motor, visual, and cognitive impairments associated with these critical driving skills. METHODS Subjects. Patients with manifest HD were recruited from 2008 to 2012 in the Movement Disorders Clinic of the University Hospitals Leuven.7 Age- and sex-matched controls were recruited from the general population through word-of-mouth advertisement. Inclusion criteria for all participants were (1) valid driver’s license before diagnosis, (2) driving at least 300 km in the previous year, and (3) minimum binocular acuity of 10/20 (20/40) in accordance with Belgian legislation. Further inclusion criteria for the HD group were (4) an expanded (.39) CAG repeat in the HTT gene and (5) clinical observation of chorea. Participants with HD were not eligible if they had serious comorbidity affecting safe driving. Exclusion criteria for the control group were diagnosis of major neurologic, internal, visual, or psychiatric diseases that may interfere with safe driving. Thirty of 39 persons with HD who were screened for eligibility consented to participate. The demographic characteristics, driving survey, and medical history were collected at the University Hospitals Leuven. The road test and off-road assessment battery were administered at the Center for Evaluation of Fitness to Drive and

Car Adaptations of the Belgian Road Safety Institute and executed in random order. The control group completed only the demographic survey, the driving history, and the on-road evaluation.

Demographic and disease characteristics. Demographic and disease characteristics were documented, including the Mini-Mental State Examination,8 TFC section of the UHDRS,9 disease duration, CAG repeats, and disease burden.10 The road test. The on-road driving evaluation was administered in an Opel Astra with dual controls for reasons of standardization and safety. The on-road assessor was a certified fitness to drive specialist who was blinded to the performance of participants in the off-road tests, but not to the diagnosis of HD. The on-road assessor gave directions, talked with the driver at regular time intervals to evaluate the effect of distraction on driving, and intervened when necessary. The standardized 20-km road test comprised a route through the residential area of Brussels, continued to a 2-way, 4-lane highway section, proceeded to an urban section, and terminated at the evaluation center. On-road driving performance was administered using the Test Ride for Investigating Practical fitness to drive (TRIP) checklist. The TRIP has established reliability11 and validity12 and has been used to evaluate driving deficits in older drivers,13 Parkinson disease,14,15 and stroke.16 This checklist assessed 13 critical on-road driving skills: lateral position on the road at speed (I) below and (II) above 50 km/h; (III) mechanical operations; speed adaptations at speed (IV) below and (V) above 50 km/h; gap distance at speed (VI) below and (VII) above 50 km/h; (VIII) lane position change; (IX) anticipation and perception of road signs and traffic signals; (X) visual behavior and communication; (XI) understanding, insight, and quality of traffic participation; (XII) turning left; and (XIII) merging into traffic stream. The 13 items were categorized into 4 clusters13 that can be mapped onto existing theoretic frameworks of driving skill.17 The operational cluster (items I–III) represented skills of vehicle control that had to be executed within milliseconds, including use of pedals and steering wheel and position of the car on the road.13,17 The tactical cluster (items IV–VIII) reflected driving actions that needed to be executed within seconds and involved executive functions including attention, flexibility, adaptation strategies, and adequate judgment.13,17 The visuo-integrative cluster (items IX–XI) comprised visuospatial, visuoperceptual, and higher-order cognitive skills.13 The mixed cluster (items XII and XIII) consisted of complex driving skills that required a combination of operational, tactical, and visuo-integrative skills.13 The TRIP scored 49 subitems of driving skill on a 4-point ordinal scale of 1 (poor), 2 (insufficient), 3 (sufficient), and 4 (good). A pass/fail decision (main outcome) was derived immediately after the on-road evaluation. Patients were assigned to the pass category when the on-road assessor observed no functional deficits that interfered with driving or when patients showed adequate compensation strategies for their functional deficits. Those allocated to the fail category showed poor overall performance on the TRIP evaluation or incurred serious adverse events that necessitated physical intervention of the onroad examiner (emergency brake, taking over the steering wheel). Off-road testing battery. The motor examination consisted of UHDRS–motor section9 and movement time of the 4-choice reaction time test.18 Visual sensory information was gathered using the binocular acuity,19 kinetic vision,19 and contrast sensitivity tests.20 The neuropsychological battery was composed of a wide array of tests including decision time of the 4-choice reaction time test,18 the UHDRS cognitive section (Letter Verbal Fluency Test; Stroop Color, Word, Interference; and SDMT),9 TMT A and B,21 executive control,22 incompatibility (cognitive flexibility),22 Neurology 82

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Useful Field of View (UFOV),23 divided attention,22 copy test of Rey-Osterrieth Complex Figure,24 and visual scanning.22

Standard protocol approvals, registrations, and patient consents. The study was approved by the Ethics Committee of the University Hospitals Leuven. All participants signed the informed consent form.

Statistical analysis. Kolmogorov-Smirnov tests were used to determine normality of the variables. The x2, Fisher exact, Wilcoxon rank-sum, and independent t tests were used to investigate differences in on-road and off-road variables between groups. Spearman r correlations were calculated between off-road and on-road items in the HD group. Correlations were considered to be weak below 0.10, moderate between 0.10 and 0.49, and strong between 0.50 and 1.00.25 Variables that were significantly correlated with the pass/fail outcome were entered in a multivariate stepwise logistic regression analysis. All statistical analyses were performed with SAS version 9.3 and SAS Enterprise software (SAS Institute, Inc., Cary, NC); p values ,0.05 were considered significant.

Patients with HD were on average 50.20 (12.40) years old; the average age of the control subjects was 50.26 (12.64) years (t test, p 5 0.98). In both groups there were 8 women. The HD group performed worse on the Mini-Mental State Examination (median 5 27, interquartile range [IQR] 5 25–28) than the control group (median 5 30, IQR 5 29–30; Wilcoxon ranksum test, p , 0.0001). Patients had very mild to mild disease severity. Nineteen patients were in Shoulson and

RESULTS

Table 1

Fahn stage 1 (TFC scores 11–13) and 11 in stage 2 (TFC scores 7–10). Six persons scored 13, 10 scored 12, 3 scored 11, 4 scored 10, 3 scored 9, 3 scored 8, and 1 scored 7. Median duration since onset of motor symptoms was 24 (12–48) months; median number of CAG repeats in HTT was 43 (41–45), and median disease burden was 383 (330–425). None of the controls failed the evaluation. In the HD group, 16 (53%) passed the on-road evaluation and 14 (47%) failed. Scores on the TFC were lower in the subjects with HD who failed the road test (median 5 10, IQR 5 9–11) than in those who passed (median 5 12, IQR 5 12–12.50; Wilcoxon rank-sum test, p 5 0.04). Six of 19 (31%) in Shoulson and Fahn stage 1 failed the road test, whereas 8 of 11 (72%) in stage 2 failed. Two of 6 individuals with maximal TFC scores failed the evaluation. The control group obtained submaximal to maximal scores on the road test (table 1). The HD group scored worse on each cluster and item of the road test. The pass group obtained submaximal to maximal scores on the road test (table 2). The fail group scored worse on all driving items, except for “gap distance at speed below 50 km/h.” Total TRIP score correlated highly with the on-road decision (r 5 20.64, p 5 0.0001). The correlations of the clusters with the on-

Difference in on-road driving performance between HD and control groups

Variable Total TRIP score Operational cluster

Range

Subjects with HD (n 5 30), median (Q1–Q3)

Controls (n 5 30), median (Q1–Q3)

p Valuea

49–196

186 (157–196)

196 (196–196)

,0.0001 ,0.0001

7–28

27 (20–28)

28 (28–28)

I

Lateral position on the road at speed ,50 km/h

2–8

8 (6–8)

8 (8–8)

