How does the "crumbling skull" doctrine affect the award of damages for negligence?

The analysis in this case requires a consideration of two related doctrines: the “thin skull” and the “crumbling skull”. In Athey v. Leonati,[17] Justice Major summarized the two 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 that 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…