Abstract

Drowsiness and falling asleep at the wheel are now identified as the reasons for many fatal crashes and traffic accidents. For years, fatigue has been associated with the risk of accidents but the causes of fatigue were unclear. Extended hours and nocturnal driving have been associated with accidents, but few reports differentiate fatigue from sleepiness. In the early 1990s, researchers using epidemiologic data began investigating sleepiness and sleep deprivation as causes of accidents. Falling asleep at the wheel associated with sleep restriction and nocturnal driving has been incriminated in 20% of traffic accidents. Drugs affecting the central nervous system (i.e., benzodiazepines, narcotic analgesics, and antihistamines), nocturnal breathing disorders, and narcolepsy also have been associated with an increased risk of accidents. Interestingly, chronic daytime sleepiness in monotonous circumstances (i.e., watching television or reading) is not associated with accident risk but situational sleepiness (i.e., drowsy driving) increases the risk of accidents very significantly. Several articles show that objective measures of sleepiness such as the Multiple Sleep Latency Test or Maintenance of Wakefulness Test can predict accident rates or deteriorated driving performance. Further studies before and after treatment are required in patients with obstructive sleep apnea as well as other types of sleep disorders involving drowsy driving (e.g., periodic leg movement syndrome, hypersomnia, insomnia, circadian rhythm disorders). This issue of evaluation is particularly crucial because studies show a major intersubject variability in response to sleep loss or sleep fragmentation. Treatments improving daytime vigilance (e.g., continuous positive airway pressure [CPAP] in obstructive sleep apnea) significantly reduce the risk of traffic accidents for a reasonable cost. On the other hand, drugs that promote alertness have never been evaluated regarding their protective effects with respect to sleep-related accidents. Many countries systematically evaluate medical disorders in professional drivers, but the criteria are still very heterogeneous; and an effort toward harmonization would be very beneficial, especially in Europe and the United States where drivers can cross several countries or states, each of which having different rules. Drowsy driving is still underdiagnosed, and sleep disorders are not well enough explored and treated in the exposed population of sedentary men. Sleep physicians also tend to pay insufficient attention to the problem of road safety in their clinical evaluation of nonprofessional drivers, and better awareness by the health community should help in reducing the risk of accidents. Public campaigns integrating notions of sleep hygiene and countermeasures (naps and coffee) as well as promotion of sleep medicine to combat drowsy driving could significantly improve road safety.

Traffic accidents are an increasing cause of death and injury in the world, and major countries have launched road safety campaigns in recent decades to decrease mortality and morbidity on their roads.¹ Alcohol and excessive speed have been highlighted in the past years as major killers, but quite surprisingly the health status of drivers has received little attention as a cause of accidents.

For many years fatigue has been associated with an increased risk of accidents, but the causes were unclear. Extended or nocturnal work or driving was associated with accidents, but few reports differentiated fatigue, which is usually seen as due to driving time, from sleepiness, which is due to reduced sleep,² extended time awake and/or being awake at the circadian trough,³ or drugs.

Epidemiologic studies from the 1990s showed that sleep-related accidents represent up to 20% of all traffic accidents in industrial societies.⁴^–⁶ Although drowsiness⁷^–⁹ has been identified as the reason behind fatal road crashes and many industrial accidents,¹⁰ many people drive when alertness is at its lowest level. A case-control study⁶ has shown that driving between 2 and 5 o’clock in the morning multiplies 5.6-fold the risk of traffic accidents and that being sleepy at the wheel multiplies eightfold the risk of accidents, which gives a clear measure of the associated risk of drowsy driving.

An Internet-based survey ¹¹ showed that inappropriate line crossings related to drowsy driving was a good predictor of future sleep-related accidents, and a Swedish epidemiologic study showed that more than 40% of the reported incidents involved crossing of the far right line before awaking and 16% involved crossing of the center line.¹²

Although both the European Union and the United States have launched public campaigns to make their citizens aware of the risk of drowsy driving (e.g., the U.S. National Sleep Foundation “Drive Alert–Arrive Alive” campaign), a major problem remains in the identification of patients or behaviors at risk for traffic accidents and the best way to reduce this risk by appropriate countermeasures.

In this review we present an update on the relationship between the extrinsic or intrinsic sleep disorders, drug intake, and traffic accidents, the present state of knowledge, and what major studies are needed to improve our patients’ safety.

Prevalence and Associated Risks

Rest/Activity Patterns

Behavioral changes affecting the sleep–wake pattern help explain sleep-related accidents. Studying large populations of drivers,¹³^,¹⁴ we demonstrated that long-distance driving was very frequently associated with sleep curtailment. Several experimental studies showed that sleep restriction¹⁵ or nocturnal driving¹⁶ could drastically alter driving performances (i.e., number of inappropriate highway line crossings).

Sleep deprivation affects not only the general population of automobile drivers but also many professional drivers all over the world. A study on professional truck drivers ¹⁷ demonstrated a mean duration of sleep of 4.78 hours/day in a 5-day period. Fifty-six percent of drivers presented at least 6 noncontinuous minutes of electroencephalographic (EEG)-recorded sleep while driving. The vast majority of these microsleep episodes occurred during the late night and early morning.

