The recently decided case of Bailey (by father and litigation friend) v Min. of Defence and another  EWCA Civ 883 re-examined the “but for” test in causation.
The first defendant managed a hospital at which the claimant underwent a procedure to explore and treat a possible gall stone. It was accepted that the procedure and post-operative recovery were negligent. Her condition deteriorated, partly due to the effect of pancreatitis. She underwent further procedures and was transferred to a hospital managed by the second defendant where she aspirated vomit which lead to cardiac arrest and hypoxic brain damage. The claimant’s case was that, notwithstanding the pancreatitis, it was the negligence of the first defendant that so weakened the claimant that she could not prevent herself aspirating vomit. The first defendant argued that as the clamant was at the same time suffering from, and was weakened by pancreatitis, which they did not cause, any negligence on their part was only one of two contributing factors and causation was not proved.
They relied heavily on the case of Wilsher v Essex Area Health Authority  1AC 1074 where negligent exposure to excess oxygen was one of several factors that could have caused the claimant in that case to develop retrolental fibroplasia, and where the court took that view that as excess oxygen could not be identified as an actual contributing factor in the development of the condition, causation was not established.
In Bailey the Court of Appeal made it clear that what was required was that there should be a “materially contributing cause”, and that it did not matter that it could not be said that one of two contributing causes (the pancreatitis and the negligent procedure) was more or less significant than the other. All that was necessary was that there had to be a material contribution on the balance of probabilities.
Lord Justice Waller summarised the position in relation to cumulative causes is as follows. If the evidence demonstrates, on a balance of probabilities, that the injury would have occurred as a result of a non-tortious cause or causes in any event, the claimant will fail to establish that the tortious cause contributed. The Hotson case exemplifies such a situation. If the evidence demonstrates that “but for” the contributions of the tortious cause the injury probably would not have occurred, the claimant will have discharged the burden of proving causation. In a case where medial science cannot establish the probability that “but for” an act of negligence the injury would not have happened but can establish that the contribution of the negligent cause was more than negligible, the “but for” test is modified and the claimant will succeed.