Zheng Chang, PhD; Paul Lichtenstein, PhD; Brian M. D’Onofrio, PhD; Arvid Sjölander, PhD; Henrik Larsson, PhD. “Serious Transport Accidents in Adults With Attention-Deficit/Hyperactivity Disorder and the Effect of Medication: A Population-Based Study” JAMA Psychiatry. doi:10.1001/jamapsychiatry.2013.4174. Published online January 29, 2014.

This study examines the association of adult ADHD with transport accidents and potential effects of ADHD treatment. The authors note that transport accidents exert a substantial burden on the world economy, such that they account for 2% of the global GNP. Untreated adult ADHD has been previously associated with increased rates of transport accidents or poor driving (increased accidents, false brakes and speeding) when adults with ADHD are examined on a driving stimulator. ADHD pharmacotherapy has also been shown to ameliorate performance on the driving simulator in these adults (Barkley RA, Cox D. J Safety Res. 2007;38(1):113-28. Epub 2007 Feb 15.) However, these studies are of relatively small sample sizes and concerns have been raised regarding potential referral bias influencing the study results.

Chang and co-authors examined over 17,000 individuals with ADHD over a four year period in the Swedish national registries. Cox proportional hazards regressions were used to examine the association between ADHD and serious traffic accidents (resulting in death or emergency room visit). Stratified Cox regressions were used within patients to examine the rates of accidents on and off medication. ADHD treatment was defined as filling a prescription for an ADHD medication within a six month period.

The study found significantly higher rates of transport accidents in men (adjusted hazard ratio, 1.47; 95%CI, 1.32-1.63) and women (1.45; 1.24-1.71) with ADHD, compared to the control group. Medication treatment was associated with a 58% risk reduction in men, but no significant risk reduction in women. The potential reasons for this gender difference is not fully known, but the 40% lower base rate of accidents in women versus men may have created a floor effect, where the base rates were lower in women to an extent which minimized potential ameliorative effects of medication. It was estimated that continuous medication treatment would have cut accident rates almost in half, presumably from positive effects on ADHD related symptoms and impairment and resulting improvement in driving (the ameliorative effects of medication on accidents is associative not causative in this trial).

This is a very important trial as it extends observations of increased motor vehicle accidents in untreated adults with ADHD to a large community based sample. It further highlights the ameliorative effects of ADHD pharmacotherapy observed in driving simulator studies to a large population based sample, which further emphasizes the importance of adults with ADHD receiving treatment. Of note, the definition of being on medication in the study, of filling a prescription in the last six months, is a fairly liberal definition of adherence to treatment; therefore, the potential beneficial effects of medication may if anything be under-estimated by this study given the long period of time individuals presumed to be treated with medication might have been off medication. Furthermore, the authors did examine whether defining medication treatment as filling a prescription within three months, would influence the findings; this tighter definition of treatment yielded similar results.

Overall adherence rates to treatment in ADHD are fairly poor, whereby most adults and young adults do not fill beyond their second prescription of psychostmulants. (Adler LD, Nierenberg AA. Review of medication adherence in children and adults with ADHD. Postgrad Med. 2010 Jan;122(1):184-91. doi: 10.3810/pgm.2010.01.2112.) It is important, given the earlier findings and results of the current study, for ADHD adults and young adults to be treated pharmacologically in periods when they are driving motor vehicles. Overall treatment principles for adults and young adults with ADHD suggest treatment throughout the day, to ensure symptomatic relief for longer periods of time during the day and especially during periods when driving (Adler LA, Barkley RA, Newcorn JH. Performance improvement CME: adult ADHD. J Clin Psychiatry. 2011 Apr;72(4):e15. doi: 10.4088/JCP.9066pi4c.). Clinicians may use the increased risk of motor vehicle accidents in untreated adults with ADHD as means of reinforcing the importance of medication adherence, when discussing adherence to pharmacotherapy with patients.