How have courts treated the issue of pre-existing conditions in questions of causation?

In Blackwater v. Plint, supra, Brenner C.J. addressed the problem of pre-existing conditions in questions of causation, and their treatment via the “thin skull” and “crumbling skull” doctrines, at paras. 381-385: Athey also addresses the relevance of pre-existing conditions. In British Columbia the "thin skull" and "crumbling skull" principles have traditionally addressed the issue of a pre-existing condition. Although the two principles are related, they differ in terms of the assessment of damages. In a thin skull case, a plaintiff will recover full damages for the injuries suffered. In a crumbling skull case, a reduction in the quantum of damages will be made to account for the plaintiff's pre-existing condition... ... Significantly, the discussion in Athey avoids this "thin skull/crumbling skull" terminology. Major J. outlined the principles at paras. 34-35: The "crumbling skull" doctrine is an awkward label for a fairly simple idea. It is named after the well-known "thin skull" rule, which makes the tortfeasor liable for the plaintiff's injuries even if the injuries are unexpectedly severe owing to a pre-existing condition. The tortfeasor must take his or her victim as the tortfeasor finds the victim, and is therefore liable even though the plaintiff's losses are more dramatic than they would be for the average person. The so-called "crumbling skull" rule simply recognizes that the pre-existing condition was inherent in the plaintiff's "original position". The defendant need not put the plaintiff in a position better than his or her original position. The defendant is liable for the injuries caused, even if they are extreme, but need not compensate the plaintiff for any debilitating effects of the pre-existing condition which the plaintiff would have experienced anyway. The defendant is liable for the additional damage but not the pre-existing damage. ... Likewise, if 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, then this can be taken into account in reducing the overall award. ... This is consistent with the general rule that the plaintiff must be returned to the position he would have been in, with all of its attendant risks and shortcomings, and not a better position.