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Cochrane Review: Helmets for preventing head and facial injuries in bicyclists

Thompson RS, Rivara FP & Thompson DC. Cochrane Database of Systematic Reviews, The Cochrane Library.

Full review (Note: Access to the Cochrane database may be denied from some countries)	Download or print this commentary
Authors' abstract

Introduction

The Cochrane database has established a reputation as a key resource for medical research. This particular review, however, has attracted considerable criticism. Some of this criticism is included with the reviewers' paper on the Cochrane database and thus forms an integral part of the review. However, submissions were sometimes shortened and summarised in a way that omits key concerns. See here for the full version of one detailed critique [1].

Further peer-reviewed criticism has appeared in the medical press, in which the studies selected for the review were accused of not possessing the necessary scientific rigour to be a reliable guide to helmet effectiveness, particularly with regard to efficacy against fatal and disabling injury to the brain [2].

Studies on which the review is based

Injury patterns in cyclists attending an accident and emergency department: a comparison of helmet wearers and non-wearers.
Maimaris C, Summers CL, Browning C, Palmer CR. BMJ, 1994;308(6943):1537-1540.
See: Commentary

The effectiveness of bicyclist helmets: a study of 1701 casualties.
McDermott FT, Lane JC, Brazenore GA, Debney EA. Journal of Trauma, 1993;34(6):834-845.

Effectiveness of bicycle helmets in preventing head injury in children.
Thomas S, Acton C, Nixon J, Battistutta D, Pitt WR, Clark R. BMJ, 1994;308:173-176.

Effectiveness of bicycle safety helmets in preventing head injuries: a case-control study.
Thompson DC, Rivara FP, Thompson RS. JAMA, 1996;276(24):1968-1973.

A case-control study on the effectiveness of bicycle safety helmets.
Thompson RS, Rivara FP, Thompson DC. New England Journal of Medicine, 1989;320:1361-1367.
See: Commentary

A case-control study on the effectiveness of bicycle safety helmets in preventing facial injury.
Thompson DC, Thompson RS, Rivara FP, Wolf ME. American Journal of Public Health, 1990;80(12):1471-1474.

Effectiveness of bicycle safety helmets in preventing serious facial injury.
Thompson DC, Nunn ME, Thompson RS, Rivara FP. JAMA 1996a;276(24):1974-1975.

Criticisms

Principal criticisms of the review are:

• The review is not independent. Four of the seven papers selected for inclusion were the work of the reviewers themselves and their data dominate the analysis, comprising 77% of the cyclists on whom the review is based. Furthermore, these four papers are based on only two data sets and have themselves been much criticised for fundamental methodological shortcomings [3].
• Only case-control studies were considered for inclusion, although non-randomised studies of this type are acknowledged to be prone to bias because of the difficulty in controlling for the many independent variables [4].
• The paradox presented by the failure of other types of studies (e.g. whole population and time-series data) to show any benefit from large increases in helmet use - let alone the substantial benefits predicted by the studies reviewed - is left unstated and unaccounted for [5].
• The authors are dismissive of the possibility of risk compensation. However, it has subsequently been demonstrated that child cyclists often ride more riskily and suffer more crashes when wearing a cycle helmet [6] and that adults are more likely to ride on busier roads if helmeted [7].
• Similarly no consideration is given to rotational injuries, which dominate the most serious injuries. Helmets have not been shown to mitigate rotational brain injury and there is evidence they may increase the risk and/or severity of rotational injury.[8].
• Claims are accepted of efficacy for which no plausible mechanism exists (e.g. the prevention of mid-face injuries), where the reviewed data set is too small to make reliable inference (e.g. the result of collisions with motor vehicles), and which would not be possible even if helmets prevented all head injuries (e.g. an increase of 35% in cyclists wearing helmets leading to 66% fewer head injuries).
• There is misleading interpretation of 'odds ratio', which is used interchangeably in the review with 'percentage reduction in head injuries'. This exaggerates the predicted benefit of helmets and masks the fact that studies of this type are not truly predictive, being essentially the authors' estimate of what proportion of the observed differences between two groups can be assigned to a single factor. Furthermore, the reviewed paper showing the least benefit from helmets is omitted from computation of odds ratio, thus again exaggerating benefit.

References

[1] Robinson DL. Helmets for preventing head and facial injuries in bicyclists - critique. 2000
     The full version of a paper submitted to the Cochrane Review, only a summary of which has been included in the review itself. 2000.
[2] Curnow WJ. The Cochrane Collaboration and bicycle helmets. Accident Analysis & Prevention, 2005;37(3):569-573.
[3] For example, see commentary on: A case-control study of the effectiveness of bicycle helmets.
[4] See: Contradictory evidence about the effectiveness of cycle helmets .
[5] For example, see the outcomes of Helmet Laws where these have been enforced.
[6] Mok D, Gore G, Hagel B, Mok E, Magdalinos H, Pless B. Risk compensation in children’s activities: A pilot study. Paediatr Child Health, 2004;9(5):327-330.
[7] Gregory K, Inwood C, Sexton B. Cycle helmet wearing in 2002. Transport Research Laboratory, report TRL578. 2003.
[8] See: Cycle helmets and rotational injuries.