Legal Causation and Scientific Proof

There can be something of a culture clash when doctors enter the world of lawyers to act as medical experts. One of the areas of clash is what amounts to proof.

In civil claims, a judge will find something to be proved when on the evidence it was more likely than not. A 50.1% likelihood is enough (although 50% falls short). There may be a dispute as to what a patient told the doctor. The account which is more than 50% likely will be accepted. A judge treats it as having happened that way despite the uncertainty. It may be unclear whether a patient had condition A or condition B; she is regarded as having condition A if the court concludes it was the more likely of the 2. In considering whether a breach of duty caused a particular harm, there may be real doubt. But if the evidence suggests that it did – more likely than not – the court will find that, as a matter of fact. This is so even if the evidence does not meet what doctors might regard as a high enough standard of scientific proof.

Doctors are often uncomfortable with this. They are used to a higher standard of proof. But in giving expert evidence, they are required to say what they think was more likely than not.

The case of Wood v Ministry of Defence[i] (MOD) provides an interesting example.

Mr Wood developed a multi-system atrophy, a neurological condition similar to Parkinson’s disease (PD). He blamed a high level of exposure to solvents whilst in the RAF. He brought a claim against the MOD for breach of duty in exposing him to this risk. The MOD admitted taking inadequate steps to protect him but denied that solvents had caused his condition.

It accepted that the solvents were neurotoxic. It admitted they were capable of causing temporary damage to the nervous system or in some extreme cases, death. But it argued that there was no evidence that it could cause permanent neurological damage short of death.

Mr Wood produced evidence from a Professor Seaton, a specialist in Environmental and Occupational Medicine. He had published research on the effects of solvents on the nervous system. His said that although the evidence fell short of ‘scientific proof’ there was probably a causal link between high levels of solvent exposure and neurological damage. The MOD relied on evidence from 2 neurologists. The judge found inconsistencies in the neurologists’ evidence and was persuaded by Professor Seaton. Mr Wood therefore won his claim at trial.

The MOD appealed. It contended that Professor Seaton’s opinion was not soundly based in scientific research and that he was an evangelist for the link between solvents and neurological damage. (There is a hint of criticism of the expert’s evangelistic zeal notwithstanding the court’s acceptance of his evidence.)

In passing, it is unfortunate that the judgment does not distinguish an ‘association’ or ‘link’ from causation causation. The 2 concepts are not the same and what a claimant has to prove is causation. However, it seems that when the judge and Court of Appeal were using the terms to suggest causal links.

The Court of Appeal looked carefully at Professor Seaton’s evidence and the grounds for his opinion.

There were several areas of evidence for the ‘association’.

The first was a series of papers Professor Seaton had published triggered by his own experience of patients exposed to high levels of solvents in naval dockyards. Some had developed neurological symptoms which did not fit a pattern of recognised neurological disease. Those closer to the source of exposure developed more severe symptoms. These were small scale studies but they convinced him there was a causal link. He accepted that the methodology was not perfect but they were of ‘some value’.

A second was a multi-centre study of patients with PD who had been exposed to a wide range of chemicals. It showed an association between PD and pesticides. It did not show an association with solvents except in the presence of certain genetic factors and did not include subjects with the high levels of exposure found in his naval dockyard studies. This study did not undermine his view.

A third was evidence of some painters in the RAF who developed a Parkinsonian syndrome at a young age. The presence of ‘clusters’ of patients with the same condition in this study and in his, particularly where the condition occurred at a younger age than usual, suggested an association.

Finally, the timing of the onset of symptoms which was consistent with the exposure blamed, even on the evidence of the MOD’s experts.

Professor Seaton was cross-examined and it was put to him that his view lacked scientific proof. He pointed out that it was unlikely there would ever be an epidemiological study demonstrating a clear association between solvent-exposure and chronic neurological damage. It would be virtually impossible to produce such a study. Most exposure was in low concentration, insufficient to cause permanent damage and solvent-induced brain damage was a rare event. But in his view this did not mean one could not infer an association.

He accepted that the evidence fell short of scientific proof but still contended that a doctor (or the court) could properly conclude that there probably was such a link. As Dame Janet Smith said in the Court of Appeal, he was correctly drawing a distinction between the standard required by epidemiologists for scientific proof and the standard of proof in a claim for damages. In her words:

‘Whereas an epidemiologist will not declare that there is an association unless a study shows that it is 95% certain that the apparent association cannot be the result of chance, the judge in a civil claim need only be satisfied that it is more likely than not that the condition has been caused by the alleged exposure. The thrust of the Professor’s argument was that, even where there is no supportive epidemiology, there are other means by which causation or a causal link could be established.’

The Court of Appeal therefore upheld the trial judge’s findings. From a legal point of view, causation was established. On the balance of probabilities, solvent-exposure had caused Mr Wood’s condition even though the evidence fell short of scientific proof.

The case is provides a reminder that medical experts sometimes need to change the way they approach causation when they enter the legal arena. Experts sometimes say in reports that they are cannot be ‘certain’ or are ‘unsure’. It is fine to say that provided they do not confuse certainty with the balance of probabilities as the standard of proof in civil claims. The courts do not need certainty or a high level of scientific proof. They need to know what is the case on the balance of probabilities.

Paul Sankey

Paul Sankey is an experienced clinical negligence solicitor and partner at Enable Law. He is one of Inspire MediLaw’s trainers and regularly speaks at our conferences.

Paul Sankey – Partner | Enable Law

[i] [2011] EWCA Civ 792