How does the "crumbling skull" doctrine work?

In Athey v. Leonati, supra, Major J. described at 243-4 the “crumbling skull” principle which is a subset of the “thin skull” principle: 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 short-comings, and not a better position.