0.002

II

Lateral position on the road at speed .50 km/h

2–8

8 (6–8)

8 (8–8)

0.003

III

Mechanical operations

Tactical cluster

3–12

11 (8–12)

12 (12–12)

,0.0001

13–52

50 (42–52)

52 (52–52)

0.002

IV

Speed adaptations at speed ,50 km/h

2–8

7 (6–8)

8 (8–8)

0.003

V

Speed adaptations at speed .50 km/h

2–8

6.5 (6–8)

8 (8–8)

,0.0001

VI

Gap distance at speed ,50 km/h

2–8

8 (6–8)

8 (8–8)

0.002

VII

Gap distance at speed .50 km/h

2–8

8 (6–8)

8 (8–8)

,0.0001

VIII

Lane position change

Visuo-integrative cluster

5–20

20 (18–20)

20 (20–20)

0.01

14–56

53 (41–56)

56 (56–56)

,0.0001

IX

Anticipation and perception of signs and traffic lights

4–16

16 (12–16)

16 (16–16)

0.0002

X

Visual behavior and communication

8–32

31 (23–32)

32 (28–32)

,0.0001

XI

Understanding, insight, and quality of traffic participation

2–8

8 (6–8)

8 (8–8)

0.0003

15–60

60 (51–60)

60 (60–60)

0.0004

Mixed cluster XII

Turning left

9–36

36 (33–36)

36 (36–36)

0.0009

XIII

Joining the traffic stream

6–24

24 (19–24)

24 (24–24)

,0.0001

Abbreviations: HD 5 Huntington disease; Q 5 quartile; TRIP 5 Test Ride for Investigating Practical fitness to drive. a Wilcoxon rank-sum test.

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Table 2

Difference in on-road driving performance between HD pass and fail groups and correlation with pass/fail decision

Variable Total TRIP score

Score range

HD pass group (n 5 16), median (Q1–Q3)

HD fail group (n 5 14), median (Q1–Q3)

Between-group Correlation with pass/fail p valuea

49–196

195 (184.5–196)

154 (114–187)

,0.0001

20.64

0.0001

p Valueb

7–28

28 (25.5–28)

21 (16–27)

0.003

20.60

0.004

I

Lateral position on the road at speed ,50 km/h

2–8

8 (8–8)

6 (4–8)

0.02

20.62

0.0002

II

Lateral position on the road at speed .50 km/h

2–8

8 (8–8)

6 (4–8)

0.002

20.74

,0.0001

III

Mechanical operations

3–12

12 (10.5–12)

10.5 (6–11)

0.01

20.47

0.008

13–52

52 (49.5–52)

43 (29–50)

0.004

20.59

0.005

Operational cluster

Tactical cluster IV

Speed adaptations at speed ,50 km/h

2–8

8 (8–8)

5 (4–7)

0.004

20.66

,0.0001

V

Speed adaptations at speed .50 km/h

2–8

8 (7–8)

5 (2–6)

0.001

20.72

,0.0001

VI

Gap distance at speed ,50 km/h

2–8

8 (8–8)

8 (5–8)

0.11

20.32

0.09

VII

Gap distance at speed .50 km/h

2–8

8 (8–8)

8 (4–8)

0.03

20.44

0.01

VIII

Lane position change

5–20

20 (20–20)

19 (15–20)

0.02

20.47

0.009

14–56

56 (53–56)

38.50 (32–52)

0.001

20.66

,0.0001

Visuo-integrative cluster IX

Anticipation and perception of signs and 4–16 traffic lights

16 (16–16)

12 (9–14)

0.001

20.68

,0.0001

X

Visual behavior and communication

8–32

32 (29–32)

23.5 (16–31)

0.008

20.53

0.002

XI

Understanding, insight, and quality of traffic participation

2–8

8 (8–8)

5 (4–8)

0.002

20.72

,0.0001

15–60

60 (58.5–60)

50 (33–60)

