One of the principles of personal injury law is that the persona who caused the injury must take the victim as he finds him (Dulieu v. White,  2 K.B. 669 at 679 (D.C.)):
If a man is negligently run over or otherwise negligently injured in his body, it is no answer to the sufferer’s claim for damages that he would have suffered less injury, or no injury at all, if he had not had an unusually thin skull or an unusually weak heart.
The distinction between “thin skull” and “crumbling skull” was described in Athey v. Leonati,  3 S.C.R. 458 at para. 34, as follows:
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.
The Court went on to state that 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 anticipated deterioration can be taken into account in reducing the overall award.
In McKelvie v. Ng, 2001 BCCA 384, the Court ordered a new trial because the trial judge improperly evaluated the plaintiff’s original position by failing to take into account the impact of the plaintiff’s pre‑existing medical conditions.
In Whyte v. Morin, 2007 BCSC 1329, following Resurfice v. Hanke, the Court stated at para. 49:
[I]n the quantification of the plaintiff’s damages I do take into account the fact that, even if the plaintiff had not been injured in April 2004, other factors may have affected her ability to earn income since the accident and into the future. In the assessment of damages, the plaintiff is not entitled to be placed in a better position than she would have been but for the accident.
The fact that the plaintiff has met the burden of proof of a causative link does not necessarily mean that the defendant is responsible for the full amount of the damages. As set out by the Supreme Court of Canada in Blackwater v. Plint, 2005 SCC 58 at para. 78
It is important to distinguish between causation as the source of the loss and the rules of damage assessment in tort. The rules of causation consider generally whether “but for” the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities. Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damages, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff would have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway…