Wilsher v Essex AHA [1987] UKHL 11

A premature baby was given excess oxygen due to a doctor's negligence and subsequently went blind. However, there were other potential innocent causes. The House of Lords held the plaintiff must prove causation on the balance of probabilities, not simply that negligence increased the risk.

Facts

The plaintiff, Martin Wilsher, was born three months prematurely, weighing only 1.2 kg. He required care in a special care baby unit and was placed on oxygen therapy. A junior doctor negligently inserted a catheter into a vein rather than an artery, leading to monitoring equipment providing falsely low readings of the arterial oxygen tension in the plaintiff’s blood. As a result, the plaintiff was administered excess oxygen. He subsequently developed retrolental fibroplasia (RLF), a condition of the eyes which resulted in near total blindness. Medical evidence showed that RLF could be caused by an excess of oxygen, but there were four other potential causes unrelated to the excess oxygen, all of which were known risk factors for premature babies, including apnoea, hypercarbia, intraventricular haemorrhage, and patent ductus arteriosus. The trial judge found the health authority liable, a decision upheld by a majority in the Court of Appeal, which applied the principle from McGhee v National Coal Board.

Issues

The central legal issue was that of causation. Where an injury (RLF) had a number of different potential causes, one of which was the defendant’s tortious conduct (excess oxygen), but the others were non-tortious, could the plaintiff succeed by showing that the defendant’s breach of duty materially increased the risk of the injury occurring? Or did the plaintiff have to prove, on the balance of probabilities, that the tortious conduct was the actual cause of the injury?

Judgment

The House of Lords unanimously allowed the appeal, overturning the Court of Appeal’s decision. Lord Bridge of Harwich delivered the leading speech. He held that the ‘material increase in risk’ principle established in McGhee v National Coal Board could not be applied to this case. The burden of proof remained on the plaintiff to demonstrate that the defendant’s negligence was, on the balance of probabilities, the cause of his injury.

Lord Bridge’s Reasoning

Lord Bridge distinguished the facts from those in McGhee. In McGhee, there was a single causal agent (brick dust). The uncertainty was not about what caused the dermatitis, but whether the ‘guilty’ dust (from lack of showers) or the ‘innocent’ dust (from the workday) was responsible. In Wilsher, there were multiple distinct and independent potential causal agents. The defendant’s negligence was responsible for only one of these agents (excess oxygen). The other four potential causes were natural incidents of a premature birth.

Lord Bridge rejected the Court of Appeal’s expansive interpretation of McGhee, stating that it did not reverse the burden of proof in causation. He explained the distinction as follows:

The position, to my mind, is wholly different from that in McGhee v. National Coal Board [1973] 1 W.L.R. 1, where the plaintiff’s dermatitis was medically proved to have been caused by the brick dust with which he was brought into contact in the course of his employment. The only uncertainty was whether the dermatitis was caused by the dust to which he was exposed during the working day, for which the employer was not liable, or by the continued presence of the dust on his skin after work, for which, by his failure to provide showers, the employer was held to be responsible. There was only one “causal agent”… In the instant case, on the other hand, the medical evidence was that the plaintiff’s RLF could have been caused by any one of a number of different factors. The defendant’s breach of duty was in relation to only one of those factors.

He concluded that where there are multiple discrete causes, the plaintiff cannot succeed merely by showing the defendant’s breach added another potential cause to the list. The plaintiff must still prove that the defendant’s breach was the operative cause that resulted in the damage.

Implications

The decision in Wilsher v Essex AHA is of major importance in the law of tort, particularly in medical negligence cases. It significantly limited the application of the ‘material increase of risk’ test from McGhee. The judgment affirmed that, in cases involving multiple independent and competing causes, the traditional ‘but for’ test for causation remains the standard. A claimant must prove that ‘but for’ the defendant’s negligence, the harm would not have occurred. This case makes it more difficult for claimants to succeed in complex medical cases where the aetiology of a condition is uncertain and several factors could have contributed to the harm.

Verdict: Appeal allowed. The House of Lords set aside the order of the Court of Appeal and ordered that the issue of causation be remitted to a judge for a new trial.

Source: Wilsher v Essex AHA [1987] UKHL 11