How is the crumbling skull rule applied in medical malpractice cases?

The defendant underlined the problem of assessing damages in a “crumbling skull” situation. The risk of future harm is not to be proven on the balance of probabilities but is to be given weight according to the probability of the occurrence happening: Zacharias v. Leys, 2005 BCCA 560. In Zacharias the Court of Appeal said at para. 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." [Emphasis in original.]