Not only do professional drivers have to drive under sleep restriction, health care workers are also affected by sleep loss and drive frequently after a night on call. A prospective survey ¹⁸ in 2737 residents reported detailed information about work hours, work shifts of an extended duration, and documented motor vehicle accidents. The odds ratios for reporting a motor vehicle accident and for reporting a near-miss incident after an extended work shift, as compared with a shift that was not of extended duration, were 2.3 (95% confidence interval [CI], 1.6 to 3.3) and 5.9 (95% CI, 5.4 to 6.3), respectively.

In 2008, researchers in one study ¹⁹ showed that extensive nocturnal driving dramatically worsens driving performance. Eight hours of nocturnal driving increase sixfold the number of inappropriate highway line crossings compared with a 2-hour nocturnal driving session (3 to 5 AM). These results suggest that fatigue related to driving duration is amplified at night so maximal driving duration should be shorter at night than during the day.

Sleep Disorders

Of all sleep disorders, obstructive sleep apnea syndrome is possibly the most studied pathologic process with regard to traffic accidents. Indeed, several studies performed in the past 20 years show a clear relationship between sleep disorders and traffic accidents.²⁰^–²⁶

In a study comparing patients with sleep apnea to controls to evaluate the additional accident risk related to sleep-disordered breathing,²⁷ patients with an apnea-hypopnea index (AHI) of 10 or higher had an odds ratio of 6.3 (95% CI, 2.4 to 16.2) for having a traffic accident as compared with those without sleep apnea. Researchers in another study²⁸ found increased accident risk in 460 apneic patients, although only the most severe patients (AHI > 30) presented an accident risk factor higher than that of the controls. In a third study,²⁹ the investigators performed an integrated analysis of recordings of sleep-related breathing disorders and self-reported automotive and company-recorded automotive accidents in 90 commercial long-haul truck drivers. In this study, truck drivers with sleep-disordered breathing had a twofold higher accident rate per mile than drivers without sleep-disordered breathing. In contrast to the study cited earlier,²⁸ accident frequency was not dependent on the severity of the sleep-related breathing disorder.³⁰

In another study on professional drivers,⁸ over 20% of long-haul drivers reported having dozed off at least twice while driving. Near misses due to dozing off had occurred in 17% of these drivers. There is debate about the best predictive symptom for risk of sleep-related accidents. Surprisingly, excessive daytime sleepiness per se as measured by the Epworth Sleepiness scale (ESS) has not been associated with accident risk in apneic patients.²⁷ This finding could be explained by a low percentage of chronically sleepy drivers (9% scored themselves above 10 at the ESS) in the studied population. In a case-control study³¹ of a series of 189 consecutive patients and a control group of 40 hospital staff workers, the best predictors of traffic accidents were self-reported sleepiness while driving (odds ratio [OR] 5; 95% CI, 2.3-10.9), having quit driving because of sleepiness (OR 3; 95% CI, 1.1-8.6), and being currently working (OR 2.8; 95% CI, 1.1-7.7).

In a similar vein, a sample of 4002 randomly selected drivers ³² were interviewed to define the prevalence of drivers who are habitually sleepy while driving. The habitually sleepy drivers reported a significantly higher frequency of vehicular accidents than control subjects (adjusted OR 13.3; CI, 4.1-43.0) and had a significantly higher prevalence of respiratory sleep disorders than control subjects. The authors concluded that habitually sleepy drivers are a large group (1 in 30 drivers) who are involved in severalfold more vehicular accidents than control subjects.

More recently, in a meta-analysis ³³ on sleep apnea and driving risk, 23 of 27 studies and 18 of 19 studies with control groups found a statistically significant increased risk, with many of the studies finding a twofold to threefold increased risk.

Sleep apnea is not the only disease responsible for excessive daytime sleepiness. Narcolepsy is a major disorder responsible for excessive daytime sleepiness, and it has also been studied as a risk factor for traffic accidents. Narcoleptic patients present a higher risk of sleep-related accidents than apneic subjects.²¹ The proportion of individuals with sleep-related accidents are 1.5- to 4-fold greater in the hypersomnolent patients than in the control group. Apneic and narcoleptic individuals account for 71% of all sleep-related accidents. The results of multiple sleep latency tests (MSLT) did not correlate with the rate of accidents among sleepy patients. However, the number of patients in the study with MSLTs was quite limited (46 apneic, 22 narcoleptic, 17 other causes of excessive daytime sleepiness), which could explain the lack of power of the study. It is worth noting that in all sleep disorders, victims of accidents presented with sleep latencies shorter than those of controls.

Elevated risks for motor vehicle accidents due to sleepiness and cataplexy have been reported for persons with untreated narcolepsy.³⁴ In one study,³⁵ researchers compared the performance on the Divided-Attention Driving Test (DADT) of 21 male patients with obstructive sleep apnea, 21 sex-matched controls, and 16 narcoleptic patients. Narcoleptic patients were younger and sleepier than the obstructive sleep apnea patients. The tracking error was much worse in patients than in control subjects (228 ± 145 cm for obstructive sleep apnea vs. 196 ± 146 for narcolepsy vs. 71 ± 31 for controls; P < .001). Further studies on narcolepsy and hypersomnia are urgently needed to improve the understanding of the driving risk of these patients.

Drugs

Even if many publications ³⁶^–⁴⁰ associate central nervous system drugs and risk of accidents, very few data show a link between sleep-related accidents and drug intake. Indirect factors such as the type of drugs responsible for traffic accidents (i.e., hypnotics and benzodiazepines) suggest that sleepiness could be the major cause for drug-related accidents, but further evidence is needed.

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