Reports

This report is based on medical evidence presented at sanctioned medical congress, from peer reviewed literature or opinion provided by a qualified healthcare practitioner. The consumption of the information contained within this report is intended for qualified Canadian healthcare practitioners only.

Based on an article in Pediatrics 2006;118:e704-10.

February 2008

With:

Timothy S. Bilkey, MD, FRCPC Director, Bilkey ADHD Clinics Barrie, Ontario Adjunct Professor of Psychiatry Schulich School of Medicine and Dentistry University of Western Ontario London, Ontario

Derryck H. Smith, MD, FRCPC Medical Director Mental Health Programs, Children’s Hospital Head, Division of Child and Adolescent Psychiatry University of British Columbia Vancouver, British Columbia

ADHD and Accidents: A Statistical Pile-Up

A substantial body of literature, ranging from reports compiled by government agencies to scientific studies, confirms that young drivers—those under 20 years of age—are at high risk of traffic violations and collisions. Over the last several years, numerous studies have revealed and elucidated a link between attention-deficit hyperactivity disorder (ADHD) and poor driving performance, which may explain at least a portion of this high accident rate in young drivers. (Older drivers with ADHD are not immune to poor driving performance, but there is some evidence that over time, they develop compensatory mechanisms; and the majority of studies have focused on younger subjects.)

According to recent meta-analyses, individuals with ADHD are 54% to 88% more likely than average to have been involved in a motor vehicle accident (Truls, www.immortal.or.at/index.php, Jerome et al. Curr Psychiatry Rep 2006;8:416-26). One of these reports, which evaluated six studies, indicated they are also 35% more likely to receive citations for traffic violations, 57% more likely to drive without a license, and 49% more likely to drive under the influence of alcohol (Jerome et al. 2006). Various studies also have demonstrated that drivers with ADHD are four times more likely than average to be at fault when they have a motor vehicle accident and up to eight times more likely to have their license revoked (Barkley et al. Pediatrics 1993; 92:212-8, Pediatrics 1996 (6pt1):1089-95). Perhaps unsurprisingly, the severity of ADHD in childhood appears to influence the risk of later poor driving performance and the frequency of accidents. Comorbid conditions such as conduct disorder and oppositional defiance disorder further increase the risk.

Links can be drawn between risky driving behaviours and inattention and distractibility. Patients with ADHD may also display poor risk perception and lack of judgment and reasoning while driving (Jerome et al. 2006). In a number of reports, observers have found that drivers with ADHD make more impulsive errors and have a reduced ability to adapt to changing circumstances. For example, one recent study of 147 individuals with ADHD and 71 controls determined that in simulated driving settings, those with ADHD had slower or more variable reaction times as well as greater steering variability and a higher number of crashes or scrapes against roadside obstacles (Fischer et al. Accid Anal Prev 2007;29:94-105). Individuals with ADHD also appear to be more likely to demonstrate poor driving competency due to quick anger and aggression while behind the wheel.

Why Is Driving Ability Affected?

In a recent review (J Safety Res 2007;38:113-28), Barkley and Cox point out that driving is a form of multitasking; it involves simultaneous operational, tactical and strategic competency. In this model, deficiencies at operational levels—for example, in concentration, attention, spatial perception, visual scanning, visual-motor integration, cognitive processing or reaction times—will have a negative impact on tactics (decisions on speed and passing, for example) and strategy (planning a route and making pre-emptive decisions). Impairment in any of the three domains heightens the risk of poor driving behaviour.

The executive function capacity of the brain is immature even in normal adolescents; this incomplete development, combined with inexperience, is believed to contribute to excessive risk-taking and accidents in young drivers. Recent research suggests that the elevated risk of poor driving in patients with ADHD is the result of a greater or more persistent neurobiologically-based deficiency in executive functioning or elementary cognitive abilities. This deficit may manifest as inattention/distractibility, impulsivity/lack of inhibition or poor adherence/application of the rules of the road. For example, patients with ADHD display substantially less ability to sustain attention on a continuous driving performance test (Barkley et al. J Abnormal Psych 2002; 111:279-89). Study subjects with ADHD have also exhibited less complete knowledge of drivers’ regulations than that shown by controls (Barkley et al. 1996); however, this finding might also be interpreted as a slower ability to process and implement that knowledge while driving.

