Abstract
The effect of helmet use on reducing the risk of death in cyclists appears to be distorted by some variables (potential confounders, effect modifiers, or both). Our aim was to provide evidence for or against the hypothesis that cycling area may act as a confounder and effect modifier of the association between helmet use and risk of death of cyclists involved in road crashes. Data were analysed for 24,605 cyclists involved in road crashes in Spain. A multiple imputation procedure was used to mitigate the effect of missing values. We used multilevel Poisson regression with province as the group level to estimate the crude association between helmet use and risk of death, and also three adjusted analyses: (1) for cycling area only, (2) for the remaining variables which may act as confounders, and (3) for all variables. Incidence–density ratios (IDR) and their 95% confidence intervals were calculated. Crude IDR was 1.10, but stratifying by cycling area disclosed a protective, differential effect of helmet use: IDR = 0.67 in urban areas, IDR = 0.34 on open roads. Adjusting for all variables except cycling area yielded similar results in both strata, albeit with a smaller difference between them. Adjusting for cycling area only yielded a strong association (IDR = 0.42), which was slightly lower in the adjusted analysis for all variables (IDR = 0.45). Cycling area can act as a confounder and also appears to act as an effect modifier (albeit to a lesser extent) of the risk of cyclists’ death after a crash.
Highlights
The effect of helmet use on reducing the risk of death in cyclists appears to be distorted by some variables
Crude Incidence–density ratios (IDR) estimation yielded a point estimate of 1.10, but this ratio was less than 1 when it was calculated separately for the two strata defined by cycling area
In line with most previous studies[1,2], our final results show an inverse relationship between cyclists’ helmet use and death. The magnitude of this association (IDR 0.45 after adjustment for all variables; i.e. risk reduction of 55%) was very similar from that observed in previous meta-analyses and not very different from that reported in Australia after helmet laws were introduced[11]
Summary
The effect of helmet use on reducing the risk of death in cyclists appears to be distorted by some variables (potential confounders, effect modifiers, or both). Cyclist- or environment-related characteristics may influence the strength of the true causal relationship between helmet use and death, acting as effect modifiers An example of this phenomenon is the speed of the collision. Cycling area (open road vs urban setting) is an excellent example of a variable which can act a priori as both as a confounder and (indirectly) as an effect modifier in the causal link between helmet use and death. Cycling area strongly affects the travelling speed of both cyclists and other vehicles on the road, which in turn is the main determinant of cyclists’ risk of death after a collision These facts make cycling area a potentially strong confounder, opening a backdoor (non-causal) path between helmet use and death, and biasing toward the null any estimates of the causal path. The speed at the time of the crash would be related to the amount of kinetic energy at impact[10] and, as hypothesized above, may in turn modify the magnitude of the causal association between helmet use and death
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