Page:ATSB RO-2018-004 - Collision of passenger train A42 with buffer stop.pdf/22

ATSB – RO-2018-004 It is considered possible that the driver experienced a pre-syncope or syncopal episode. It is reported that up to 50% of the population will lose consciousness at some point in their life due to a syncopal episode. Sydney Trains agrees with an independent medical assessment that, considering all of the medical information, the most likely diagnosis was a blackout of undetermined cause.

In the context of human performance, fatigue is a physical and psychological condition primarily caused by prolonged wakefulness and/or insufficient or disturbed sleep. Fatigue can have a range of influences on performance, such as decreased short-term memory, slowed reaction time, decreased work efficiency, reduced motivational drive, increased variability in work performance, and increased errors of omission. Transport accident investigation agencies have identified fatigue impairment as a contributory factor in many accidents and incidents.

Extensive medical tests, including neurological and cardiology tests, were conducted on the driver following the incident. One test detected a problem. A sleep test (a home polysomnography sleep study) conducted in July 2018 led to the driver being diagnosed with moderate obstructive sleep apnea. This was followed by an in-clinic test on 8 October 2018 that showed mild obstructive sleep apnea and a maintenance of wakefulness (MWT) test conducted on 9 October 2018 that was normal, with no suggestion of sleepiness. He was advised by his medical practitioner to use a continuous positive airway pressure (CPAP) machine. In December 2018, the driver reported that since using the CPAP machine, his overall feeling of well-being had improved. He was unable to say if he was feeling more alert although he had not resumed train-driving duties.

Research has shown that sleep apnea increases the accident rate in motor vehicle drivers between two and seven times due to sleepiness and/or due to altered blood gases and hypoxia affecting mental function. Obstructive sleep apnea involves repetitive obstruction to the upper airway during sleep. Throughout the sleep period, the breathing of a person can stop from a few seconds to over a minute. These episodes, which can occur many times during the night, are known as apneas. The person can be unaware that it has happened during the night but will frequently wake feeling tired. An increase in sleepiness can result from obstructive sleep apnea.

It is possible that the driver was fatigued and experienced a microsleep. A microsleep is a very short period of sleep when the brain disengages from the environment and slips uncontrollably into light non-REM sleep. Microsleeps have been shown to correlate with periods of low performance and they occur most frequently during conditions of fatigue. It is possible the driver experienced a microsleep due to an increased level of fatigue due to a combination of obstructive sleep apnea, a cumulative lack of sleep, and an early morning shift start.

Even though the driver stated that he felt normal on the day of the incident, research has shown that patients with sleep disorders may not be aware of an impending sleep episode and that there may be a lack of realisation by a sleep-deprived person as to how fatigued they actually are. This driver had not reported any clinical features of sleep apnea prior to the incident and

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