0.02

20.47

0.01

Mixed cluster XII

Turning left

9–36

36 (36–36)

30.5 (22–36)

0.009

20.62

0.0002

XIII

Joining the traffic stream

6–24

24 (22.5–24)

19.5 (12–24)

0.02

20.45

0.01

Abbreviations: HD 5 Huntington disease; Q 5 quartile; TRIP 5 Test Ride for Investigating Practical fitness to drive. a Wilcoxon rank-sum test. b Spearman r correlation.

road decision varied between 20.47 for the mixed cluster and 20.66 for the visuo-integrative cluster. The road clusters were entered in the logistic regression analysis. Only the visuo-integrative cluster emerged as significant variable in the multivariate analysis (odds ratio 5 0.84, 95% confidence interval 0.73–0.96, p 5 0.01). The individual road items showed variable strengths of association, ranging between 20.32 and 20.74. Five (items II, IV, V, IX, and XI) of the 13 individual item scores correlated better with the on-road decision than the total on-road score (table 2). The off-road tests showed variable strengths of association with on-road driving performance (table 3). Strong correlations were found between total score of the TRIP and motor functions (UHDRS–motor section), information processing (Stroop Color and Word), executive (executive control), UFOV, divided attention, and visual tracking and working memory (SDMT). The operational cluster correlated strongly with visual sensory functions (kinetic vision), motor symptoms (UHDRS–motor section), and cognitive functions including speed of processing (Stroop Word), UFOV total score, UFOV selective attention, divided attention, and SDMT. All off-road measures except the

selective component of UFOV failed to correlate strongly with the tactical and mixed cluster. Finally, the visuo-integrative clusters correlated strongly only with measures of the cognitive domain: speed of processing (Stroop Color, Stroop Word), executive functions (executive control), UFOV total score and selective attention, and visual tracking and working memory (SDMT). Visual measures such as binocular acuity and contrast sensitivity failed to correlate strongly with any of the onroad measures. Similarly, decision time, Letter Verbal Fluency Test, Stroop Interference, incompatibility, speed of processing component of UFOV, Rey-Osterrieth Complex Figure, and visual scanning did not show strong correlations with on-road driving performance. All off-road variables that correlated strongly with on-road performance were entered in the stepwise logistic regression analysis to determine pass/fail decision. UFOV selective attention emerged as the only predictor of pass/fail (odds ratio 5 1.17, 95% confidence interval 1.02–1.34, p 5 0.02). DISCUSSION In this study, we examined the critical driving impairments and the key motor, visual, and Neurology 82

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Table 3

Correlation matrix of on-road total score and clusters with performance on visual, motor, and neuropsychological tests in Huntington disease (n 5 30) Total score r

