The Battlefield of Bicycle Helmet Efficacy

Scientists are arguing over studies of the efficacy of bicycle helmets. We hope that such controversy leads to a positive outcome: more studies, more results, more helmets developed and worn, more heads saved.

The defense: The Cochrane Collaboration evaluates interventions in health care by doing systematic reviews of the efficacy of scientific studies. These are usually randomized controlled trials because they provide the data most closely approximating the reliability of a scientific experiment when humans are concerned. Case-control studies are considered less reliable. Both types should relate data to a hypothesis that links a defined intervention with measurable effects. Cochrane reviews have shown whether or not interventions are effective, thus guiding medical professionals and the public to adopt or discard them. The Cochrane review of bicycle helmets concludes that five studies establish scientific evidence that standard helmets reduce the risk of head injury by 88%and the risk of severe brain injury by 75% (Thompson et al. 1989, 1996;  McDermott et al. 1993;  Thomas et al. 1994; Maimaris et al 1994).

The attack: Curnow (2005) writes that since three of the five studies were done, helmets have changed from hard shell to soft shell, which may not give the same amount of protection. He also posits that certain mechanisms of injury have been ignored in helmet testing. He goes on to claim that neither the Cochrane review nor the five studies address the four hypotheses they imply concerning hard and soft shells and focal and diffuse injury; rather the hypothesis is simply that helmets reduce brain injury. Curnow further states that the control groups were not a random sample of the population. For example, cyclists who wore helmets may have been more careful than non-wearers and were over-represented in the control groups compared to the population. He argues that cases were more involved with motor vehicles than controls. He says the studies assume that hitting the head is necessary for the brain to be injured, yet they don’t discriminate between the various types of injuries possible. In conclusion he says that the design of helmets has not been guided by research on mechanisms of brain injury, that none of the studies have had the requisite scientific vigor, and that because the studies were done on hard shell helmets, whereas now helmets are soft shell, the Cochrane review is not a reliable guide to interventions.

Confounding the battlefield: Gandavia (2005) comments that, “Wearing helmets by all cyclists should also seem common sense.”  And Anand (2005), “I suggest that common sense dictates the use of protection, but not necessarily protection with a helmet” going on to suggest that an epidemiological study be done of the value of a turban in protecting against head injury.

Aiding the defense: Hagel and Pless (2006) support the findings of the five studies that Curnow criticizes. Thompson et al (1989, 1996) used non-head injured cyclists who needed treatment in the emergency department as controls – they were not careful enough to avoid injury. In their 1996 study, Thompson et al controlled the analysis of the helmet effect for crash involvement with a motor vehicle, surface type hit, speed of the bicycle, and damage to the bicycle. So, in fact, the controls were not more cautious or did they involve in less severe crashes. Thompson et al (1996) also negate Curnow’s criticism that helmeted riders sustain greater frequency and severity of injuries because they take more risks than unhelmeted riders. Hagel and Pless (2006) accuse Curnow of taking the position that when empirical data do not support a theory, the assumptions underlying the theory must be questioned, if not dismissed. His view is that the positive results of the studies are biased: because helmets have been shown to be effective in reducing severe brain injury, the evidence must be flawed. Curnow prefers large population studies over well-conducted case-control studies. However, large studies provide weaker evidence because they attribute group level observations to individual level associations. It is impossible to know if injured individuals were or were not wearing a helmet. Hagel and Pless end with, “. . . . Curnow’s paper would not comply with the rigorous criteria required of a Cochrane systematic review because he fails to present all relevant evidence for the effect of bike helmet use and legislation in a balanced way.”

Editor’s note: Curnow criticism of the Cochrane Collaboration analysis of bicycle helmets flies in the face of the solid case control epidemiological studies conducted during the past two decades. Unlike the Cochrane Collaboration, he has “hand picked” studies that may underscore his criticisms, but he fails to consider other studies that do not support his position. He has not objectively criticized the Cochrane Collaboration’s findings that five studies scientifically establish evidence that bicycle helmets are effective in preventing head injury.

References
Curnow WJ. The Cochrane Collaboration and bicycle helmets. Accid Analy Prev May 2005; 37(3):569-573.
Thompson RS, Rivara FP, Thompson DC. A case-control study on the effectiveness of bicycle safety helmets. New Engl J Med 1989; 320:1361-1367.
Thompson, DC, Rivara FP, Thompson RS. Effectiveness of bicycle safety helmets in preventing head injuries: a case-control study. J Amer Med Ass 1996; 276:1968-1973.
McDermott FT, Lane JC, Brazenor GA, Debney EA, The effectiveness of bicycle helmets: a study of 1710 casualties. J Trauma 1993; 34:834-845.
Thomas S, Acton C, Nixon J, Battistutta D, Pitt WR, Clark R. Effectiveness of bicycle helmets in preventing head injuries in children; case-control study. Br Med J 1994; 308:173-176.
Maimaris C, Summers CL, Browning C, Palmer CR, Injury patterns in cyclists attending an accident and emergency department: a comparison of helmet wearers and non-wearers. Br Med J 1994; 308:1537-1540.
Anand JK. Compulsory helmets for cyclists. J R Soc Med 2005; 98:86.
Ganevia S. Compulsory helmets for cyclists. J R Soc Med 2005; 97:612.
Hagel BE, Barry Pless I. A critical examination of arguments against bicycle helmet use and legislation.Accid Anal Prev 2006; 38 (1).

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