Causation for injuries in a motor vehicle accident case
Reasons for judgment were released yesterday in the case of McKenzie v. Lloyd. This case involved the claim for damages following injury in four motor vehicle accidents. A few days after one of the accidents the plaintiff was discovered to have suffered from a stroke. It is the cause of the stroke which was the fundamental issue in the case. 18 days after the accident that purportedly caused the stroke the plaintiff struck his head on the doorframe of a cedar play house. The morning after he hit his head he woke up with vertigo and severe nausea and he was taken by ambulance to the hospital. 2 days later the plaintiff had an emergency CT scan at which time the stroke was identified.
The plaintiff’s GP provided an opinion to the court in which she in part stated that in her opinion the accident was the cause of the stroke. The neurologist that treated the plaintiff provided the opinion that it was unlikely that the stroke would have occurred were it not for the motor vehicle accident. An epidemiologist also provided an opinion that in the absence of the motor vehicle accident the plaintiff would not have suffered a stroke. ICBC relied on the opinion o a neurosurgeon who opined that the stroke was more likely than not caused by the bump to the head. The court had to weigh these opinions and apply the law to determine whether the stroke was caused by the accident or not.
The Court began the analysis of causation with the following summary of the law:
 The “but for” test is the general test for factual causation: the plaintiff must prove on a balance of probabilities that but for the defendant’s negligence, he would not have suffered his injuries. As affirmed by the Supreme Court of Canada in Clements v. Clements, 2012 SCC 32 at paras. 8 – 10, this is a factual enquiry and causation must be established by the plaintiff on a balance of probabilities, having regard for all the evidence.
 The Court in Clements at para. 9, affirmed Mr. Justice Sopinka’s statement on the law of causation from Snell v. Farrell,  2 S.C.R. 311 at para. 29, where he said that causation need not be determined by scientific precision and that the “but for” test must be applied in a “robust common sense fashion.”
 The following statements from Snell at paras. 33 – 35 have application to the case at bar:
 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 Manfield’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)
 It is not therefore essential that the medical experts provide a firm opinion supporting the plaintiff’s theory of causation. Medical experts ordinarily determine causation in terms of certainties whereas a lesser standard is demanded by the law…
 In Harvey, Medical Malpractice (1973), the learned author states at p. 169 – Some courts have assumed an unrealistic posture in requiring that the medical expert state conclusively that a certain act caused a given result. Medical testimony does not lend itself to precise conclusions because medicine is not an exact science.
The trial judge ultimately concluded that it was more likely than not that the car accident caused the stroke. This case provides an excellent example of the manner in which our courts determine causation.