The SCC declined to adopt either approach and outlined what it hoped would be the definitive word on causation, while staying within the boundaries of the traditional approach to causation. Justice Sopinka acknowledged that proving causation in medical malpractice cases is often difficult for the patient, and the physician is usually in a better position to know the cause of the injury.3 While the proof of causation in medical malpractice cases may be difficult for the plaintiff, the difficulty in proof arose from a “too rigid” adoption of the requirements of proof.4
Justice Sopinka, in speaking for the Court, stated as follows:
In many malpractice cases, the facts lie particularly within the knowledge of the defendant. In these circumstances, very little affirmative evidence on the part of the plaintiff will justify the drawing of an inference of causation in the absence of evidence to the contrary… The legal or ultimate burden remains with the plaintiff, but in the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced. If some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take account of Lord Mansfield’s famous precept. This is, I believe, what Lord Bridge had in mind in Wilsher when he referred to a “robust and pragmatic approach to the… facts” (p. 569).5
Justice Sopinka recognized the challenges with requiring precise conclusions on causation from medical experts because “medicine is not an exact science.”6 Following Snell, it was not necessary to prove causation with scientific precision.
Athey v. Leonati (1996)
In Athey v. Leonat7 (“Athey”), the SCC elaborated on the Snell decision, stating that causation can be established where the defendant’s negligence “materially contributed” to the injury, even if it was not the sole cause.
In this case, the plaintiff was an autobody repairman with a predisposition to back problems. He was involved in a motor vehicle accident that left him with a stiff back and neck, requiring transition to a position requiring less physical work. Months later, he was involved in a second motor vehicle accident which further shifted his position to involve less physical work. On his doctor’s advice, he engaged in exercises and felt a sudden “pop” in his low back. He was hospitalized, undergoing surgery for disc herniation. He was unable to return to work in the same position.
The trial judge found that the disc herniation was causally related only in a minor way to the motor vehicle accidents and the defendants should pay only 25% of the damages. The plaintiff’s appeal to the British Columbia Court of Appeal (BCCA) was dismissed. The plaintiff successfully appealed to the SCC which unanimously allowed him to recover 100% of his damages. While the accidents only contributed 25% to the disc herniation and 75% was still attributed to the plaintiff’s latent weakness, the Court found that the accidents nonetheless caused the disc herniation.
Justice Major emphasized that causation is typically established on the basis of the “but for” test, and sufficient causal connection exists if but for the defendant’s negligence, the plaintiff would not have suffered the injury.8 The plaintiff need not prove that the defendant’s negligence was the sole causal factor.9 It was sufficient to establish that the defendant’s conduct materially contributed to the injury, beyond the de minimis range. As outlined in Snell, causation was essentially a practical question of fact and a matter of common sense, which in some circumstances, an inference of causation may be drawn without positive scientific proof.10
