What is the “crumbling skull” rule?

The “crumbling skull” rule was explained by Major J. in Athey v. Leonati at para [35]: 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: [References deleted] 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: [References deleted] 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. B. The Situation in this Case