What is the test for determining whether a plaintiff has a crumbling skull?

In Zacharias v. Leys, 2005 BCCA 560, a judgment pronounced in the interval between Athey and Resurfice, our Court of Appeal addressed the distinction between weighing evidence of causation and considering evidence going to the measure of damages. The distinction is important, particularly in cases where the plaintiff is alleged to have had a “crumbling skull”: 16 The crumbling skull rule is difficult to apply when there is a chance, but not a certainty, that the plaintiff would have suffered the harm but for the defendants' conduct. Major J. addressed this issue in Athey when he wrote, at paragraph 35, that damages should be adjusted only when there is a "measurable risk that the pre-existing condition would have detrimentally affected the plaintiff in the future, regardless of the defendant's negligence." Such a risk of harm need not be proved on a balance of probabilities, which is the appropriate standard for determining past events but not future ones. Future or hypothetical events should simply be given weight according to the probability of their occurrence. At paragraph 27, Major J. wrote that "if there is a 30 percent chance that the plaintiff's injuries will worsen, then the damage award may be increased by 30 percent of the anticipated extra damages to reflect that risk." In the same paragraph, he went on to say that a future event should be taken into account as long as it is a "real and substantial possibility and not mere speculation."