Efficacy of Pharmacotherapy

Numerous studies undertaken in the last several years have analyzed whether and how successfully stimulant medications for ADHD—which unquestionably improve inattention, distractibility and impulsivity—influence patients’ ability to operate a motor vehicle. Most such studies have evaluated the subjects’ own impression of their driving competency as well as a more objective measure such as performance on a virtual-reality driving simulator. A consistent limitation for the majority of the trials is small sample size, while some may be criticized for possible selection bias, overrepresentation of males, short duration, insufficient randomization or attention to treatment compliance. However, in the aggregate, their results make a persuasive case for ensuring patients with ADHD are under the effects of appropriate medication while driving.

In the first such study, immediate-release methylphenidate (MPH) was shown to improve simulated driving performance as compared with placebo in males aged 19 to 25 (Cox et al. J Nerv Ment Dis 2000;188:230-4). As might be expected, a limitation of immediate-release MPH is the tendency for its effect to wear off. In a study comparing the immediate-release medication (given at 8 a.m., noon and 4 p.m.) and sustained-release (OROS) MPH (given once daily at 8 a.m.), six teenage male drivers with ADHD were evaluated at four time points in the afternoon, evening and night. With immediate-release MPH, they exhibited improved driving at 2 p.m. but increasing impairment as the day progressed. At 8 p.m., they had significantly more impaired driving scores as measured by inappropriate braking, missed stop signs or lights, collisions and rapid changes in speed. When they used OROS MPH, they had improved scores over the evaluation period (Cox et al. J Am Acad Child Adolesc Psychiatry 2004;43:269-75). Another study that evaluated actual vs. simulated driving performance by 12 male teenage patients with ADHD determined that OROS MPH significantly reduced errors related to inattentiveness; however, it did not influence speeding or impulsive errors (Cox et al. J Am Board Fam Pract 2004;17:235-9).

Overall, controlled studies of MPH vs. placebo have demonstrated improved driving with active treatment. Recently published pediatric psychiatry guidelines (J Am Acad Child Adolesc Psychiatry 2007;46(7):894-921) cite the finding by Cox and colleagues that long-acting MPH may have better effects on adolescents’ driving performance than the short-acting agent. The advantage of the OROS formulation likely relates to consistency of blood levels, although driving performance than may deteriorate if the agent’s effects begin to wane at the end of the dosing period. Beneficial effects from either short- or long-acting agents likely decrease as the medication is metabolized.

Fewer data are available on the efficacy of extended-release mixed amphetamine salts (MAS XR) vs. placebo. In a six-week crossover study of 15 patients aged 19 to 25 years, treatment led to significant improvements in performance (Figure 1), including fewer collisions, better crash avoidance, and less speeding, swerving out of lane and tailgating.

Figure 1. Driving Simulator Study: Driving Safety Score (intent-to-treat population)

Cox and colleagues published a comparison of the relative benefits on simulated driving performance of two long-acting stimulants, OROS methylphenidate and MAS XR (Pediatrics 2006;118:704-09). The study population included 19 males and 16 females aged 16 to 19; there was a preponderance of individuals with combined (n=12) and inattentive (n=21) subtype of ADHD. In this crossover study, the subjects took OROS MPH titrated to 72 mg/day and MAS XR titrated to 30 mg/day (doses deemed by the investigators to be effective, well tolerated and approximately equivalent) for 17 days each. Driving evaluations were conducted on days 10 and/or 17 at 5 p.m., 8 p.m. and 11 p.m. Both the subjects’ self-ratings and that of the investigators were assessed.

Although both treatments improved driving performance at various time points, the authors concluded that OROS MPH resulted in the best overall driving performance, as measured by an impaired driving score encompassing multiple variables of speed, steering and braking. As a practical analogy, they suggested that drivers taking OROS MPH performed approximately as well as drivers aged 55 to 59, while those taking placebo drove as well as individuals in their late 70s.