Binocular acuity Kinetic vision Contrast sensitivity

Operational p

0.14

r

0.48

0.41

0.54

a

p

p

r

p

0.7

0.05

0.8

20.03

0.89

0.14

0.38

0.04

0.39

0.03

0.28

0.28

0.14

0.13

0.49

0.004a

20.54a

0.002a

20.38

0.04

20.42

0.02

20.41

0.02

Movement time

20.24

0.19

20.44

0.014

20.15

0.44

20.30

0.11

20.28

0.13

Decision time

20.31

0.09

20.31

0.09

20.29

0.13

20.37

0.04

20.27

0.15

0.25

0.18

0.18

0.33

0.35

0.06

0.21

0.25

a

0.40

0.03

a

0.43

0.02

Stroop Word

0.24

0.17

0.002

r 20.07 a

0.2

0.54

a

0.55

a

a

0.002

0.38

0.43

a

0.50

0.002

0.31

r

Mixed

20.51a

Stroop Color

0.14

0.16

Visuo-integrative

UHDRS–motor section

LVFT

0.26

p 0.27

0.03

Tactical

0.02 a

0.005

0.1

0.46 a

a

0.01

0.41

0.59

0.02

0.59

a

0.0006 0.0006

Stroop Interference

0.39

0.03

0.43

0.02

0.20

0.28

0.43

0.02

0.25

0.18

Executive control

0.52a

0.003a

0.49

0.006

0.39

0.03

0.58a

0.0008a

0.47

0.01

20.20

0.30

20.21

0.27

0.001

20.48

0.007

0.39

20.11

0.58

Incompatibility

20.24

UFOV

20.53

UFOV 1

20.13

UFOV 2

20.44

UFOV 3

20.61

20.21

0.20 a

a

0.002

20.69

0.48

20.27

a

0.0003

20.73

a

,0.0001 0.15

20.48

0.01 a

0.27 a

20.38

0.04

20.56

0.06

0.74

20.16

20.40

0.007 a

20.24

20.49

0.03

20.49

0.006

,0.0001a

0.40

0.03

0.49

0.006

0.45

0.01

20.62

a

a

0.006

20.50

0.005

a

a

a

,0.0001

a

0.3 a

0.0003

20.51

a

0.004a

Divided attention

0.53a

0.003a

0.66a

ROCF

0.29

0.13

0.32

0.09

0.21

0.28

0.37

0.05

0.23

0.23

Visual scanning

20.24

0.19

20.25

0.19

20.16

0.39

20.29

0.12

20.23

0.22

TMT A

20.39

0.04

20.47

0.008

20.27

0.16

20.45

0.01

20.37

0.04

TMT B

20.41

20.31

0.10

20.49

20.39

0.03

SDMT

20.47

0.02 a

0.55

a

0.002

0.55

0.008 a

0.002

a

0.45

0.01

0.64

a

0.007 a

0.0001

0.47

0.009

Abbreviation: LVFT 5 Letter Verbal Fluency Test; ROCF 5 Rey-Osterrieth Complex Figure; SDMT 5 Symbol Digit Modalities Test; TMT 5 Trail Making Test; UFOV 5 Useful Field of View; UHDRS 5 Unified Huntington’s Disease Rating Scale. a Strong correlations (r . 0.50).

cognitive deficits associated with impaired driving in HD. We found deficits across all levels of driving skill, even in drivers with very mild symptoms of HD. Impairments in selective attention and higher-order visuo-integrative driving skills are most indicative for on-road failure in HD. The strength of our study is the use of a standardized road test with established reliability11 and validity12 (including inherent face validity26) to investigate driving safety in HD. Previous studies used different outcomes to measure the impact of HD on driving, such as qualitative interviews,5,6,27 functional assessment scales,6 driving simulators,4,28 neurologist’s appraisal of fitness to drive,3 or self-reported and state-registered car crashes.4 The lack of other on-road studies in HD prevents direct comparison. In a driving simulator experiment, drivers with HD were found to incur more errors than control drivers in operational and tactical skills including signaling, steering, braking, accelerator use, and maintaining appropriate speed.4 Our findings indicate that HD affects the whole spectrum of driving 960

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skills on the road, ranging from basic operational maneuvers such as vehicle control to the most complex driving skills including left turns at intersections and merging into traffic. Failure rate in the road test in this relatively mild HD sample was high: 14 participants in the HD group failed whereas none of the healthy controls performed poorly on the road test. As expected, the total on-road score was strongly associated with the pass/fail decision. Most TRIP items and all TRIP clusters correlated with the pass/fail decision. Modeling of TRIP clusters revealed that the visuo-integrative cluster represented the most important driving skill deficits resulting in failure on the road test in HD. We found significant associations between performance on the road test and deficits in visual, motor, speed of processing, attention, executive, and visuospatial functions consistent with the complex nature of driving and the wide spectrum of neurologic involvement in HD. The strengths of associations between off-road tests and on-road clusters depended on the complexity level of

