Killer cures

What kind of moron does not wear a helmet whilst riding a bike? Anyone that stupid deserves to have their brains scrapped off the road. —Dave, bloke commenting on the failed Melbourne bike share.

Cycle in London without a helmet?  You’d need your head examined… —Ross Lydall, Evening Standard transport correspondent.

The BMA, as a part of its policy to improve safe cycling supports compulsory wearing of cycle helmets when cycling for children and adults. —The British Medical Association

I know a lot of you find the whole helmets thing — whether they “help” or “work” or not — tiresome and unimportant.  Well tough.  Bicycle helmets are a medical intervention — a special kind of medical intervention — and whether or not medical interventions work and are worthwhile is always a fascinating subject.  More importantly, a large proportion of the general public and of journalists assume that helmets work, and the British Medical Association campaigns for compulsory bicycle helmet laws.  What the BMA does matters.  If the BMA endorses a medical intervention, we can’t dismiss arguments about it as tiresome and unimportant.

Archie Cochrane, the influential champion of modern evidence based medicine and one of history’s most underrated heroes, is said to have played a mischievous prank on colleagues.  In an age when doctor knew best, Cochrane managed to organise a randomised trial of two care regimens for recovering heart attack patients: extensive hospital care (which every doctor knew was what a heart attack patient needed) versus home care.  A few months into the trial he convened his colleagues in the monitoring group to break the bad news that eight home care patients had died versus four hospital care patients.  His colleagues’ fears had been proven correct: hospital treatment was clearly far superior to home treatment and the trial must be stopped immediately as it would simply be unethical to continue to subject patients to dangerous home care.  At which point Cochrane took another look at his notes and declared that, to his great embarrassment of course, he had misread his shorthand: eight hospital patients had died for only four home care patients.  After the awkward silence, the monitoring group all agreed that it was far too early to draw any conclusions from such small numbers and at such an early stage — it could be pure chance that more patients died in hospital care.  The trial went on and never did provide any evidence that hospital care is any better than home care.

It seems obvious that bicycle helmets are a good thing.  They save lives.  They prevent life-changing head injuries.  If your head is fast approaching concrete, you want something to intervene.  It’s common sense, right?  You’d be mad not to wear one.

But Cochrane and his fellow mid-20th century proponents of evidence-based medicine showed that facts do not always match common sense.  The obvious answer is not always the correct one.  The obvious common sense fact that hospital care is better than home care for recovering heart attack patients turned out not to be correct.  As a new generation of doctors recognised the importance of evidence-based medicine, randomised controlled trials were retrospectively carried out on nearly everything that doctors do.  And, oops, they discovered that a lot of practices that doctors had considered to be simple obvious common sense had actually been harming their patients, ruining lives and sometimes killing people.

For a long time I took a Pascal’s Wager on bike helmets: while I had been given various reasons to believe that even if there was a benefit from wearing one it was probably marginal, there was no good reason not to wear one.  But the lesson from Cochrane — that common sense can kill you — is that there could be a very good reason for not wearing one.  What if wearing a bicycle helmet actively increases your risk of injury and death while riding a bicycle?  We can’t just assume that it doesn’t.

How could bicycle helmets possibly be bad for you?  Concrete meets head: intervention surely a good thing?  As that great 21st century populariser of evidence-based medicine would say: I think you’ll find it’s a bit more complicated than that.  In helmets, as in most transport issues, we seem to be obsessed with the engineering and overlook the way that people behave.  Helmet efficacy is as much a question of psychology as it is physics.

Because the interesting aspect of helmet research is not so much how they affect your chances surviving an accident, but how they affect your chances of having an accident.  It all comes back to how road users behave, and there are reasons to believe that helmet use could change people’s behaviour in a way that increases the accident rate.  Many readers will already be familiar with the two most established lines of research: risk compensation and the safety-in-numbers effect.  I’ll look at those in more detail another time, but briefly, risk compensation proposes that we adjust our behaviour according to perceived risks — in this case, the cyclist wearing the helmet perceives himself to be at reduced risk, and happily cycles with less care; more importantly, the car, bus and truck drivers around him drive with less care.  The safety-in-numbers effect proposes that cyclists are safer when there are more cyclists on the road — both in that specific time and place, as other vehicles will have to slow and use caution around them; and in general, as other road users will be expecting to see cyclists and are more likely to know how to behave around them.  If the perception is that cycling is a dangerous extreme sport that requires a helmet, and if that perception puts people off cycling, then the safety-in-numbers effect is diminished.

It’s easy to dismiss these things without considering them: helmets are hard but simple; behaviour is soft but complicated.  It’s easier to go with common sense.  But common sense is often what bad science is made of, and common sense can kill you.

That doesn’t mean we can just assume that helmets are ineffective or bad.  With a medical intervention, you start from scratch, collect the data, and follow the evidence wherever it takes you.  This introductory post and its title are not supposed to bias our exploration of the evidence one way or another, only to get us beyond the unexamined assumption that helmets work.

So what’s the best evidence on bicycle helmets?  Named in honour of the pioneer Archie Cochrane, the Cochrane Collaboration systematically reviews the evidence for medical interventions.  A Cochrane Review looks carefully at all of the research that has been conducted on an intervention, considers the factors affecting the quality of each piece of research, and synthesises the results of all of the research to a conclusion which will generally be considered by medical practitioners to be the best knowledge we currently have on that intervention.  In a field that must always remain skeptical of the status quo and open to new evidence, a Cochrane Review is in practice considered to be the closest approximation we have to The Truth.  Good doctors don’t use their common sense, they use Cochrane Reviews.

The Cochrane Collaboration have reviewed the evidence for bicycle helmet efficacy.  This weekend, I’ve got half a dozen posts looking at that evidence, the way that it is presented by the Collaboration, and the evidence that the Collaboration has chosen to omit.

le in London without a helmet? You’d need your head examined…Cycle in London

14 thoughts on “Killer cures”

  1. An interesting and well times post, what with the AA handing out helmets to (supposedly) protect cyclists from AA members. I have written a post this morning about the recent Elvik review of the literature and how this has been treated by the news outlets just this morning

  2. I think this is a very well balanced description of the issues. Personally, I nearly always wear a helmet.

    When I rode recently without one, I felt that I was being a bit more cautious, but then that could have been an artifact because I was riding the most “relaxed” bike that I own.

    I’m particularly interested to know more about how other road users change their attitudes towards cyclists based on if the rider has a helmet or not. There’s a single study that’s often quoted (Dr. Ian Walker’s) about more space being given if you’re helmetless or appear to be female. However, I don’t know how rigorous that study was, if it it’s been replicated or if there’s anything that looks at perceptions of groups of cyclists. It also looks like the space differences are in the margin of 10%-20%

    It’s a complicated area, that’s for sure. Looking forward to what you have to show.

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