Results

During the data collection period 366 sleep studies were booked. Of these, 25 patients did not attend, 95 were attending for a second study (usually a CPAP titration study) and were therefore excluded, and 96 patients refused consent to participate, leaving 150 patients in the study. The demographic data of these patients are shown in table 1 and indicate that they were representative of all patients undergoing sleep studies over the 6 month period. Eighty percent had an RDI of >15/hour or an Epworth score greater than 12; 25% reported an accident, 35% a near miss event, and 15% reported nodding off at the wheel in the previous 3 years; 8% of the subjects took sleeping tablets, 46% drank more than three cups of coffee per day, and the median (IQR) amount of alcohol consumed per week was 8 (2–16) units. The mean (SD) age of passing the driving test was 21.5 (7.6) years, 55% drove more than 10 000 miles per year, and 15% classified themselves as a professional driver.

The driving simulator results revealed a median (IQR) tracking error of 0.321 (0.218–1.172), response time of 2.85 (1.99–4.23) seconds, and number of off-road events per hour of 24 (3–104).

Discussion

This study shows that performance on a driving simulator is influenced by a number of factors. Those not directly related to on-road driving or OSA (age, female sex, and admitted alcohol consumption) have the strongest influence, but a history of near miss accidents is an independent predictor of a poor tracking error and number of off-road events per hour on the simulator.

The fact that performance on a simple simulator is affected by factors which are not directly related to driving history may explain the lack of relationship between simulator performance (using a steer clear vigilance test) and accident history in a previous study.8Patients may perform poorly on the simulator simply because of a lack of ability to participate in a “computer game” irrespective of their ability to drive; older subjects performed less well and it is perhaps not surprising that they have less familiarity and confidence in the use of computers and electronic “games”. Patients who admitted to a higher weekly consumption of alcohol also performed less well and they are more likely to have had a drink prior to attending for their study. The reason why women perform less well is not clear, since other studies suggest they perform better at tasks involving hand/eye coordination.29-31

However, despite these limitations, performance on the driving simulator did show some relationship to accident history. After controlling for confounding factors, data derived from the simulator still showed a relationship with accidents in the previous year. Patients who performed well on the simulator were unlikely to have had an accident in the previous year but there was no relationship between those who performed poorly and accident history. None of the patients with fewer than 10 off-road events/hour on the simulator reported having an accident in the previous year whereas only 10% of patients with more than 10 off-road events per hour on the simulator reported an accident in the previous year. Self-reported sleepiness (Epworth score) was associated with near misses and episodes of falling asleep at the wheel in the previous 3 years. OSA severity (RDI) was not associated with either adverse driving events or driving simulator performance, confirming the observation in other studies that classical indicators of severity are in fact poor indicators of the impact of OSA.8 Although, as a group, patients with OSA have been found to perform less well than matched controls on a simulator, there is a considerable overlap which confirms that other factors are important.8 ,18-20 ,25 ,27 We also found that there was no correlation between severity of OSA (RDI and Epworth score) and simulator performance. Most patients attending sleep clinics are middle aged men and therefore, because increasing age and female sex were independently associated with poor simulator performance, we performed a further analysis excluding these subjects. Unfortunately, this did not make a difference to the utility of the model.

There are several limitations to our methodology. Firstly, although we endeavoured to study consecutive patients, a large number of patients refused to take part which may have introduced a selection bias. Patients who thought they were safe drivers or those who felt more confident performing computer based tasks (for example, younger patients) may have been more likely to agree to participate in the study. Those who thought they had a problem with driving may have been reluctant to take part for fear of losing their licence, although they were told that the data derived from either the questionnaire or the simulator would not be used to advise on their safety to drive or become part of their medical records. However, the study group was not significantly different from the total population attending for sleep studies over the same period in terms of demographic, clinical, and sleep study data. The fact that 39% of patients refused to participate confirms that this is a sensitive issue and may limit the usefulness of simulators in routine practice.

Secondly, we relied on a questionnaire to obtain information about driving history and accidents, thus potentially introducing recall bias. It has been shown that patients with OSA are reluctant to report accidents32 and may under report symptoms.33While this is a limitation, information from police, vehicle licensing authorities, or insurers may also underestimate the problem because not all accidents are reported and no information would have been available on near misses or episodes of falling asleep at the wheel, which have been found to be more likely in snorers with breathing pauses.3

Thirdly, our patients had one 5 minute practice run on the simulator which may not have been sufficient to accustom them to its use. One study27has suggested that at least three 5 minute practice runs are required to abolish any learning effect on the simulator, whereas others have used practice times ranging from 1 to 12 minutes.14 ,25 It was felt that a longer practice run may have further reduced recruitment into the study and limited the applicability of its use in routine clinical practice in the future. One single 5 minute practice run, as recommended by the supplier of the simulator, was therefore performed by all subjects.

Fourthly, we only tested patients for 20 minutes whereas actual driving is often for much longer periods; furthermore, the circumstances of the simulator reproduce motorway driving whereas accidents also happen on quiet rural roads or in close proximity to home.

Finally, all the analyses used accidents which had already occurred and this makes the assumption that what happened in the past will occur again in the future.

Limited sleep studies, using either the Densa DMS2000 or the Autoset Clinical 1, were used to establish the diagnosis of OSA and assess its severity (RDI). Studies have shown the sensitivity and specificity of the Densa to be 82% and 90% for the diagnosis of OSA34and of the Autoset to be 100% and 92%.35 However, both machines underscored hypopnoeas when compared with full polysomnography, so we may have underestimated the degree of mild OSA in terms of RDI in this study. However, the poor correlation between RDI, accident history, and simulator performance suggests that the addition of EEG monitoring would not have significantly altered the results. Furthermore, a recent meta-analysis of diagnostic tools in sleep apnoea failed to recommend standardisation of methodology36 and limited studies appeared to be most useful when investigating OSA. A further study by Douglaset al 37 concluded that recording sleep was of no diagnostic value, and that OSA could be defined as accurately using RDI expressed as time in bed as RDI expressed as time asleep.

Physicians treating patients with OSA need to advise their patients about driving. Current UK driving regulations state that patients with OSA and hypersomnolence should not drive. However, not all patients with OSA are at increased risk of RTA and there is no link with disease severity. Adopting a policy of advising all patients not to drive may have far reaching implications on their lifestyles and livelihoods. It may also prevent patients admitting that they have a problem and stop them seeking help and therefore treatment. Our results confirm that it is difficult to determine whether an individual is safe to drive. Those who do not report daytime sleepiness and do not have more than 10 off-road events/hour on the driving simulator appear to be at least risk. Poor performance on the simulator, however, does not indicate that the patient is not safe to drive. These data need to be confirmed in further studies before simple computer simulators can be used in clinical practice to advise whether or not an individual is safe to drive.