the driving skill. Motor and basic visual sensory functions correlated only with specific operational maneuvers. Motor abilities are thought to underlie basic operational skills to keep the vehicle steady on the road.29 Drivers with psychomotor impairments due to advanced age30 or neurologic conditions31 have demonstrated difficulties in this specific domain of driving skill. By contrast, the visuo-integrative cluster showed strong correlations with various functions in the cognitive domain including speed of information processing, executive function, UFOV, visual tracking, and working memory, demonstrating the complexity of the driving skills in this cluster. Very few associations were found between off-road measures and the tactical and mixed clusters, which could be attributable to the difficulty of measuring these high-order functions with available cognitive tests. Selective attention was the sole variable that correlated strongly with all clusters and the total on-road score, showing that UFOV testing at the state driving evaluation agency added additional value to a predictive offroad battery beyond paper-pencil tests such as SDMT, Stroop Word Reading, or TMT B that were used as screening measures in our previous study.7 The UFOV selective attention test requires the localization of a car embedded in an array of triangles while focusing on a central fixation task at different speeds of presentation.23 This result is consistent with the observation that impaired selective attention in early-stage HD is common and manifests as difficulty identifying a single target among a series of distractors in rapid serial visual presentation tasks.32,33 UFOV performance was also an independent predictor of driving safety in Parkinson disease34 and Alzheimer disease.35 The TFC scale of the UHDRS differentiated well between pass and fail groups. The TFC assesses impairments in daily tasks such as occupation, chores, finances, household, and the ability to live at home. Measures of functional independence have been found to predict driving status3 and to correlate with self-reported car crashes4 in HD in accordance with our findings. However, the TFC cannot replace more detailed, specific testing in road safety prediction because 2 of 6 subjects with HD who had maximal TFC scores failed the evaluation, suggesting presence of driving impairment even in the very early stages of HD. There are also limitations to this study. We did not measure fatigue, personality characteristics, familiarity with the vehicle, or road course, which might have had a role in the road performance. In conclusion, drivers with HD exhibit problems in all clusters of driving skill. The relatively high percentage of drivers with HD failing the road test and presence of individuals with high functional scores among the failed indicate that monitoring for driving safety should start in the very early stages of manifest HD. UFOV selective attention may be used as an

off-road evaluation tool to assist clinicians in this complex decision-making process because it relates well with all levels of driving skill. Confirmatory study is needed to determine the best and most sensitive tests for this endeavor. Future research is also warranted to examine whether driving difficulties manifest even in the premotor stages of HD. AUTHOR CONTRIBUTIONS Dr. Devos was involved in the conceptualization of the study, analyzed and interpreted the data, and drafted the manuscript. Dr. Nieuwboer was involved in the conceptualization of the study, interpreted the data, and revised the manuscript. Dr. Tant was involved in the conceptualization of the study and interpreted the data. Dr. De Weerdt and Dr. Vandenberghe were involved in the conceptualization of the study, interpreted the data, and revised the manuscript. Dr. Uc was involved in the conceptualization of the study, analyzed and interpreted the data, and drafted the manuscript.

STUDY FUNDING No targeted funding reported.

DISCLOSURE The authors report no disclosures. Go to Neurology.org for full disclosures.

Received September 15, 2013. Accepted in final form December 2, 2013.

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On-road driving impairments in Huntington disease Hannes Devos, Alice Nieuwboer, Wim Vandenberghe, et al. Neurology 2014;82;956-962 Published Online before print February 12, 2014 DOI 10.1212/WNL.0000000000000220 This information is current as of February 12, 2014 Updated Information & Services

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Neurology ® is the official journal of the American Academy of Neurology. Published continuously since 1951, it is now a weekly with 48 issues per year. Copyright © 2014 American Academy of Neurology. All rights reserved. Print ISSN: 0028-3878. Online ISSN: 1526-632X.

On-road driving impairments in Huntington disease.

To determine the driving skill impairments and underlying visual, motor, and cognitive deficits that lead to failure on road testing in manifest Hunti...
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