No clear evidence from countries that have enforced the wearing of helmets

Re: Objective observation of helmet use is essential

8 July 2006

Reply to Keatinge “Objective observation of helmet use is essential” (1) and two email communications to us on the same topic. Your questions and/or statements are listed below with our responses.

YOUR STATEMENTS “Cyclists without head injuries will report rates of helmet wearing much higher than their true rate. Cyclists with recent head injuries, as in the case-control studies, will not be able to deceive themselves or their interviewers, and will report much lower, truer rates of helmet wearing. This bias is enough to account for all the positive findings in case-control studies:” (1) AND “The promising results of the case-control studies could trivially be accounted for by the hypothesis that most cyclists exaggerate their helmet use, but head-injured ones do not because after a demonstrable injury to the head they can’t fool themselves in that particular way. (Robinson’s results do not rely on self -reporting.)” 22 June email AND “in one case-control study based in Seattle, 7% of the cyclists with head injuries reported wearing helmets, as compared with 24% of the emergency room controls and 23% of community cyclists who had had an accident(2) However, less than 6% of cyclists on the Seattle streets at the time were actually observed to wear helmets (3) Either helmets are a serious cause of accidents, or self-reports of helmet use are not valid” (1)

OUR ANSWERS: Your claim that there is differential reporting of helmet use by head injured (cases) and non head injured (controls) cyclists in published case-control studies is mere speculation. There is no evidence that controls, or for that matter cases, report helmet use incorrectly. There is good evidence that self-reported helmet use is accurate. Remember, all these cyclists had an injury that caused them to seek medical care. This is an important event which people remember accurately. Additionally, there were no helmet laws in Seattle at the time. Observations of helmet use in the general cycling population after helmet legislation (4) or before and after a helmet promotion campaign (3) provide no information as to whether any of these cyclists were wearing helmets when they crashed and went to the hospital. The studies of observed helmet use in the work cited above provide information on the prevalence of helmet use in the general cycling population. The case control studies provide the prevalence of helmet use in cyclists who seek ED treatment following a crash, a much different circumstance.

YOUR STATEMENT: “Robinson presents the best available evidence derived from objective assessment of helmet wearing. Her demonstration that cycle helmet laws do not work is likely to remain the definitive answer.” (1)

OUR ANSWER: The studies that Dorothy Robinson cites in her recent BMJ article (1) are time series studies or ecological designs which do not have appropriate comparison groups. We have pointed this out in our discussions with you, Dorothy Robinson, Bill Curnow and Mayer Hillman over the past ten years. Please re-read our replies to your criticisms of our Cochrane review. These are available at the end of the review (5) or on the web at the following address: www.cochrane- Additionally, Robinson selects portions of the studies which support her arguments and ignores the portions which indicate a reduction in head injuries following legislation. (6) She also omits 2 studies from North America which indicate legislation reduces head or brain injury. (7,8)

YOUR QUESTION: “I would be interested to know how this data collection was done in this study, and in particular how helmet use was ascertained. Was it done by observation, by questionnaire in hospital, or by later follow-up?" (Email of 29 May, 2006 to inquire about our Cycle Helmet studies: NEJM, 1989 and JAMA, 1996.(2,9)

OUR ANSWER: In reply, both studies were prospective case control studies so the data were collected in an ongoing fashion as the bicycle crashes occurred and the riders sought treatment in the emergency departments (ED’s) at the participating hospitals. Hospital ED’s were visited at least once a week to identify bicycle riders who were injured. Helmet use was determined by self-report using mailed questionnaires sent to each identified cyclist following the ED visit. People who did not respond promptly were interviewed by telephone using the same questionnaire. Helmet use was ascertained from these completed questionnaires.