According to their analysis, overall results with MAS XR were not better than those with placebo. However, this finding is not consistent with the documented effects of MAS on ADHD symptoms. The authors acknowledged that the study was likely underpowered to detect a significant benefit of this agent relative to placebo; and that if the doses selected for the study were not actually equivalent, a higher dose of MAS might achieve greater effects. If one applies Canadian Attention Deficit Hyperactivity Disorder Resource Alliance (CADDRA) guidelines, 54 mg of OROS MPH would be equivalent to 30 mg of MAS XR and therefore, the doses used in this study were not comparable. (Furthermore, these guidelines would suggest that appropriate doses would be up to 108 mg for OROS MPH and up to 60 mg for MAS XR.)

The benefits on driving performance with the nonstimulant medication atomoxetine vs. placebo have been assessed in a single pilot study (Barkley et al. J Atten Disord 2007;10(3):306-16). In contrast with the majority of prior studies, this assessment evaluated 18 adults with a mean age of 37 years. Although the subjects considered their ADHD symptoms and driving performance improved on a dose of 1.2 mg/kg daily, performance on a simulator or as evaluated by investigators was not significantly better than with placebo. Further study of this agent is likely to take place.

Practical Points for the Patient Who Wants to Drive

According to the Canadian Medical Association (CMA), physicians have a responsibility to ensure their patients are medically fit to operate a motor vehicle. Ideally, a patient with ADHD who has or wants to obtain a driving permit should be screened with one or more tools that can predict poor performance. A relatively simple first step for clinicians is the administration of screening questionnaires. One of these, the Jerome questionnaire, is a nine-item scale that collects the driver’s assessment of his or her own driving and additional relevant information on attention, impulsivity, alertness and emotional volatility. Referral to a driving assessment centre may frequently be appropriate but can be difficult.

A driver’s license is a source of pride and independence for most young people. To ensure they can earn and maintain the privilege, individuals with ADHD should be advised of the appalling statistics related to driving without medication, and that their driving will be less prone to potentially dangerous mishaps with proper use of medication. In addition, studies show many young drivers are keenly aware of the difference between their driving performance on and off medication, a finding that can similarly promote adherence.

In its current guidelines, the CMA states that “stimulants most likely reduce the risk of moving violations and crashes for drivers with ADHD, particularly in the first five years of driving.” In line with the association’s recommendation, the treating physician should recommend that any individual with ADHD drives only when using an appropriate medication with effects that alleviate symptoms and extend into all the hours the individual is likely to operate a vehicle. A regimen that works successfully during school or work hours but allows medication effects to wear off by 8 p.m. or so will probably not be appropriate for a young driver on the road after midnight. In some instances, the timing of the stimulant dose may be altered to reflect driving needs. Long-acting stimulants are generally preferable. Demonstrated previous response to stimulants probably points to better success with driving. As suggested by the authors of the comparative study, adolescents who enjoy a robust effect of MAS can continue to use this agent, especially if they have previously used MPH and found their response suboptimal.

Interestingly, a pilot study has suggested that driving a car with a manual transmission, because it requires more directed attention, may be useful for patients with ADHD (Cox et al. J Atten Disord 2006 10(2):212-16). However, this finding remains preliminary. Perhaps more practical are common-sense manoeuvers such as avoiding distractions (loud music, cell phone conversations) and the use of alcohol or drugs. Since many teens going out as a group are used to assigning a designated driver, it may be possible for the patient’s friends to extend this practice to ensure he or she need not drive.

Further Steps

Once the individual has a learner’s or driver’s permit, the CMA advises that licensing authorities and the parents of young drivers with ADHD should pay “close attention to speeding, red light infractions and risk-taking behaviour.” Physicians should also inquire about the patient’s driving performance in any discussion about treatment, and attempt to remain abreast of research and recommendations on reducing the risk of poor driving in their patients with ADHD.