To pay or not to pay? Stats is the question.
Disease Aware Issue 4
The Supreme Court has expressed repeated doubts about the role of epidemiology in the question of causation, notably in the Atomic Veterans’ case and later in Sienkiewicz v Greif. Although both cases involved cancer, the Supreme Court’s caution can be applied to other cases.
One disease that needs further consideration is carpel tunnel syndrome - allegedly caused by exposure to vibration. Numerous CTS claims are received by insurers. They are invariably presented as standard HAVS cases.
The claim will focus almost entirely on the levels of vibration exposure and the employer’s actions to address that exposure. But is that the right approach? The answer is - probably not. Medical science remains uncertain about the causes of CTS. A number of risk factors can contribute to its onset. The specific causative role of each is unsure. They include obesity, age, smoking, arthritis, pregnancy and physical activity.
A Scandinavian study into the prevalence of CTS (Atroshi et al 1999) identified symptoms in 8% of men age 35 to 44 rising to 15.8% for those aged 55 to 64. A similar study in the UK estimated a prevalence of between 7 and 16% in the general population. Not all these people have been exposed to vibration. The overwhelming majority will not have been exposed.
With all these risk factors and such a common condition, what is the evidence to establish a causative connection between vibration exposure and CTS? The evidence currently available establishes no causative link between vibration and CTS. The cause of CTS remains unknown. Even diagnosis remains uncertain in the absence of any definitive objective testing.
So why does the insurance industry continue to face large numbers of claims for so called vibration induced carpel tunnel syndrome? The answer lies in the historical attribution of CTS to vibration.
In 1992, the Industrial Injuries Advisory Council (IIAC) recommended the prescription of CTS where there had been prolonged and regular use of hand held vibratory equipment. IIAC’s view was based principally on a study by Palmer which found a doubling of the risk of CTS with regular and prolonged use of such equipment. The IIAC prescription of CTS does not establish a causal link, just a statistical association. This association has been given undeserved significance by being repeated and misrepresented in later guidance from the HSE and others.
For example HSG (88) from 1994 “Guidance on Hand-Arm Vibration” refers to a causal relationship for CTS in vibration exposed workers and recommends health surveillance. More recently L140 Guidance on the Control of Vibration at Work Regulations 2005 refers to CTS being caused by vibration exposure. CTS tests are once again included in health surveillance guidance. This later guidance even repeats the Palmer finding of nearly 25 years earlier of a doubling of the risk.
A number of studies since Palmer have tried to identify a causative link. The role of vibration remains contentious. While some studies do maintain an association between CTS and the use of handheld vibratory equipment they fail to establish whether the causative factor is the vibration itself or some ergonomic factor arising from tool usage. This can be crucial in determining the link between any proven breach of duty and the injury.
A report by Tanaka et al 1999 examined the risk factors and their prevalence in sufferers of CTS. The most common factor was weight. Tanaka concluded that those with a BMI over 25 are twice as likely to develop CTS. The paper found that individuals exposed to vibration had a risk factor of 1.9 (i.e. a factor less determinative than weight). Note that there is no known synergistic effect of a combination of risk factors. However, an overweight individual using vibratory tools would not be four times as likely to develop CTS. Earlier papers found an even higher association with obesity. Werner et al 1994 viewed obesity as having a relative risk factor of 4.1 compared to more slender individuals.
Because of the lack of understanding of the causation there is no evidence of a link between the extent of tool usage and the onset or severity of the condition. In that situation, how do we deal with a claim by a diabetic obese former employee who smokes? Any of those factors may have caused the condition.
How does the claimant succeed in proving causation?
Bonnington Castings Ltd v Wardlaw  AC 613 starts the story. Here, a steel dresser contracted pneumoconiosis following exposure to silica dust from both a pneumatic hammer and swing grinders. A statutory duty applied to the grinders, but not the hammer. The issue was whether the dust that caused the injury came from the grinders or the hammer. It was held that, on the balance of probabilities, dust from the grinders had materially contributed to the injury (even if it could not be established that the offending dust came partially or wholly from the grinder). On that basis causation had been established. The material contribution test was born.
Later came Wilsher v Essex Area Health Authority  1 AC 1074. Here, a premature baby developed fibroplasia, resulting in blindness. One of the possible causes was negligent care. The medical evidence established that there were four other possible causes. The court decided for the defendant saying that the ‘but for’ test applied. The plaintiff had to prove that but for the defendant’s negligence, his injury would not have occurred. He could not.
Wilsher affirmed that the onus of proving causation rested with the plaintiff. Mr Bonnington’s silicosis could only have been caused by silica dust. Baby Wilsher’s fibroplasia had five possible causes.
In Bailey v Ministry of Defence & Anor  EWCA Civ 883, Lord Justice Waller tried to reconcile the two lines of authority. He considered that the ‘but for’ test was appropriate where there were different and distinct factors that could have caused the injury, but where it was impossible to establish which had actually done so (Wilsher). However, Bonnington established that any factor that could be shown to be a contributory factor to a material extent should be treated as causing the injury.
The right approach to causation continues to be the subject of conflicting decisions even to present date. In the Atomic Veterans Supreme Court decision, the Justices agreed with the Court of Appeal (CoA) that the veterans’ claims had no real prospect of success, Lord Phillips noting: “There is no known basis for concluding that the exposure will have gone so far as to double that risk. On the law as it stands, merely proving an increase in risk will not establish a good cause of action. To succeed a veteran must show that, on balance of probability, the injury would not have been sustained had it not been for the exposure.”
So, can our chubby, smoking CTS claimant realistically expect to satisfy the ‘but for’ test? Some claimants refer back to Palmer, arguing that use of vibratory tools can double the risk - and where the risk of injury is doubled and the injury occurs, causation is established. The other studies highlighted above (Tanaka etc) question this facile reasoning. Is there really a doubling of the risk from vibratory equipment? What level of tool usage actually doubles the risk? Is that risk significantly greater than the risks identified from obesity, smoking, diabetes etc?
If we refer back to Lord Justice Phillips, we may have a claim type here which is built upon a false premise. Even the use of epidemiological evidence of a doubling of the risk is itself dubious. The degree by which the risk of CTS is doubled by use of vibratory tools is also very much open to challenge. As always in disease cases, a defendant is only as good as their medical expert. Any medical expert being asked to consider CTS causation must employ detailed consideration of all the available material.
Arguments about doubling the risk will remind some insurers of other, more potentially expensive diseases. As ever, careful deliberation must be given to wider considerations when deploying causation arguments in disease cases.