YOUR STATEMENTS: “I would be interested in any information you can give on the degree of independence of these two sources of data on helmet wearing, and on any records of whether emergency room staff actually saw the helmets themselves.” AND “The hypothesis that most cyclists exaggerate their helmet use, but the head-injured ones do not because after a demonstrable injury to the head they can’t fool themselves in that particular way.” (22 June Email )

OUR ANSWER: We also abstracted the medical record of those cyclists in the study and tabulated information about helmet use as recorded by ED personnel. The medical record was written when the patient was treated in the ED. This ascertainment is independent of helmet use reported subsequently on the research questionnaire. This permitted independent corroboration of reported helmet use. We compared the report of helmet use in the medical record to the report of helmet use on the questionnaire and assessed the agreement between the ED record and the questionnaire report of helmet use. Agreement was almost 100% for both cases and controls. (9)

In many cases the attending physician or emergency staff saw the helmets.

In addition, independent laboratory analysis of 500 of the helmets which were damaged in the crashes indicated that cyclists reported helmet type correctly. This is a more complex memory task then reporting whether a helmet was worn. (10) In conclusion, the multiple methods used to ascertain helmet use provide solid evidence for the validity of self reported helmet use.

SUMMARY : The question here for you and other doubters at this point is; Are you really trying to pursue the science assessing cycle helmet efficacy and population effectiveness or is this some sort of crusade on your part to fit the science to your beliefs? Over the years my colleagues and I have explained case control studies and other epidemiologic methods of conducting studies and analyzing statistics. (See our Cochrane review, our comments replying to numerous criticisms, Peter Cummings’ recent article in Accident Analysis and Prevention,” Misconceptions regarding case-control studies of bicycle helmets and head injury”.(5,11) You could also read the article by Brent Hagel et al “Arguments against helmet legislation are flawed” which is published the same March issue of BMJ as Dorothy Robinson’s article. The rapid responses from anti-helmet writers are simply offering the same old arguments, opinions and speculations that have been written by the same people for years. If you truly wish to move the science forward in this area, we respectfully request that you all take some formal courses in the use of Epidemiological methods, non- experimental design and biostatistics. After that you should review the whole body of peer-reviewed published information with an open mind and see where your review leads you. We predict it will lead you to the conclusion that we have reached, that cycle helmets prevent head and brain injuries and increased wearing rates result in decreased head and brain injuries in cyclists. (5,6)

References 1. Keatinge,RM. Objective observation of helmet use is essential. BMJ 14 May 2006.

2. Thompson RS, Rivara FP,Thompson DC. A case-control study of the effectiveness of bicycle safety helmets. NEJM 1989;320:1361-7.

3. DiGuiseppi CG, Rivara FP, Koepsell T, Polissar L. Bicycle helmet use by children. Evaluation of a community-wide helmet campaign JAMA 1989;262:2256-2261.

4. Robinson DL. No clear evidence from countries that have enforced the wearing of helmets. BMJ 2006;332:722-5.

5. Thompson DC, Rivara FP, Thompson RS. Helmets for preventing head and facial injuries in bicyclists. Cochrane Database Syst Rev 2000;(2): CD001855.

6. Hagel B, Macpherson A, Rivara FP, Pless B. Arguments against helmet legislation are flawed. BMJ 2006:332:725-6.

7. Lee BH, Schofer JL, Koppelman FS. Bicycle safety helmet legislation and bicycle-related non-fatal injuries in California. Accid Anal Prev 2005;37:93-102

8. Macpherson AK, To TM, Macarthur C, Chipman ML, Wright JG, Parkin PC. Impact of mandatory helmet legislation on bicycle-related head injuries in children: a population-based study. Pediatrics 2002;110:e60.

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

10. Ching RP, Thompson DC, Thompson RS, Thomas DJ, Chilcott WC, Rivara FP. Acc Anal Prev 1997;29:555-562.

11. Cummings P, Rivara FP, Thompson DC, Thompson RS. Misconceptions regarding case-control studies of bicycle helmets and head injury. Accid Anal Prev 2006;38:636-643.

Competing interests: None declared

Competing interests: None declared

Diane Thompson, Epidemiologist (retired)

Robert S. Thompson

Harborview Injury Prevention & Research Center, Univ of Washington, Seattle, WA 98104 USA

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