If I have a stroke right after a minor collision, will ICBC say the events are totally unrelated? What if I was totally healthy before the collision and then had headaches leading up to the stroke?
In the recent case McKenzie v Lloyd (2016 BCSC 1745), the plaintiff was involved in three collisions. The first collision was relatively minor and the plaintiff did not visit his doctor. He did, however, testify that he suffered from headaches following the crash. 18 days after that first collision, the plaintiff struck his head on a doorframe and was left with a cut on his head. The next day, he awoke with vertigo and severe nausea and attended the hospital. He did not report the collision nor the recent head bump to the emergency room physician, and was sent home with anti-nausea medication.
The next day, he followed up with his general practitioner, who was concerned enough about the plaintiff’s symptoms to put the wheels in motion for the plaintiff to have an emergency CT scan, which revealed that the plaintiff had suffered a stroke. Whether or not the collision had caused or contributed to the stroke was the significant issue at trial.
Justice Russell reviewed the law on causation before ultimately finding that the collision had not caused or contributed to the plaintiff’s stroke:
 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 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).
 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. As pointed out in Louisell, Medical Malpractice, vol. 3, the phrase “in your opinion with a reasonable degree of medical certainty,” which is the standard form of question to a medical expert, is often misunderstood. The author explains, at p. 25-57, that:
Many doctors do not understand the phrase … as they usually deal in “certainties” that are 100% sure, whereas “reasonable” certainties which the law requires need only be more probably so, i.e., 51%.
 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.
 Both the plaintiff and the defendants in this case submitted medical expert reports arguing opposing theories with regard to causation. The plaintiff submits that MVA#1 caused his stroke, while the defendants have submitted contradictory evidence asserting that the head bump of May 15, 2012 caused it. As noted above, the ultimate burden rests with the plaintiff to prove that the MVA is the more likely cause. He is not required to prove his case with absolute precision or with medical certainty.
 Keeping the foregoing in mind, I have considered and weighed the evidence adduced in this case, including the testimony of witnesses and experts, along with the expert reports. For the following reasons, I find that the plaintiff has established that it is more probable than not that MVA#1 caused the stroke and that “but for” this MVA he would not have suffered a stroke.